Derivative Financial Instruments (Summary of aggregate contractual or notional amounts and gross estimated fair values related to derivative financial instruments) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Contractual/notional amount	$ 113	$ 46
Estimated fair value asset		1
Estimated fair value (liability)	(1)	(1)
Without Hedge Designation [Member]
Contractual/notional amount	65	46
Estimated fair value asset		1
Estimated fair value (liability)	(1)	(1)
Without hedge designation [Member] | Credit default swaps - purchased protection [Member]
Contractual/notional amount	20	20
Estimated fair value asset
Estimated fair value (liability)	(1)	(1)
Without hedge designation [Member] | Currency forwards [Member]
Contractual/notional amount	40	22
Estimated fair value asset		1
Estimated fair value (liability)
Without hedge designation [Member] | Equity warrants [Member]
Contractual/notional amount	5	4
Estimated fair value asset
Estimated fair value (liability)
Trading Activities [Member] | Futures sold, not yet purchased [Member]
Contractual/notional amount	48
Estimated fair value asset
Estimated fair value (liability)