Securities Available for Sale - Additional Information (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Dec. 31, 2013 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Sep. 30, 2014 Other Than Temporarily Impaired Securities [Member] Mortgage-backed Securities, Issued by Private Enterprises [Member]	Sep. 30, 2013 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	$ 7,800,000,000	$ 11,000,000,000
Securities AFS Pledged Transferee Can Transfer	370,000,000	0
Available-for-sale Securities	$ 26,162,000,000	$ 22,542,000,000	$ 132,000,000	$ 154,000,000	$ 19,000,000	$ 23,000,000