Summary of Significant Accounting Policies - Software and Website Development Costs (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Technology and development
Property, Plant and Equipment [Line Items]
Capitalized software costs write off amount	$ 300,000	$ 1,200,000
Software and Software Development Costs
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Capitalized software costs	$ 55,700,000	43,900,000
Accumulated amortization of computer software	35,300,000	20,600,000
Capitalized software costs write off amount			$ 300,000
Accumulated amortization written off			100,000
Acceleration of amortization	2,100,000		$ 200,000
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