Securities - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2015	Sep. 30, 2014
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities transferred to held-to-maturity securities			$ 536,000,000
Unrealized net loss on securities transferred from available-for-sale to held-to-maturity, Gross			$ 13,300,000
Realized gains or losses on the sale of securities	$ 0	$ 591,000		$ 0	$ 938,000
Corporate debt securities, callable, amortized cost	55,000,000			55,000,000
Corporate debt securities, callable, estimated fair value	$ 48,100,000			$ 48,100,000
Floating-rate securities, purchased period				1998
Floating-rate debt securities, fixed interest rate period				Over 90-day LIBOR
Principal maturity year				2028 or prior if called by the issuer