| Leases - Supplemental Balance Sheet Information (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Jan. 01, 2019 | Dec. 31, 2018 | 
|---|---|---|---|
| Operating Lease [Abstract] | |||
| Operating lease right-of-use assets | $ 428 | $ 347 | $ 0 | 
| Operating lease current liabilities | 46 | 30 | 0 | 
| Operating lease liabilities | 397 | $ 339 | $ 0 | 
| Total operating lease liabilities | 443 | ||
| Finance Lease [Abstract] | |||
| Property and equipment, gross | 31 | ||
| Accumulated depreciation | (22) | ||
| Property and equipment, net | 9 | ||
| Current maturities of debt | 4 | ||
| Long-term debt | 1 | ||
| Total finance lease liabilities | $ 5 | ||
| Weighted average remaining lease term, operating leases (years) | 10 years | ||
| Weighted average remaining lease term, finance leases (years) | 2 years | ||
| Weighted average discount rate, operating lease (as a percent) | 5.30% | ||
| Weighted average discount rate, finance lease (as a percent) | 1.90% | 
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- Definition Finance Lease, Right-Of-Use Asset, Accumulated Depreciation No definition available. 
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- Definition Finance Lease, Right-Of-Use Asset, Gross No definition available. 
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- References No definition available. 
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- Definition Present value of lessee's discounted obligation for lease payments from finance lease. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Present value of lessee's discounted obligation for lease payments from finance lease, classified as current. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Present value of lessee's discounted obligation for lease payments from finance lease, classified as noncurrent. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Amount of lessee's right to use underlying asset under finance lease. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Weighted average discount rate for finance lease calculated at point in time. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Weighted average remaining lease term for finance lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- References No definition available. 
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- Definition Present value of lessee's discounted obligation for lease payments from operating lease. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Present value of lessee's discounted obligation for lease payments from operating lease, classified as current. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Present value of lessee's discounted obligation for lease payments from operating lease, classified as noncurrent. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Amount of lessee's right to use underlying asset under operating lease. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Weighted average discount rate for operating lease calculated at point in time. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Reference 1: http://www.xbrl.org/2003/role/exampleRef 
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