Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 78,703
Due after five but within ten years, Amortized Cost		27,079
Due after one but within five years, Amortized Cost		14,087
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	707,953
Due within one year, Amortized Cost		84,452
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		996
Due after ten years, Amortized Cost		42,290
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	705,873
Amortized Cost		794,075	$ 790,082
Due within one year, Estimated Fair Value		997
Due after one but within five years, Estimated Fair Value		14,176
Due after five but within ten years, Estimated Fair Value		27,555
Due after ten years, Estimated Fair Value		35,975
Total Debt Securities, Estimated Fair Value		786,246	778,612
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		7,434	7,076
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		15,263	18,546
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		13,839	15,143
Total Debt Securities, Estimated Fair Value		15,184	16,617
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		1,352	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		792,405	788,412
Total Debt Securities, Estimated Fair Value		784,576	776,942
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		7,434	7,076
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		15,263	18,546
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		694,100
Total Debt Securities, Estimated Fair Value		690,700
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		792,405
Total Debt Securities, Estimated Fair Value		784,576
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		33,648	33,292
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		16,404	6,319
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		27,555	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		1,096	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		690,689	676,853
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		13,800
Total Debt Securities, Estimated Fair Value		$ 15,184	$ 16,617

[1] Mortgage-Backed Securities include an amortized cost of $13.8 million and a fair value of $15.2 million for Obligations of U.S.