Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Net Investment Income [Line Items]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value		$ 34,529
Due after five but within ten years, Amortized Cost		4,970
Due after one but within five years, Amortized Cost		23,248
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	[1]	913,530
Due within one year, Amortized Cost		33,815
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost		5,597
Due after ten years, Amortized Cost		0
Corporate/Mortgage-Backed Securities, Amortized cost	[1]	938,097
Amortized Cost		947,345	$ 896,495
Due within one year, Estimated Fair Value		5,601
Due after one but within five years, Estimated Fair Value		23,536
Due after five but within ten years, Estimated Fair Value		5,392
Due after ten years, Estimated Fair Value		0
Securities available for sale, at fair value		972,626	902,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		25,496	8,906
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		215	3,109
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Amortized Cost		670,300
Securities available for sale, at fair value		689,200
Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		947,345
Securities available for sale, at fair value		972,626
Corporate Debt Securities [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value		23,498	23,962
Obligations of States and Political Subdivisions [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value		10,027	17,909
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value		1,004	1,000
Residential Mortgage Backed Securities [Member]
Net Investment Income [Line Items]
Securities available for sale, at fair value		689,224	$ 661,947
US Government Agencies Debt Securities [Member]
Net Investment Income [Line Items]
Amortized Cost		243,300
Securities available for sale, at fair value		$ 248,900

[1] Mortgage-Backed Securities include an amortized cost of $243.3 million and a fair value of $248.9 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $670.3 million and a fair value of $689.2 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.