Derivative Instruments - Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative Instruments:
Derivative instruments, notional amount	$ 46,911,800	$ 40,109,880
Net estimated fair value of derivative instruments - liabilities	(2,584,907)	(2,552,687)
Derivative Liabilities [Member]
Derivative Instruments:
Derivative instruments, notional amount	46,911,800	40,109,880
Net estimated fair value of derivative instruments - liabilities	(2,584,907)	(2,552,687)
Derivative Assets [Member]
Derivative Instruments:
Net estimated fair value of derivative instruments - assets