Securities - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Sep. 30, 2013	Jun. 30, 2017	Jun. 30, 2016
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 on the sale of available-for-sale securities	$ 0	$ 75,000		$ 0
Realized losses on sale of available-for-sale securities	0	$ 0		0	$ 87,000
Corporate debt securities, callable, amortized cost	60,500,000			60,500,000
Corporate debt securities, callable, estimated fair value	$ 56,800,000			$ 56,800,000
Floating-rate corporate debt securities, purchased period				1998
Floating-rate corporate debt securities, fixed interest rate spread				over 90-day LIBOR
Principal maturity year				2028 or prior if called by the issuer