Derivative and Hedging Activities - Schedule of Notional Amounts and Gross Fair Values of Derivative Assets and Derivative Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Notional amount	$ 2,466,898	$ 2,215,056
Fair value,asset	43,049	45,898
Liability	43,319	46,160
Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	200,000	200,000
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,080,889	972,002
Credit Risk Contract [Member]
Derivatives, Fair Value [Line Items]
Fair value,asset	10
Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Liability	16
Subject to Master Netting Arrangement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,282,099	1,173,212
Fair value,asset	2,375	2,296
Liability	42,812	45,985
Subject to Master Netting Arrangement [Member] | Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	200,000	200,000
Fair value,asset	1,867	2,109
Liability	1,869	2,330
Subject to Master Netting Arrangement [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,080,889	972,002
Fair value,asset	491	140
Liability	40,943	43,655
Subject to Master Netting Arrangement [Member] | Credit Risk Contract [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,210	1,210
Fair value,asset	17	47
Subject to Master Netting Arrangement [Member] | Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Fair value,asset	17	47
Not Subject to Master Netting Arrangement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,184,799	1,041,844
Fair value,asset	40,674	43,602
Liability	507	175
Not Subject to Master Netting Arrangement [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,080,889	972,002
Fair value,asset	40,664	43,602
Liability	377	128
Not Subject to Master Netting Arrangement [Member] | Credit Risk Contract [Member]
Derivatives, Fair Value [Line Items]
Notional amount	102,700	68,632
Fair value,asset	10
Liability	114
Not Subject to Master Netting Arrangement [Member] | Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,210	1,210
Liability	$ 16	$ 47