Employee Benefit Plans - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended									12 Months Ended							12 Months Ended							12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012 restricted_stock_unit	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Subsidiaries [Member]	Dec. 31, 2011 Subsidiaries [Member]	Dec. 31, 2010 Subsidiaries [Member]	Dec. 31, 2012 Nonqualified 401k Excess Plan [Member]	Dec. 31, 2011 Nonqualified 401k Excess Plan [Member]	Dec. 31, 2010 Nonqualified 401k Excess Plan [Member]	Dec. 31, 2012 Pension Plans, Defined Benefit	Dec. 31, 2011 Pension Plans, Defined Benefit	Dec. 31, 2010 Pension Plans, Defined Benefit	Nov. 15, 2011 Pension Plans, Defined Benefit	Nov. 14, 2011 Pension Plans, Defined Benefit	Dec. 31, 2012 Other Postretirement Benefit Plan, Defined Benefit, Pre-tax [Member]	Dec. 31, 2011 Other Postretirement Benefit Plan, Defined Benefit, Pre-tax [Member]	Dec. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2011 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2010 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2012 Other Postretirement Benefit Plan, Defined Benefit, After-tax [Member]	Dec. 31, 2011 Other Postretirement Benefit Plan, Defined Benefit, After-tax [Member]	Dec. 31, 2012 Salary Shares [Member]	Dec. 31, 2011 Salary Shares [Member]	Dec. 31, 2012 Restricted Stock [Member]	Dec. 31, 2011 Restricted Stock [Member]	Dec. 31, 2010 Restricted Stock [Member]	Dec. 31, 2012 Participants Younger Then 65 [Member]	Dec. 31, 2012 Participants Older Then 65 [Member]
Long Term Incentive Deferred Cash Expense		$ 155,000,000	$ 116,000,000	$ 77,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Total Share-based Liabilities Paid																							4,000,000	7,000,000
Common Stock, Capital Shares Reserved for Future Issuance		21,000,000																							17,000,000
Share Based Compensation Arrangement By Share Based Payment Award Options Available For Additional Grant		17,941,440
Share Based Compensation Arrangement By Share Based Payment Award Restricted Stock Available For Additional Grant		10,754,365
Number of Restricted Stock Units Granted		574,257
Fair Value of Restricted Stock Units Per Unit		21.67
Balance of Restricted Stock Units		12,000,000
Fair value of options granted per share		$ 7.83	$ 10.51	$ 12.78
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value		15,000,000	1,000,000
Total grant date fair value of restricted shares vested																									31,000,000	55,000,000	85,000,000
Unrecognized stock compensation expense related to nonvested stock option and restricted stock		67,000,000	63,000,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		26,000,000	22,000,000	21,000,000
Stock-based compensation expense recognized in noninterest expense for these equity plans					8,000,000	8,000,000	13,000,000
Matching Contribution To Employee Maximum		2.00%						6.00%	5.00%	5.00%
Compensation expese related to defined contribution plan		96,000,000	81,000,000	74,000,000
Defined Contribution Plan, Cost Recognized, Special Contribution	38,000,000		28,000,000
Effective interest crediting rate to each Personal Pension Account		3.00%
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments		0	88,000,000	0							0	88,000,000	0					0	0	0
Defined Benefit Plan, Curtailments											0	96,000,000						0	0
Accumulated benefit obligation for the Pension Benefits		2,800,000,000	2,700,000,000
Defined Benefit Plan Basis Used To Determine Overall Expected Long Term Rate Of Returns On Assets Assumption Historical											7.00%			7.25%	7.75%	6.25%	6.75%	3.25%			4.06%	4.39%
Expected long-term rate of return on plan assets											7.00%							5.00%
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year		26,000,000
Defined Benefit Plan Effect Of 25 Basis Point Decrease In Discount Rate		1,000,000
Defined Benefit Plan Effect Of 25 Basis Point Decrease In Expected Return On Plan Assets		$ 7,000,000
Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year																												8.00%	7.50%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate		5.00%																											5.00%
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition		2 years 2 months