Derivative Instruments (Details 4) (AFS [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Interest rate swaps [Member]
Summary of notional amounts of AFS's trading derivative products
Derivative products, Notional Amount	$ 978,781	$ 1,370,995
Interest rate swaps pay-fixed/receive-variable [Member]
Summary of notional amounts of AFS's trading derivative products
Derivative products, Notional Amount	2,094,301	3,798,305
Interest rate swaps basis swaps [Member]
Summary of notional amounts of AFS's trading derivative products
Derivative products, Notional Amount	175,835	175,835
Currency Swaps [Member]
Summary of notional amounts of AFS's trading derivative products
Derivative products, Notional Amount	3,549	13,559
Future contracts [Member]
Summary of notional amounts of AFS's trading derivative products
Derivative products, Notional Amount	295,000	53,500
Other contracts [Member]
Summary of notional amounts of AFS's trading derivative products
Derivative products, Notional Amount	$ 118,450	$ 118,930