Fair value measurements - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	$ 1,300,000,000	$ 1,100,000,000	$ 1,300,000,000
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 residential mortgage - backed securities with internal modeling, at cost		1,100,000,000
Fair value of privately issued residential mortgage-backed securities valued with internal modeling		1,000,000,000
Weighted-average market yield		6.80%
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	44,000,000	43,000,000	44,000,000
Fair value of securities issued by financial institutions and other entities backed by trust preferred securities	53,000,000	62,000,000	53,000,000
Other-than-temporary impairment losses		5,000,000	12,000,000	5,000,000
Loans measured at fair value on nonrecurring basis	419,000,000	335,000,000	419,000,000	746,000,000
Change in fair value of nonrecurring fair value measured loans for charge-offs and impairment reserves		67,000,000	158,000,000	224,000,000
Nonrecurring fair value measurement	51,000,000	34,000,000	51,000,000
Other-than-temporary impairment charge related to investment in BLG	79,000,000		79,000,000
Investment in BLG	20.00%		20.00%
Carrying value of minority interest investment in Bayview Lending Group LLC	115,000,000	93,000,000	115,000,000
Percentage of mortgage asset			31.00%
Percentage of weighted-average loss severity			75.00%
Investment fair value percent		20.00%
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount margin		4.00%
Discount rates for fair value estimations		10.00%
Discount rates		8.00%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount margin		11.00%
Discount rates for fair value estimations		85.00%
Discount rates		12.50%
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	262,000,000	207,000,000	262,000,000	476,000,000
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	$ 157,000,000	$ 128,000,000	$ 157,000,000	$ 270,000,000