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Investment securities - Additional Information (Detail) (USD $)
3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended
Sep. 30, 2011
Sep. 30, 2011
Mar. 31, 2011
Government issued or guaranteed [Member]
Sep. 30, 2012
Mortgage-backed securities [Member]
Sep. 30, 2011
Mortgage-backed securities [Member]
Jun. 30, 2011
Mortgage-backed securities [Member]
Sep. 30, 2012
Mortgage-backed securities [Member]
Sep. 30, 2011
Mortgage-backed securities [Member]
Jun. 30, 2012
Trust preferred securities [Member]
Jun. 30, 2012
Collateralized debt obligations [Member]
Investment Holdings [Line Items]                    
Gross realized gains on investment securities $ 90,000 $ 150,000,000                
Aggregate amortized cost on mortgage-backed securities     484,000,000     1,000,000,000     136,000,000 100,000,000
Pre-tax gain on sale of mortgage-backed securities     39,000,000     66,000,000     25,000,000 20,000,000
Pre-tax other-than-temporary impairment losses       $ 6,000,000 $ 10,000,000   $ 33,000,000 $ 52,000,000