Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Available-for-sale Securities, Debt Maturities, Single Maturity Date		$ 86,644
Due after five but within ten years, Amortized Cost		27,068
Due after one but within five years, Amortized Cost		20,596
Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Amortized Cost Basis	[1]	787,808
Due within one year, Amortized Cost		94,665
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis		2,601
Due after ten years, Amortized Cost		44,400
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	801,348
Amortized Cost		884,679	$ 891,111
Due within one year, Estimated Fair Value		2,601
Due after one but within five years, Estimated Fair Value		20,640
Due after five but within ten years, Estimated Fair Value		27,832
Due after ten years, Estimated Fair Value		35,571
Total Debt Securities, Estimated Fair Value		890,198	886,560
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		16,924	11,926
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		11,405	16,477
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		19,380	20,034
Total Debt Securities, Estimated Fair Value		21,700	22,092
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		2,320	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		882,473	888,941
Total Debt Securities, Estimated Fair Value		887,992	884,390
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		16,924	11,926
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		11,405	16,477
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		768,400
Total Debt Securities, Estimated Fair Value		779,600
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		882,473
Total Debt Securities, Estimated Fair Value		887,992
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		33,273	35,658
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		6,336	2,319
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		27,832	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		19,203	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		23	28
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Total Debt Securities, Estimated Fair Value		779,625	777,577
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		19,380
Total Debt Securities, Estimated Fair Value		$ 21,700	$ 22,092

[1] Mortgage Backed Securities include an amortized cost of $19.4 million and a fair value of $21.7 million for Obligations of U.S.