Summary of Credit Agreement (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Letters of credit issued	$ 11,130	$ 4,135
Borrowing capacity	48,770	55,865
Revolving credit facility
Line of Credit Facility [Line Items]
Credit limit	60,000	40,000
Outstanding balance on revolving credit facility	100
Interest rate	5.00%
Term Loan
Line of Credit Facility [Line Items]
Credit limit	59,000	40,000
Outstanding principal balance on the term loan, less unamortized discount of $573 and $444, respectively, due November 2014	$ 58,177	$ 19,306
Interest rate	3.80%	3.60%