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Securities - Additional Information (Detail) (USD $)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2012
Securities
Security
Sep. 30, 2012
Securities
Security
Dec. 31, 2011
Security
Schedule of Available-for-sale Securities [Line Items]      
Debt securities were determined to have either initial other-than-temporary impairment or changes to previous estimates relating to the credit component   $ 0 $ 0
Percentage of debt securities issued by US Treasury, US Government Agencies and government sponsored entities 89.00% 89.00% 84.00%
Temporary impairment losses recognized credit loss 0 0  
Non-credit component unrealized loss amounts recognized 0 0  
Remaining non-credit component unrealized loss recognized 0 0  
Impairment losses other-than-temporary recognized related to credit loss 0 0  
Assets Backed Securities in Available for Sale Portfolio 28 28 45
Assets Backed Securities Wrapped by Monoline Insurance Company 8 8 9
Value of Asset Backed Securities Backed by Insurance Company 328,000,000 328,000,000 349,000,000
Fair value of Assets Backed Securities 593,000,000 593,000,000 818,000,000
Unrealized Losses on Assets backed securities, Gross 96,000,000 96,000,000 121,000,000
Fair Value of Financial Guarantee from Insurance Company for the purpose of evaluating other then temporary Impairment 104,000,000 104,000,000 114,000,000
Security wrapped by below investment grade monoline insurance was deemed to be other-than-temporarily impaired 0 0 1
Other Than Temporary Impaired Monoline Wrapped Security     1,000,000
Federal Home Loan Bank [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Investments in stock were included in other assets 143,000,000 143,000,000 133,000,000
Federal Reserve Bank [Member]
     
Schedule of Available-for-sale Securities [Line Items]      
Investments in stock were included in other assets $ 483,000,000 $ 483,000,000 $ 483,000,000