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	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Contractual/notional amount	$ 69	$ 84
Estimated fair value asset
Estimated fair value (liability)	(1)	(3)
Derivative Instruments Without Hedge Designation [Member]
Derivative [Line Items]
Contractual/notional amount	52	84
Estimated fair value asset
Estimated fair value (liability)	(1)	(3)
Derivative Instruments Without Hedge Designation [Member] | Credit default swap - purchased protection [Member]
Derivative [Line Items]
Contractual/notional amount	20	20
Estimated fair value asset
Estimated fair value (liability)		(1)
Derivative Instruments Without Hedge Designation [Member] | Currency forwards [Member]
Derivative [Line Items]
Contractual/notional amount	27	59
Estimated fair value asset
Estimated fair value (liability)	(1)	(2)
Derivative Instruments Without Hedge Designation [Member] | Equity warrants [Member]
Derivative [Line Items]
Contractual/notional amount	5	5
Estimated fair value asset
Estimated fair value (liability)
Trading activities [Member] | Futures Sold, Not yet Purchased [Member]
Derivative [Line Items]
Contractual/notional amount	17
Estimated fair value asset
Estimated fair value (liability)