Derivative Instruments - Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Derivative instruments, notional amount	$ 40,100,000	$ 27,100,000
Net estimated fair value of derivative instruments - assets		2,561
Net estimated fair value of derivative instruments - liabilities	(2,552,687)	(754,439)
Derivative Liabilities [Member]
Derivative [Line Items]
Derivative instruments, notional amount	40,109,880	26,882,460
Net estimated fair value of derivative instruments - liabilities	(2,552,687)	(754,439)
Derivative Assets [Member]
Derivative [Line Items]
Derivative instruments, notional amount		200,000
Net estimated fair value of derivative instruments - assets		$ 2,561