Derivative and Hedging Activities - Schedule of Notional Amounts and Gross Fair Values of Derivative Assets and Derivative Liabilities (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Notional amount	$ 3,134,482,000	$ 2,893,041,000
Fair value, asset	88,006,000	53,202,000
Liability	82,219,000	51,605,000
Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	250,000,000	250,000,000
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,373,959,000	1,262,964,000
Credit Risk Contracts [Member]
Derivatives, Fair Value [Line Items]
Fair value, asset	22,000	7,000
Subject to Master Netting Arrangement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,625,139,000	1,514,144,000
Fair value, asset	7,893,000	3,197,000
Liability	81,966,000	51,453,000
Subject to Master Netting Arrangement [Member] | Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	250,000,000	250,000,000
Fair value, asset	7,859,000	3,178,000
Liability	1,341,000	962,000
Subject to Master Netting Arrangement [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,373,959,000	1,262,964,000
Fair value, asset		1,000
Liability	80,625,000	50,491,000
Subject to Master Netting Arrangement [Member] | Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,180,000	1,180,000
Fair value, asset	34,000	18,000
Not Subject to Master Netting Arrangement [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,509,343,000	1,378,897,000
Fair value, asset	80,113,000	50,005,000
Liability	253,000	152,000
Not Subject to Master Netting Arrangement [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,373,959,000	1,262,964,000
Fair value, asset	80,091,000	49,998,000
Liability		1,000
Not Subject to Master Netting Arrangement [Member] | Equity Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	1,180,000	1,180,000
Liability	34,000	18,000
Not Subject to Master Netting Arrangement [Member] | Credit Risk Contracts [Member]
Derivatives, Fair Value [Line Items]
Notional amount	134,204,000	114,753,000
Fair value, asset	22,000	7,000
Liability	$ 219,000	$ 133,000