Securities - Amortized Cost and Fair Value of Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	$ 766,950	$ 748,356
Held-to-Maturity, Gross Unrecognized Gains	13,231	3,399
Held-to-Maturity, Gross Unrecognized Losses	(2,131)	(11,766)
Held-to-Maturity, Fair Value	778,050	739,989
U.S. Government Sponsored Entities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	3,885	3,873
Held-to-Maturity, Gross Unrecognized Gains	25	7
Held-to-Maturity, Gross Unrecognized Losses	(4)	(20)
Held-to-Maturity, Fair Value	3,906	3,860
Residential Mortgage-Backed Securities (Issued by Government-Sponsored Entities) [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	585,906	567,766
Held-to-Maturity, Gross Unrecognized Gains	9,663	2,354
Held-to-Maturity, Gross Unrecognized Losses	(1,996)	(9,653)
Held-to-Maturity, Fair Value	593,573	560,467
Corporate [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	22,993	22,993
Held-to-Maturity, Gross Unrecognized Gains	352	234
Held-to-Maturity, Gross Unrecognized Losses	(77)	(326)
Held-to-Maturity, Fair Value	23,268	22,901
Small Business Administration Loan Pools [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	1,585	1,746
Held-to-Maturity, Gross Unrecognized Gains	44
Held-to-Maturity, Gross Unrecognized Losses		(18)
Held-to-Maturity, Fair Value	1,629	1,728
State and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Held-to-Maturity, Amortized Cost	152,581	151,978
Held-to-Maturity, Gross Unrecognized Gains	3,147	804
Held-to-Maturity, Gross Unrecognized Losses	(54)	(1,749)
Held-to-Maturity, Fair Value	$ 155,674	$ 151,033