Summary of Significant Accounting Policies - Software and Website Development Costs Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Technology and development
Property, Plant and Equipment [Line Items]
Capitalized software costs write off amount	$ 100,000	$ 300,000
Software and Software Development Costs
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Capitalized software costs	$ 71,200,000	55,700,000
Accumulated amortization of computer software	48,800,000	35,300,000
Acceleration of amortization	1,700,000	2,100,000
Software and Software Development Costs, Assets Not Yet Placed in Service
Property, Plant and Equipment [Line Items]
Accumulated amortization written off	0	0
Acceleration of amortization	$ 0	$ 0