Fair value measurements (Details Textual) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012 ValuationSources	Jun. 30, 2011	Jun. 30, 2012 ValuationSources	Jun. 30, 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	$ 344,000,000	$ 478,000,000	$ 344,000,000	$ 478,000,000
Fair value measurements (Textual) [Abstract]
Number of independent pricing sources used to value privately issued residential mortgage backed securities valued with internal modeling	2		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	1,100,000,000		1,100,000,000
Fair value of privately issued residential mortgage-backed securities valued with internal modeling	1,200,000,000		1,200,000,000
Weighted-average market yield			7.30%
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		44,000,000
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	55,000,000		55,000,000		53,000,000
Other-than-temporary impairment losses	1,000,000	2,000,000	5,000,000	11,000,000
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves	21,000,000	61,000,000	34,000,000	91,000,000
Change in fair value of nonrecurring fair value measured foreclosed assets	7,000,000	11,000,000	10,000,000	12,000,000
Assets taken in foreclosure of defaulted loans measured at fair value on a nonrecurring basis	47,000,000	31,000,000	47,000,000	31,000,000
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount Margin	5.00%		5.00%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount Margin	10.00%		10.00%
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	234,000,000	324,000,000	234,000,000	324,000,000
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	$ 110,000,000	$ 154,000,000	$ 110,000,000	$ 154,000,000