Securities Available for Sale - Additional Information (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Asset-backed Securities [Member]	Dec. 31, 2012 Asset-backed Securities [Member]	Mar. 31, 2013 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Dec. 31, 2012 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Mar. 31, 2013 Impaired Securities Mortgage-backed Securities, Issued by Private Enterprises [Member]	Mar. 31, 2012 Impaired Securities Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 10,000,000,000	$ 10,600,000,000
Securities AFS Pledged Transferee Cannot Transfer	0	0
Pledged Assets Separately Reported, Securities Pledged for Repurchase Agreements, at Fair Value	864,000,000
Trading Securities Pledged as Collateral	950,000,000	727,000,000
Amount of Repurchase Agreements Secured by Trading Assets and Other Marketable Securities	813,000,000	703,000,000
Available-for-sale Securities	$ 23,823,000,000	$ 21,953,000,000	$ 156,000,000	$ 216,000,000	$ 202,000,000	$ 209,000,000	$ 2,000,000	$ 114,000,000