Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 79,818
Due after five but within ten years, Amortized Cost		27,081
Due after one but within five years, Amortized Cost		14,088
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	698,530
Due within one year, Amortized Cost		84,675
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		997
Due after ten years, Amortized Cost		42,509
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	697,156
Amortized Cost		784,875	$ 790,082
Due within one year, Estimated Fair Value		997
Due after one but within five years, Estimated Fair Value		14,214
Due after five but within ten years, Estimated Fair Value		27,515
Due after ten years, Estimated Fair Value		37,092
Total Debt Securities, Estimated Fair Value		778,644	778,612
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		7,051	7,076
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		13,282	18,546
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		783,205	788,412
Total Debt Securities, Estimated Fair Value		776,974	776,942
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		7,051	7,076
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		13,282	18,546
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		662,900
Total Debt Securities, Estimated Fair Value		660,800
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		783,205
Total Debt Securities, Estimated Fair Value		776,974
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		34,629	33,292
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		16,578	6,319
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		27,514	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		1,097	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		660,771	$ 676,853
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		35,700
Total Debt Securities, Estimated Fair Value		$ 36,385

[1] Mortgage-Backed Securities include an amortized cost of $35.7 million and a fair value of $36.4 million for Obligations of U.S.