Financing Agreement - Additional Information (Detail) (USD $)				1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Aug. 31, 2011	Dec. 31, 2010 Master Notes Senior Unsecured Notes, 3.81 Percent, Due May 2018	Dec. 31, 2010 Master Notes Senior Unsecured Notes, 4.60 Percent, Due May 2021
Line of Credit Facility [Line Items]
Revolving loan facility, maximum capacity			$ 350,000,000
Issuance of senior notes				25,000,000	75,000,000
Interest rate for purchase agreement				3.81%	4.60%
Maturity date				5/31/2018	5/31/2021
Capital lease obligations	7,601,000	7,179,000
Capital lease obligations, current	$ 3,297,000	$ 2,887,000