Changes in Allowance for Loan Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	$ 51	$ 50	$ 54	$ 53
Charge-offs	(5)	(5)	(13)	(14)
Recoveries	1		2	1
Provision for loan losses	10	8	14	13
Balance at end of period	57	53	57	53
Residential real estate mortgages
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	34	34	40	38
Charge-offs	(2)	(2)	(6)	(8)
Recoveries	1		2
Provision for loan losses	2	6	(1)	8
Balance at end of period	35	38	35	38
Home equity lines of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	17	16	14	15
Charge-offs	(3)	(3)	(7)	(6)
Recoveries				1
Provision for loan losses	8	2	15	5
Balance at end of period	$ 22	$ 15	$ 22	$ 15