Loans and Allowance for Loan Losses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended									3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2012		Dec. 31, 2011		Dec. 31, 2011 Pass [Member]	Dec. 31, 2011 Criticized [Member]	Sep. 30, 2012 Home equity and other consumer loans [Member]	Jun. 30, 2012 Home equity and other consumer loans [Member]	Dec. 31, 2011 Home equity and other consumer loans [Member]	Sep. 30, 2012 Commercial portfolio [Member]	Sep. 30, 2012 Commercial portfolio [Member]	Dec. 31, 2011 Commercial portfolio [Member]	Sep. 30, 2012 Consumer portfolio [Member]	Sep. 30, 2012 Consumer portfolio [Member]	Dec. 31, 2011 Consumer portfolio [Member]	Sep. 30, 2012 Nonaccrual loans excluding FDIC covered loans [Member]	Dec. 31, 2011 Nonaccrual loans excluding FDIC covered loans [Member]	Sep. 30, 2012 Nonaccrual loans excluding FDIC covered loans [Member] Maximum [Member]	Sep. 30, 2012 Loans Receivable [Member]	Sep. 30, 2012 Fdic Covered [Member] Commercial portfolio [Member]	Dec. 31, 2011 Fdic Covered [Member] Commercial portfolio [Member]	Sep. 30, 2012 Fdic Covered [Member] Consumer [Member]	Dec. 31, 2011 Fdic Covered [Member] Consumer [Member]	Sep. 30, 2012 Other [Member]	Dec. 31, 2011 Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value of other acquired loans																										$ 30	$ 47
Nonaccrual loans	534		534		638				52	21	24	127	127	282	377	377	309	504	591
Total loans 90 days or more past due and still accruing	1		1		1
Total loans held for investment	55,410	[1]	55,410	[1]	53,540	[1]																460	864	64	85
Revised pass and criticized loans							266	87
Commitment to lend additional funds to borrowers with Loan modifications classified as TDRs	41		41
Troubled debt restructured loans	67		67
Nonaccrual troubled debt restructured loans			35
Incremental charge offs			17
Minimum defaulting period	60		60
Interest income recognized for impaired loans												5	9		5	8		1		1
Transfer of loans from held for investment to held for sale																					200
Loans receivable sold during the period																					$ 233

[1] Includes $6 million and ($30) million at September 30, 2012 and December 31, 2011, respectively, for net unamortized discounts and premiums and deferred fees and costs.