Derivative and Hedging Activities - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Derivative [Line Items]
Notional amount	$ 3,134,482,000		$ 2,893,041,000
Derivative assets	88,006,000		53,202,000
Derivative liabilities	82,219,000		51,605,000
Amounts reclassified from AOCI	(395,000)
Additional amount in excess of posted collateral required in case of breached agreements	1,894,000		1,333,000
Interest Rate Contracts [Member]
Derivative [Line Items]
Notional amount	$ 250,000,000		250,000,000
Period to reclassification of cash flow hedge gain loss	12 months
Maximum length of time hedged in interest rate cash flow hedge	7 years
Derivative gains to be reclassified within twelve months	$ 1,979,000
Derivative gains to be reclassified within twelve months, net of tax	1,286,000
Derivative gain or loses excluded from assessment of hedge effectiveness	0
Hedge ineffectiveness	0	$ 0
Gains or losses from cash flow hedge derivatives reclassified to earnings	0	$ 0
Interest Rate Contracts [Member] | Interest Income - Loans and Leases [Member]
Derivative [Line Items]
Derivative financial instrument, net	687,000		809,000
Interest Rate Contracts [Member] | Interest Income - Loans and Leases [Member] | Amount Reclassified from Other Comprehensive Income [Member]
Derivative [Line Items]
Derivative financial instrument, net	687,000
Amounts reclassified from AOCI	446,000
Interest Rate Contracts [Member] | Interest Expense - Short-term Borrowings [Member]
Derivative [Line Items]
Derivative financial instrument, net	150,000
Interest Rate Contracts [Member] | Interest Expense - Short-term Borrowings [Member] | Amount Reclassified from Other Comprehensive Income [Member]
Derivative [Line Items]
Derivative financial instrument, net	150,000
Amounts reclassified from AOCI	97,000
Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	1,373,959,000		1,262,964,000
Credit Risk Contract Sold [Member]
Derivative [Line Items]
Notional amount	$ 83,010,000
Risk participation agreements, minimum term	1 year
Risk participation agreements, maximum term	14 years
Maximum exposure under credit risk agreement assuming customer default	$ 219,000		133,000
Not for Trading Assets [Member] | Interest Rate Contracts [Member]
Derivative [Line Items]
Derivative assets	7,859,000		3,178,000
Trading Assets [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets	80,091,000		49,999,000
Not for Trading Liabilities [Member] | Interest Rate Contracts [Member]
Derivative [Line Items]
Derivative liabilities	1,341,000		962,000
Trading Liabilities [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative liabilities	$ 80,625,000		$ 50,492,000