Fair value measurements (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2012 ValuationSources	Mar. 31, 2011	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured at fair value on nonrecurring basis	$ 363,000,000	$ 550,000,000
Fair value measurements (Textual) [Abstract]
Number of independent pricing sources used to value privately issued residential mortgage backed securities valued with internal modeling	2
Privately issued residential mortgage backed securities at cost	1,300,000,000		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	1,300,000,000
Fair value of privately issued residential mortgage-backed securities valued with internal modeling	1,100,000,000
Weighted-average market yield	7.60%
Minimum discount margin	5.00%
Maximum discount margin	11.00%
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	44,000,000		44,000,000
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	58,000,000		53,000,000
Other-than-temporary impairment losses	4,000,000	9,000,000
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves	19,000,000	48,000,000
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis	10,000,000	19,000,000
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	208,000,000	267,000,000
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	$ 155,000,000	$ 283,000,000