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