Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) MortgageLoan Positions	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MortgageLoan Positions
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized loss attributable to collateralized mortgage obligations	$ 62,000,000		$ 62,000,000
Number of fixed rate collateralized mortgage obligations | MortgageLoan	56		56
Weighted-average maturity collateralized mortgage, in years			4 years 2 months 12 days
Weighted-average maturity mortgage-backed securities, in years			5 years 2 months 12 days
Impairment recognized in earnings	$ 0	$ 0
Pledged available for sale and held-to-maturity securities	$ 6,500,000,000		$ 6,500,000,000
Other Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized loss attributable to mortgage-backed securities			$ 1,000,000
Number of other mortgage-backed securities positions | Positions	17		17