Annual report pursuant to Section 13 and 15(d)

Composition of Certain Financial Statement Captions - Narrative (Details)

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Composition of Certain Financial Statement Captions - Narrative (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Property, Plant and Equipment [Abstract]      
Number of owned securities that were in an unrealized loss position | security 0    
Unrealized losses deemed to be other-than-temporary $ 0    
Property, Plant and Equipment [Line Items]      
Depreciation 46,200,000 $ 34,200,000 $ 20,400,000
Capitalized cost related to internal use software and website development costs 7,400,000 7,500,000  
Accumulated amortization 130,789,000 89,385,000  
Software developed for internal use      
Property, Plant and Equipment [Line Items]      
Capitalized cost related to internal use software and website development costs 50,900,000 25,700,000  
Accumulated amortization 23,400,000 9,300,000  
Amortization expense $ 14,100,000 5,300,000 $ 2,200,000
Minimum      
Property, Plant and Equipment [Line Items]      
Expected useful lives (years) 3 years    
Minimum | Software developed for internal use      
Property, Plant and Equipment [Line Items]      
Expected useful lives (years) 3 years    
Maximum      
Property, Plant and Equipment [Line Items]      
Expected useful lives (years) 5 years    
Maximum | Software developed for internal use      
Property, Plant and Equipment [Line Items]      
Expected useful lives (years) 5 years    
Other current assets      
Property, Plant and Equipment [Line Items]      
Receivables for the reimbursement of costs of leasehold improvements in connection with operating leases $ 3,800,000 $ 9,100,000