Debt - Additional Information (Detail) (USD $)							6 Months Ended
	Jun. 30, 2013	Jan. 18, 2013	Jun. 30, 2013 Receivables Securitization Program [Member]	Dec. 31, 2012 Receivables Securitization Program [Member]	Jun. 30, 2013 Receivables Securitization Program [Member] USR [Member]	Dec. 31, 2012 Receivables Securitization Program [Member] USR [Member]	Jun. 30, 2013 2013 Credit Agreement [Member]	Jun. 30, 2013 Minimum [Member] 2013 Credit Agreement [Member]	Jun. 30, 2013 Maximum [Member] 2013 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum amount of financing upon amendment of program		$ 200,000,000
Credit facility with aggregate committed principal amount		150,000,000					700,000,000
Receivables sold to Investor	377,500,000		377,500,000	400,200,000
Credit facility					200,000,000	150,000,000
Maturity date of loan agreement							Jul. 06, 2018
Potential maximum committed principal amount							1,050,000,000
Letter of credit maximum capacity							$ 50,000,000
LIBOR-based loans rates								1.00%	2.00%
Alternate base rate loans rates								0.00%	1.00%
LIBOR-based loans rate							1.375%
Alternate base rate loans rate							0.375%
Percentage of lenders fee on unutilized portion borrowing facility								0.15%	0.35%
Percentage of outstanding debt priced at variable interest rates	100.00%