Fair Value Measurements (Quantitative Information About Level 3 Measurements) (Details) (Recurring [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	11,069		11,164
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	180		183
State And Municipal Securities [Member] | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	23	[1]	23	[1]
State And Municipal Securities [Member] | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	6.00%	[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	10.00%	[1]	10.00%	[1]
Unobservable Input, Workout period	6 years	[1]	6 years	[1]
Auction-Rate Preferred Securities [Member] | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets at fair value	153		156
Auction-Rate Preferred Securities [Member] | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	4.00%		4.00%
Unobservable Input, Workout period	2 years		2 years
Auction-Rate Preferred Securities [Member] | Maximum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable Input, Discount rate	6.00%		6.00%
Unobservable Input, Workout period	5 years		4 years

[1] Primarily auction-rate securities.