Shareholders' Equity (Details Textual) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended								3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Stock Options [Member]	Dec. 31, 2012 Restricted Stock [Member]	Dec. 31, 2012 Stock incentive plan [Member]	Mar. 31, 2012 Annual Co-CEO Equity Bonus Plan [Member]	Mar. 31, 2011 Annual Co-CEO Equity Bonus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock reserved for issuance under stock incentive plans								13,700,000
Other benefit plans, shares									111,270	188,302
Common Stock issued, fair value per share									$ 38.38	$ 33.99
Unrecognized compensation expense related to equity-based awards that have yet to vest						$ 23	$ 8
Weighted average period of cost expected to be recognized						3 years 3 months 18 days	2 years 3 months 18 days
Restricted Common Stock vesting period				3 years	4 years
Stockholders' Equity (Textual) [Abstract]
Rate of options exercisable per year, commencing one year after grant	20% per year commencing one year after grant
Duration of options expiration, after date of grant	10 years
Total intrinsic value of options exercised	25	23	19
Cash received from the exercise of stock options	40	33	27
Tax benefit related to exercise of stock options	8	7	6
Fair value per share of options granted	$ 13.02	$ 12.49	$ 8.90
Total compensation expense related to stock incentive plans	26	22	20
Tax benefit related to compensation costs	8	7	6
Net of tax unrealized gains (losses) related to credit-only impaired securities	$ 33	$ (16)	$ (11)