Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 63,617
Due after five but within ten years, Amortized Cost		26,337
Due after one but within five years, Amortized Cost		15,727
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	764,832
Due within one year, Amortized Cost		62,686
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		5,099
Due after ten years, Amortized Cost		15,523
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	747,590
Amortized Cost		829,188	$ 740,846
Due within one year, Estimated Fair Value		5,073
Due after one but within five years, Estimated Fair Value		15,541
Due after five but within ten years, Estimated Fair Value		26,409
Due after ten years, Estimated Fair Value		16,594
Total Debt Securities, Estimated Fair Value		812,877	731,358
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		4,474	4,854
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		20,785	14,342
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		827,518	739,176
Total Debt Securities, Estimated Fair Value		811,207	729,688
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		4,474	4,854
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		20,785	14,342
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		681,300
Total Debt Securities, Estimated Fair Value		664,800
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		827,518
Total Debt Securities, Estimated Fair Value		811,207
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		14,132	23,646
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		21,226	16,493
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		27,161	27,410
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		1,098	1,097
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		664,788	$ 625,555
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		83,500
Total Debt Securities, Estimated Fair Value		$ 82,800

[1] Mortgage-Backed Securities include an amortized cost of $83.5 million and a fair value of $82.8 million for Obligations of U.S.