Financing Agreement - Additional Information (Detail) (USD $)	1 Months Ended				1 Months Ended
	Aug. 31, 2011	Sep. 30, 2012	Dec. 31, 2011	Dec. 31, 2010 Master Notes	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]
Unsecured revolving loan facility	$ 350,000,000
Loan agreement expiration date	2016-08
Revolving loan facility, maximum capacity	250,000,000
Issuance of senior notes					25,000,000	75,000,000
Interest rate for purchase agreement					3.81%	4.60%
Maturity date					2018-05	2021-05
Issuance date of senior notes				2011-05
Capital lease obligations		7,299,000	7,253,000
Current portion of capital lease obligations		$ 2,607,000	$ 2,653,000