Derivative Instruments - Fair Value (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Derivative instruments, notional amount	$ 35,500,000	$ 27,100,000
Net estimated fair value of derivative instruments - assets		2,561	[1]
Net estimated fair value of derivative instruments - liabilities	1,035,215	754,439	[1]
Derivative Assets [Member]
Derivative [Line Items]
Derivative instruments, notional amount		200,000
Net estimated fair value of derivative instruments - assets		2,561
Derivative Liabilities [Member]
Derivative [Line Items]
Derivative instruments, notional amount	35,525,230	26,882,460
Net estimated fair value of derivative instruments - liabilities	$ (1,035,215)	$ (754,439)

[1] Derived from the audited consolidated financial statements at December 31, 2010.