Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 94,112
Due after five but within ten years, Amortized Cost		27,077
Due after one but within five years, Amortized Cost		27,386
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	740,825
Due within one year, Amortized Cost		99,838
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		3,300
Due after ten years, Amortized Cost		42,075
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	736,972
Amortized Cost		842,333	$ 790,082
Due within one year, Estimated Fair Value		3,294
Due after one but within five years, Estimated Fair Value		27,361
Due after five but within ten years, Estimated Fair Value		27,360
Due after ten years, Estimated Fair Value		36,097
Total Debt Securities, Estimated Fair Value		832,754	778,612
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		7,242	7,076
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		16,821	18,546
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		14,585	15,143
Total Debt Securities, Estimated Fair Value		16,018	16,617
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		1,440	1,481
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		7	7
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		840,663	788,412
Total Debt Securities, Estimated Fair Value		831,084	776,942
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		7,242	7,076
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		16,821	18,546
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		726,200
Total Debt Securities, Estimated Fair Value		721,000
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		840,663
Total Debt Securities, Estimated Fair Value		831,084
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		33,741	33,292
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		16,369	6,319
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		27,360	27,229
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		16,642	16,631
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		0	1
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		720,954	676,853
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		14,600
Total Debt Securities, Estimated Fair Value		$ 16,018	$ 16,617

[1] Mortgage-Backed Securities include an amortized cost of $14.6 million and a fair value of $16.0 million for Obligations of U.S.