Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total securities, Less than 12 months, Estimated Fair Value	$ 25,887	$ 120,181
Total securities, Less than 12 months, Unrealized Losses	(55)	(932)
Total securities, 12 months or longer, Estimated Fair Value	342,424	622,974
Total securities, 12 months or longer, Unrealized Losses	(6,477)	(20,427)
Total securities, Estimated Fair Value	368,311	743,155
Total securities, Unrealized Losses	(6,532)	(21,359)
Held-to-Maturity Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	25,887	118,203
Held-to-maturity, Less than 12 months, Unrealized Losses	(55)	(924)
Held-to-maturity, 12 months or longer, Estimated Fair Value	300,137	556,536
Held-to-maturity, 12 months or longer, Unrealized Losses	(6,273)	(19,467)
Held-to-maturity, Total, Estimated Fair Value	326,024	674,739
Held-to-maturity, Total, Unrealized Losses	(6,328)	(20,391)
Held-to-Maturity 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	7,409	5,326
Held-to-maturity, Less than 12 months, Unrealized Losses	(37)	(25)
Held-to-maturity, 12 months or longer, Estimated Fair Value	96,822	163,977
Held-to-maturity, 12 months or longer, Unrealized Losses	(4,040)	(6,786)
Held-to-maturity, Total, Estimated Fair Value	104,231	169,303
Held-to-maturity, Total, Unrealized Losses	(4,077)	(6,811)
Investment securities - U.S. government and agency obligations | Held-to-Maturity 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	0
Held-to-maturity, Less than 12 months, Unrealized Losses	0	0
Held-to-maturity, 12 months or longer, Estimated Fair Value	9,968	14,845
Held-to-maturity, 12 months or longer, Unrealized Losses	(11)	(130)
Held-to-maturity, Total, Estimated Fair Value	9,968	14,845
Held-to-maturity, Total, Unrealized Losses	(11)	(130)
Mortgage-backed securities | Held-to-Maturity Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	18,478	112,877
Held-to-maturity, Less than 12 months, Unrealized Losses	(18)	(899)
Held-to-maturity, 12 months or longer, Estimated Fair Value	203,315	392,559
Held-to-maturity, 12 months or longer, Unrealized Losses	(2,233)	(12,681)
Held-to-maturity, Total, Estimated Fair Value	221,793	505,436
Held-to-maturity, Total, Unrealized Losses	(2,251)	(13,580)
Mortgage-backed securities | Held-to-Maturity 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	0	46,615
Held-to-maturity, Less than 12 months, Unrealized Losses	0	(159)
Held-to-maturity, 12 months or longer, Estimated Fair Value	73,099	147,763
Held-to-maturity, 12 months or longer, Unrealized Losses	(864)	(4,951)
Held-to-maturity, Total, Estimated Fair Value	73,099	194,378
Held-to-maturity, Total, Unrealized Losses	(864)	(5,110)
Mortgage-backed securities | Held-to-Maturity Securities | FNMA
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	2,362	27,594
Held-to-maturity, Less than 12 months, Unrealized Losses	(3)	(125)
Held-to-maturity, 12 months or longer, Estimated Fair Value	78,954	185,328
Held-to-maturity, 12 months or longer, Unrealized Losses	(786)	(5,905)
Held-to-maturity, Total, Estimated Fair Value	81,316	212,922
Held-to-maturity, Total, Unrealized Losses	(789)	(6,030)
Mortgage-backed securities | Held-to-Maturity 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	16,116	35,221
Held-to-maturity, Less than 12 months, Unrealized Losses	(15)	(535)
Held-to-maturity, 12 months or longer, Estimated Fair Value	47,920	59,468
Held-to-maturity, 12 months or longer, Unrealized Losses	(506)	(1,825)
Held-to-maturity, Total, Estimated Fair Value	64,036	94,689
Held-to-maturity, Total, Unrealized Losses	(521)	(2,360)
Mortgage-backed securities | Held-to-Maturity 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	3,342	0
Held-to-maturity, 12 months or longer, Unrealized Losses	(77)	0
Held-to-maturity, Total, Estimated Fair Value	3,342	3,447
Held-to-maturity, Total, Unrealized Losses	(77)	(80)
State and municipal obligations | Held-to-Maturity 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	4,898	2,856
Held-to-maturity, Less than 12 months, Unrealized Losses	(22)	(4)
Held-to-maturity, 12 months or longer, Estimated Fair Value	44,556	106,073
Held-to-maturity, 12 months or longer, Unrealized Losses	(289)	(1,693)
Held-to-maturity, Total, Estimated Fair Value	49,454	108,929
Held-to-maturity, Total, Unrealized Losses	(311)	(1,697)
Corporate debt securities | Held-to-Maturity 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	2,511	2,470
Held-to-maturity, Less than 12 months, Unrealized Losses	(15)	(21)
Held-to-maturity, 12 months or longer, Estimated Fair Value	42,298	43,059
Held-to-maturity, 12 months or longer, Unrealized Losses	(3,740)	(4,963)
Held-to-maturity, Total, Estimated Fair Value	44,809	45,529
Held-to-maturity, Total, Unrealized Losses	(3,755)	(4,984)
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	0	1,978
Available-for-sale, Less than 12 Months, Unrealized Losses	0	(8)
Available-for-sale, 12 months or longer, Estimated Fair Value	42,287	66,438
Available-for-sale, 12 months or longer, Unrealized Losses	(204)	(960)
Available-for-sale, Total, Estimated Fair Value	42,287	68,416
Available-for-sale, Total, Unrealized Losses	(204)	(968)
Investment Securities | Investment securities - U.S. government and agency obligations
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Available-for-sale, Less thank 12 months, Estimated Fair Value	0	985
Available-for-sale, Less than 12 Months, Unrealized Losses	0	(3)
Available-for-sale, 12 months or longer, Estimated Fair Value	42,287	66,438
Available-for-sale, 12 months or longer, Unrealized Losses	(204)	(960)
Available-for-sale, Total, Estimated Fair Value	42,287	67,423
Available-for-sale, Total, Unrealized Losses	$ (204)	(963)
Investment Securities | 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)