Fair value measurements - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of independent pricing sources used to value privately issued mortgage-backed securities valued with internal modeling	2		2
Privately issued mortgage-backed securities at cost	$ 1,100,000,000
Weighted-average reliance on internal model pricing, percentage	33.00%
Average weighting placed on the values provided by the independent sources, percentage	67.00%
Privately issued mortgage - backed securities with internal modeling, at cost	1,100,000,000
Fair value of privately issued mortgage-backed securities valued with internal modeling	1,000,000,000
Weighted-average market yield	5.90%
Debt instrument weighted average basis spread on variable rate	7.00%
Securities issued by financial institutions and other entities backed by trust preferred securities at cost	43,000,000		43,000,000
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	62,000,000		62,000,000
OTTI losses	0	4,000,000
Loans measured at fair value on nonrecurring basis	227,000,000	363,000,000
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves	15,000,000	19,000,000
Nonrecurring fair value measurement	19,000,000	10,000,000
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	158,000,000	208,000,000
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	$ 69,000,000	$ 155,000,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount margin	5.00%
Discount rates for fair value estimations	10.00%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount margin	10.00%
Discount rates for fair value estimations	85.00%