Securities - Amortized Cost and Fair Value of Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	$ 665,995	$ 535,462
Held-to-Maturity, Gross Unrecognized Gains	1,363	2,622
Held-to-Maturity, Gross Unrecognized Losses	(14,346)	(5,340)
Held-to-Maturity, Fair Value	653,012	532,744
U.S. Government Sponsored Entities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	5,358	998
Held-to-Maturity, Gross Unrecognized Losses	(24)	(13)
Held-to-Maturity, Fair Value	5,334	985
Residential Mortgage-Backed Securities (Issued by Government-Sponsored Entities) [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	481,294	383,875
Held-to-Maturity, Gross Unrecognized Gains	255	573
Held-to-Maturity, Gross Unrecognized Losses	(12,521)	(4,866)
Held-to-Maturity, Fair Value	469,028	379,582
Corporate [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	22,993	22,991
Held-to-Maturity, Gross Unrecognized Gains	372	355
Held-to-Maturity, Gross Unrecognized Losses	(35)
Held-to-Maturity, Fair Value	23,330	23,346
Small Business Administration Loan Pools [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	1,884	2,048
Held-to-Maturity, Gross Unrecognized Losses	(34)	(14)
Held-to-Maturity, Fair Value	1,850	2,034
State and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	154,466	125,550
Held-to-Maturity, Gross Unrecognized Gains	736	1,694
Held-to-Maturity, Gross Unrecognized Losses	(1,732)	(447)
Held-to-Maturity, Fair Value	$ 153,470	$ 126,797