Securities - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2013	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
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	$ 2,000	$ 0
Realized losses on sale of available-for-sale securities	0	$ 0
Corporate debt securities, callable, amortized cost	59,900,000
Corporate debt securities, callable, estimated fair value	56,300,000
Securities pledged for other debt obligations, at fair value	641,300,000			$ 466,400,000
Reverse Repurchase Agreement
Schedule of Available-for-sale Securities [Line Items]
Securities pledged for repurchase agreements, at fair value	$ 85,700,000			$ 58,000,000