| Summary of Significant Accounting Policies - Cumulative Effect of Adoption of ASUs on the Consolidated Balance Sheet (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Jan. 01, 2019 | Dec. 31, 2018 | 
|---|---|---|---|---|
| ASSETS | ||||
| Operating lease right-of-use assets | $ 427 | $ 466 | $ 347 | $ 0 | 
| Liabilities: | ||||
| Accounts payable and accrued expenses | 1,223 | 1,151 | 735 | 736 | 
| Operating lease current liabilities | 48 | 46 | 30 | 0 | 
| Operating lease liabilities | 419 | 456 | 339 | 0 | 
| Other long-term liabilities | $ 149 | $ 94 | 81 | $ 102 | 
| Accounting Standards Update 2016-02 | ||||
| ASSETS | ||||
| Operating lease right-of-use assets | 347 | |||
| Liabilities: | ||||
| Accounts payable and accrued expenses | (1) | |||
| Operating lease current liabilities | 30 | |||
| Operating lease liabilities | 339 | |||
| Other long-term liabilities | $ (21) | 
| X | ||||||||||
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- Definition Accounts payable and accrued expenses excluding interest payable current. No definition available. 
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| X | ||||||||||
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- References No definition available. 
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| X | ||||||||||
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- References No definition available. 
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| X | ||||||||||
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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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| X | ||||||||||
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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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| X | ||||||||||
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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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| X | ||||||||||
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- Definition Amount of liabilities classified as other, due after one year or the normal operating cycle, if longer. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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| X | ||||||||||
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- Details 
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