Derivatives, Interest Rate Risk Management Programs (Details) (Not Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011		Sep. 30, 2011
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 18,348,043	[1]	$ 12,417,696	[1]	$ 15,801,043	[1]
Interest Rate Risk Management Program [Member]
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 66,000	[1]	$ 44,000	[1]	$ 44,000	[1]

[1] Notional amounts for commodity contracts are converted into dollar-equivalent amounts based on dollar prices at the inception of the contract.