Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, Less thank 12 months, Estimated Fair Value	$ 5,011	$ 1,978
Available-for-sale, Less than 12 Months, Unrealized Losses	(9)	(8)
Available-for-sale, 12 months or longer, Estimated Fair Value	32,363	66,438
Available-for-sale, 12 months or longer, Unrealized Losses	(110)	(960)
Available-for-sale, Total, Estimated Fair Value	37,374	68,416
Available-for-sale, Total, Unrealized Losses	(119)	(968)
Held-to-maturity, Less than 12 months, Estimated Fair Value	71,394	118,203
Held-to-maturity, Less than 12 months, Unrealized Losses	(190)	(924)
Held-to-maturity, 12 months or longer, Estimated Fair Value	178,710	556,536
Held-to-maturity, 12 months or longer, Unrealized Losses	(4,755)	(19,467)
Held-to-maturity, Total, Estimated Fair Value	250,104	674,739
Held-to-maturity, Total, Unrealized Losses	(4,945)	(20,391)
Total securities, Less than 12 months, Estimated Fair Value	76,405	120,181
Total securities, Less than 12 months, Unrealized Losses	(199)	(932)
Total securities, 12 months or longer, Estimated Fair Value	211,073	622,974
Total securities, 12 months or longer, Unrealized Losses	(4,865)	(20,427)
Total securities, Estimated Fair Value	287,478	743,155
Total securities, Unrealized Losses	(5,064)	(21,359)
Investment securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	22,526	5,326
Held-to-maturity, Less than 12 months, Unrealized Losses	(71)	(25)
Held-to-maturity, 12 months or longer, Estimated Fair Value	66,007	163,977
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,559)	(6,786)
Held-to-maturity, Total, Estimated Fair Value	88,533	169,303
Held-to-maturity, Total, Unrealized Losses	(3,630)	(6,811)
U.S. government and agency obligations | Investment securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, Less thank 12 months, Estimated Fair Value	5,011	985
Available-for-sale, Less than 12 Months, Unrealized Losses	(9)	(3)
Available-for-sale, 12 months or longer, Estimated Fair Value	32,363	66,438
Available-for-sale, 12 months or longer, Unrealized Losses	(110)	(960)
Available-for-sale, Total, Estimated Fair Value	37,374	67,423
Available-for-sale, Total, Unrealized Losses	(119)	(963)
Held-to-maturity, Less than 12 months, Estimated Fair Value	0	0
Held-to-maturity, Less than 12 months, Unrealized Losses	0	0
Held-to-maturity, 12 months or longer, Estimated Fair Value	4,999	14,845
Held-to-maturity, 12 months or longer, Unrealized Losses	(2)	(130)
Held-to-maturity, Total, Estimated Fair Value	4,999	14,845
Held-to-maturity, Total, Unrealized Losses	(2)	(130)
Mortgage-backed securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	48,868	112,877
Held-to-maturity, Less than 12 months, Unrealized Losses	(119)	(899)
Held-to-maturity, 12 months or longer, Estimated Fair Value	112,703	392,559
Held-to-maturity, 12 months or longer, Unrealized Losses	(1,196)	(12,681)
Held-to-maturity, Total, Estimated Fair Value	161,571	505,436
Held-to-maturity, Total, Unrealized Losses	(1,315)	(13,580)
Mortgage-backed securities | FHLMC
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	13,132	46,615
Held-to-maturity, Less than 12 months, Unrealized Losses	(32)	(159)
Held-to-maturity, 12 months or longer, Estimated Fair Value	41,799	147,763
Held-to-maturity, 12 months or longer, Unrealized Losses	(537)	(4,951)
Held-to-maturity, Total, Estimated Fair Value	54,931	194,378
Held-to-maturity, Total, Unrealized Losses	(569)	(5,110)
Mortgage-backed securities | FNMA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, Less thank 12 months, Estimated Fair Value		993
Available-for-sale, Less than 12 Months, Unrealized Losses		(5)
Available-for-sale, 12 months or longer, Estimated Fair Value		0
Available-for-sale, 12 months or longer, Unrealized Losses		0
Available-for-sale, Total, Estimated Fair Value		993
Available-for-sale, Total, Unrealized Losses		(5)
Held-to-maturity, Less than 12 months, Estimated Fair Value	13,475	27,594
Held-to-maturity, Less than 12 months, Unrealized Losses	(31)	(125)
Held-to-maturity, 12 months or longer, Estimated Fair Value	43,134	185,328
Held-to-maturity, 12 months or longer, Unrealized Losses	(472)	(5,905)
Held-to-maturity, Total, Estimated Fair Value	56,609	212,922
Held-to-maturity, Total, Unrealized Losses	(503)	(6,030)
Mortgage-backed securities | GNMA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	22,261	35,221
Held-to-maturity, Less than 12 months, Unrealized Losses	(56)	(535)
Held-to-maturity, 12 months or longer, Estimated Fair Value	24,936	59,468
Held-to-maturity, 12 months or longer, Unrealized Losses	(113)	(1,825)
Held-to-maturity, Total, Estimated Fair Value	47,197	94,689
Held-to-maturity, Total, Unrealized Losses	(169)	(2,360)
Mortgage-backed securities | SBA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	0	3,447
Held-to-maturity, Less than 12 months, Unrealized Losses	0	(80)
Held-to-maturity, 12 months or longer, Estimated Fair Value	2,834	0
Held-to-maturity, 12 months or longer, Unrealized Losses	(74)	0
Held-to-maturity, Total, Estimated Fair Value	2,834	3,447
Held-to-maturity, Total, Unrealized Losses	(74)	(80)
State and municipal obligations | Investment securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	22,526	2,856
Held-to-maturity, Less than 12 months, Unrealized Losses	(71)	(4)
Held-to-maturity, 12 months or longer, Estimated Fair Value	18,348	106,073
Held-to-maturity, 12 months or longer, Unrealized Losses	(188)	(1,693)
Held-to-maturity, Total, Estimated Fair Value	40,874	108,929
Held-to-maturity, Total, Unrealized Losses	(259)	(1,697)
Corporate debt securities | Investment securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	0	2,470
Held-to-maturity, Less than 12 months, Unrealized Losses	0	(21)
Held-to-maturity, 12 months or longer, Estimated Fair Value	42,660	43,059
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,369)	(4,963)
Held-to-maturity, Total, Estimated Fair Value	42,660	45,529
Held-to-maturity, Total, Unrealized Losses	$ (3,369)	$ (4,984)