Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 MortgageLoan
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized loss attributable to collateralized mortgage obligations	$ 136,000,000
Number of fixed rate collateralized mortgage obligations	67
Weighted-average maturity collateralized mortgage, in years	4 years 7 months 6 days
Weighted-average maturity mortgage-backed securities, in years	4 years 7 months 6 days
Pledged available for sale and held-to-maturity securities	7,800,000,000
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	14