Fair Value Measurements (Quantitative Information About Level 3 Measurements) (Details) (Recurring [Member], USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	$ 10,334		$ 11,164
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	175		183
State And Municipal Securities [Member] | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	25	[1]	23	[1]
State And Municipal Securities [Member] | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	5.00%	[1]	6.00%	[1]
Unobservable Input, Workout period	4 years	[1]	4 years	[1]
State And Municipal Securities [Member] | Maximum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	9.00%	[1]	10.00%	[1]
Unobservable Input, Workout period	5 years	[1]	6 years	[1]
Equity and Other Non-Debt Securities [Member] | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	$ 146	[1]	$ 156	[1]
Equity and Other Non-Debt Securities [Member] | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	6.00%	[1]	4.00%	[1]
Unobservable Input, Workout period	2 years	[1]	2 years	[1]
Equity and Other Non-Debt Securities [Member] | Maximum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	9.00%	[1]	6.00%	[1]
Unobservable Input, Workout period	4 years	[1]	4 years	[1]

[1] Auction-rate securities.