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Financing Agreement - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
1 Months Ended
Mar. 31, 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    
Issuance of senior notes       25,000 75,000
Interest rate for purchase agreement       3.81% 4.60%
Maturity date       5/31/2018 5/31/2021
Capital lease obligations 6,354 7,179      
Capital lease obligations, current $ 2,627 $ 2,887