Fair Value (Reconciliation of Assets Measured at Fair Value on a Recurring Basis Using Level 3 Inputs) (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 241	$ 417
Purchases	46	18
Sales	(21)	(2)
Net unrealized losses in accumulated other comprehensive income	(2)	0
Net realized gains in investment and other income	7	0
Transfers to held-to-maturity	0	(222)
Balance at end of period	271	211
Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	17	208
Purchases	15	0
Sales	0	0
Net unrealized losses in accumulated other comprehensive income	0	0
Net realized gains in investment and other income	0	0
Transfers to held-to-maturity	0	(201)
Balance at end of period	32	7
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	224	209
Purchases	31	18
Sales	(21)	(2)
Net unrealized losses in accumulated other comprehensive income	(2)	0
Net realized gains in investment and other income	7	0
Transfers to held-to-maturity	0	(21)
Balance at end of period	$ 239	$ 204