Derivatives and Hedging Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jun. 16, 2016	Jun. 29, 2015
Derivative [Line Items]
Derivative, Amount of Hedged Item, Loans	$ 664,089,000		$ 664,089,000		$ 699,452,000
Collateral netted against derivative liabilities	39,800,000	$ 64,200,000	39,800,000	$ 64,200,000	53,600,000
Negative NPVs	37,200,000	62,700,000	37,200,000	62,700,000	52,700,000
Over collateralization net position	11,600,000	17,900,000	11,600,000	17,900,000	25,000,000
Additional collateral held in securities	51,400,000		51,400,000
Fair Value Hedge Assets, Loans	13,296,000		13,296,000		41,919,000
Derivative, Amount of Hedged Item	(295,546,000)		(295,546,000)		(308,608,000)
Fair Value Hedge Assets	23,284,000		23,284,000		$ 9,959,000
Gain (Loss) on Foreign Currency Derivative Instruments Not Designated as Hedging Instruments	$ 1,200,000	$ 800,000	$ 2,400,000	$ 1,800,000
Subordinated Debentures Maturing July 2025 [Member] | Subordinated Debt [Member]
Derivative [Line Items]
Debt Instrument, Face Amount							$ 150,000,000.0
Subordinated Debentures Maturing July 2056 [Member] | Subordinated Debt [Member]
Derivative [Line Items]
Debt Instrument, Face Amount						$ 175,000,000.0