Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 87,503
Due after five but within ten years, Amortized Cost		27,070
Due after one but within five years, Amortized Cost		20,696
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	749,350
Due within one year, Amortized Cost		95,019
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		2,601
Due after ten years, Amortized Cost		44,652
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	766,160
Amortized Cost		846,039	$ 891,111
Due within one year, Estimated Fair Value		2,602
Due after one but within five years, Estimated Fair Value		20,780
Due after five but within ten years, Estimated Fair Value		28,036
Due after ten years, Estimated Fair Value		36,085
Total Debt Securities, Estimated Fair Value		855,333	886,560
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		19,055	11,926
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		9,761	16,477
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		16,730	20,034
Total Debt Securities, Estimated Fair Value		18,762	22,092
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		2,032	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		844,369	888,941
Total Debt Securities, Estimated Fair Value		853,663	884,390
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		19,055	11,926
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		9,761	16,477
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		732,600
Total Debt Securities, Estimated Fair Value		747,400
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		844,369
Total Debt Securities, Estimated Fair Value		853,663
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		33,723	35,658
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		6,422	2,319
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		28,036	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		19,322	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		2	28
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		747,396	777,577
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		16,730
Total Debt Securities, Estimated Fair Value		$ 18,762	$ 22,092

[1] Mortgage Backed Securities include an amortized cost of $16.7 million and a fair value of $18.8 million for Obligations of U.S.