Securities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2013
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	$ 0
Realized losses on sale of available-for-sale securities	0	$ 0
Corporate debt securities, callable, amortized cost	60,500,000
Corporate debt securities, callable, estimated fair value	$ 54,600,000
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