Fair value measurements - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of independent pricing sources used to value privately issued residential mortgage backed securities valued with internal modeling	2		2		2
Privately issued residential mortgage backed securities at cost	$ 1,200,000,000		$ 1,200,000,000		$ 1,300,000,000
Weighted-average reliance on internal model pricing, in percentage			33.00%
Average weighting placed on the values provided by the independent sources, in percentage			67.00%
Privately issued residential mortgage - backed securities with internal modeling, at cost	1,200,000,000		1,200,000,000
Fair value of privately issued residential mortgage-backed securities valued with internal modeling	1,000,000,000		1,000,000,000
Weighted-average market yield			6.50%
Debt instrument weighted average basis spread on variable rate			8.00%
Securities issued by financial institutions and other entities backed by trust preferred securities at cost	43,000,000		43,000,000		44,000,000
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	57,000,000		57,000,000		53,000,000
Other-than-temporary impairment losses			5,000,000	11,000,000
Loans measured at fair value on nonrecurring basis	320,000,000	506,000,000	320,000,000	506,000,000
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves	17,000,000	38,000,000	47,000,000	137,000,000
Nonrecurring fair value measurement	51,000,000	53,000,000	51,000,000	53,000,000
Changes in fair value recognized for those foreclosed assets held	1,000,000	6,000,000	10,000,000	17,000,000
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	221,000,000	355,000,000	221,000,000	355,000,000
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	$ 99,000,000	$ 151,000,000	$ 99,000,000	$ 151,000,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount Margin	4.00%		4.00%
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%		10.00%
Discount rates for fair value estimations			85.00%