Summary of Significant Accounting Policies - Software and Website Development Costs (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Capitalized software costs	$ 101,001,000	$ 50,655,000
Accumulated amortization	$ 29,611,000	19,924,000
Software Costs
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Capitalized software costs	$ 43,900,000	29,700,000
Accumulated amortization	20,600,000	11,900,000
Capitalized software costs write off amount	1,200,000	300,000
Accumulated amortization written off	0	100,000
Acceleration of amortization	$ 0	$ 200,000