Recorded Amounts and Estimated Fair Values of Long-term Debt and Derivative Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Total	$ 1,309,087	$ 707,472
Carrying (Reported) Amount, Fair Value Disclosure
Derivative [Line Items]
Variable rate	406,978	307,433
Fixed rate	902,109	400,039
Total	1,309,087	707,472
Estimate of Fair Value, Fair Value Disclosure
Derivative [Line Items]
Variable rate	406,978	307,433
Fixed rate	954,053	438,492
Total	$ 1,361,031	$ 745,925