Interest Rate Swaps - Netting Presentation (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative liabilities - Net estimated fair value/carrying value	$ 59,073	$ 44,467	[1]
Liabilities [Member] | Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities - Notional amount	1,355,000	950,000
Derivative instruments having gross unrealized losses, fair value	59,073	44,467
Derivative liabilities - Net notional amount	1,355,000	950,000
Derivative liabilities - Net estimated fair value/carrying value	$ 59,073	$ 44,467

[1] Derived from the audited consolidated financial statements.