Changes in Allowance for Loan Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	$ 50	$ 53	$ 54	$ 53
Charge-offs	(3)	(5)	(8)	(9)
Recoveries		1	1	1
Provision for loan losses	4	1	4	5
Balance at end of period	51	50	51	50
Residential real estate mortgages
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	37	37	40	38
Charge-offs	(1)	(3)	(4)	(6)
Recoveries			1
Provision for loan losses	(2)		(3)	2
Balance at end of period	34	34	34	34
Home equity lines of credit
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at beginning of period	13	16	14	15
Charge-offs	(2)	(2)	(4)	(3)
Recoveries		1		1
Provision for loan losses	6	1	7	3
Balance at end of period	$ 17	$ 16	$ 17	$ 16