Property and Equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Property and equipment, net
Property and equipment, gross	$ 127,863		$ 127,863		$ 103,783
Less accumulated depreciation and amortization	(51,496)		(51,496)		(43,144)
Property and equipment, net	76,367		76,367		60,639
Depreciation and amortization	7,200	$ 4,400	13,700	$ 8,300
Furniture and computer equipment
Property and equipment, net
Property and equipment, gross	55,672		55,672		48,399
Site and software development costs
Property and equipment, net
Property and equipment, gross	43,859		43,859		36,294
Leasehold improvements
Property and equipment, net
Property and equipment, gross	28,124		28,124		14,290
Construction in progress
Property and equipment, net
Property and equipment, gross	$ 208		$ 208		$ 4,800