Derivative and Hedging Activities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
Notional amount	$ 2,466,898		$ 2,466,898		$ 2,215,056
Derivative assets	43,049		43,049		45,898
Derivative liabilities	43,319		43,319		46,160
Interest income	123,767	$ 114,192	245,688	$ 222,017
Amounts reclassified from AOCI			(1,058)
Additional amount in excess of posted collateral required in case of breached agreements	1,974		1,974		1,862
Interest Rate Contracts [Member]
Derivative [Line Items]
Notional amount	200,000		$ 200,000		200,000
Period to reclassification of cash flow hedge gain loss			12 months
Maximum length of time hedged in interest rate cash flow hedge			8 years
Derivative gains to be reclassified within twelve months			$ 2,817
Derivative gains to be reclassified within twelve months, net of tax			1,831
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] | Amount Reclassified from Other Comprehensive Income [Member]
Derivative [Line Items]
Interest income			1,623
Amounts reclassified from AOCI			1,055
Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	1,080,889		1,080,889		972,002
Credit Risk Contract Sold [Member]
Derivative [Line Items]
Notional amount	74,010		$ 74,010
Risk participation agreements, minimum term			2 years
Risk participation agreements, maximum term			9 years
Maximum exposure under credit risk agreement assuming customer default	114		$ 114		25
Trading Assets [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative assets	41,155		41,155		43,742
Not for Trading Assets [Member] | Interest Rate Contracts [Member]
Derivative [Line Items]
Derivative assets	1,867		1,867		2,109
Trading Liabilities [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative liabilities	41,320		41,320		43,783
Not for Trading Liabilities [Member] | Interest Rate Contracts [Member]
Derivative [Line Items]
Derivative liabilities	$ 1,869		$ 1,869		$ 2,330