Securities Available for Sale - Additional Information (Detail) (USD $) Share data in Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2008	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Dec. 31, 2011 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Sep. 30, 2012 Other Than Temporarily Impaired Securities [Member] Mortgage-backed Securities, Issued by Private Enterprises [Member]	Sep. 30, 2011 Other Than Temporarily Impaired Securities [Member] Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 8,100,000,000		$ 8,100,000,000	$ 9,100,000,000
Securities AFS Pledged Transferee Cannot Transfer	0		0	0
Available-for-sale Securities	21,467,000,000		21,467,000,000	28,117,000,000	217,000,000	221,000,000	217,000,000	176,000,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	3		3
Cash And Marketable Securities Pledged As Collateral	939,000,000		939,000,000	770,000,000
Amount of Repurchase Agreements Secured by Other Marketable Securities	907,000,000		907,000,000	747,000,000
Sold Equity Forward Agreements, Underlying Shares	59
Executed Equity Forward Agreements, Underlying Shares		60.0
Contributed Equity Forward Agreements, Underlying Shares	1
Net Gain on Sale of Certain Other Equity Securities	1,900,000,000
Noncash Contribution Expense	$ 38,000,000		$ 38,000,000