Securities - Securities with Unrealized Losses Aggregated by Investment Category and Length of Time that Individual Securities have been in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Less than 12 Months
Fair Value	$ 9,143	$ 98,395
Unrealized Loss	(25)	(766)
12 Months or More
Fair Value	196,945	210,530
Unrealized Loss	(3,836)	(6,561)
Fair Value	206,088	309,925
Unrealized Loss	(3,861)	(7,327)
U.S. Gov’t sponsored entities
Less than 12 Months
Fair Value	4,963	14,786
Unrealized Loss	(2)	(41)
12 Months or More
Fair Value	69,440	70,676
Unrealized Loss	(743)	(1,529)
Fair Value	74,403	86,462
Unrealized Loss	(745)	(1,570)
State & political subdivisions
Less than 12 Months
Fair Value	994	13,834
Unrealized Loss	(13)	(62)
12 Months or More
Fair Value	10,389	21,080
Unrealized Loss	(165)	(511)
Fair Value	11,383	34,914
Unrealized Loss	(178)	(573)
Residential & multi-family mortgage
Less than 12 Months
Fair Value	3,186	69,015
Unrealized Loss	(10)	(656)
12 Months or More
Fair Value	87,606	87,286
Unrealized Loss	(1,798)	(2,917)
Fair Value	90,792	156,301
Unrealized Loss	(1,808)	(3,573)
Corporate notes & bonds
Less than 12 Months
Fair Value	0	0
Unrealized Loss	0	0
12 Months or More
Fair Value	9,528	9,759
Unrealized Loss	(476)	(601)
Fair Value	9,528	9,759
Unrealized Loss	(476)	(601)
Pooled SBA
Less than 12 Months
Fair Value	0	760
Unrealized Loss	0	(7)
12 Months or More
Fair Value	19,035	20,795
Unrealized Loss	(581)	(917)
Fair Value	19,035	21,555
Unrealized Loss	(581)	(924)
Other
Less than 12 Months
Fair Value	0	0
Unrealized Loss	0	0
12 Months or More
Fair Value	947	934
Unrealized Loss	(73)	(86)
Fair Value	947	934
Unrealized Loss	$ (73)	$ (86)