Concentration of Credit Risk and Off-Balance Sheet Risk - Revenue and Trade Accounts Receivable from Bidders (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Concentration Risk [Line Items]
Concentration Risk, Percentage	10.00%	10.00%	10.00%	10.00%
Sales Revenue, Net [Member] | Bidder A [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	6.00%	8.00%	8.00%	10.00%
Sales Revenue, Net [Member] | Bidder B [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	13.00%	12.00%	12.00%	12.00%
Sales Revenue, Net [Member] | Bidder C [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	8.00%	3.00%	8.00%	3.00%
Accounts Receivable [Member] | Bidder A [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage			8.00%	8.00%
Accounts Receivable [Member] | Bidder B [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage			14.00%	13.00%
Accounts Receivable [Member] | Bidder C [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage			10.00%	5.00%