Property and Equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Property and equipment, net
Property and equipment, gross	$ 145,441		$ 145,441		$ 103,783
Less accumulated depreciation and amortization	(56,799)		(56,799)		(43,144)
Property and equipment, net	88,642		88,642		60,639
Depreciation and amortization	9,000	$ 5,300	22,700	$ 13,600
Furniture and computer equipment
Property and equipment, net
Property and equipment, gross	60,881		60,881		48,399
Site and software development costs
Property and equipment, net
Property and equipment, gross	47,653		47,653		36,294
Leasehold improvements
Property and equipment, net
Property and equipment, gross	27,351		27,351		14,290
Construction in progress
Property and equipment, net
Property and equipment, gross	$ 9,556		$ 9,556		$ 4,800