Segments - Consolidated Sales Revenues by Similar Products and Services (Detail) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting, Revenue Reconciling Item [Line Items]
Net revenues	$ 134,630,171	$ 123,055,431	$ 112,183,360	$ 102,725,747	$ 103,754,645	$ 97,266,784	$ 88,844,059	$ 83,736,006	$ 86,465,653	$ 81,628,144	$ 69,378,160	$ 61,459,668	$ 472,594,709	$ 373,601,494	$ 298,931,625
Marketplace [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net revenues													331,338,080	261,997,021	217,932,815
Non-marketplace [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net revenues													$ 141,256,629	$ 111,604,473	$ 80,998,810