Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Sep. 30, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 89,386
Due after five but within ten years, Amortized Cost		27,073
Due after one but within five years, Amortized Cost		17,697
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	708,152
Due within one year, Amortized Cost		95,294
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		5,600
Due after ten years, Amortized Cost		44,924
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	722,603
Amortized Cost		805,116	$ 891,111
Due within one year, Estimated Fair Value		5,602
Due after one but within five years, Estimated Fair Value		17,756
Due after five but within ten years, Estimated Fair Value		27,951
Due after ten years, Estimated Fair Value		38,077
Total Debt Securities, Estimated Fair Value		813,659	886,560
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		17,187	11,926
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		8,644	16,477
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		16,059	20,034
Total Debt Securities, Estimated Fair Value		17,976	22,092
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		1,917	2,071
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		0	13
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		803,446	888,941
Total Debt Securities, Estimated Fair Value		811,989	884,390
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		17,187	11,926
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		8,644	16,477
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		692,100
Total Debt Securities, Estimated Fair Value		704,600
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		803,446
Total Debt Securities, Estimated Fair Value		811,989
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		35,609	35,658
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		6,519	2,319
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		27,951	27,598
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		19,307	19,118
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		1	28
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		704,626	777,577
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		16,059
Total Debt Securities, Estimated Fair Value		$ 17,976	$ 22,092

[1] Mortgage Backed Securities include an amortized cost of $16.1 million and a fair value of $18.0 million for Obligations of U.S.