Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total securities, Less than 12 months, Estimated Fair Value	$ 120,181	$ 450,421
Total securities, Less than 12 months, Unrealized Losses	(932)	(3,806)
Total securities, 12 months or longer, Estimated Fair Value	622,974	261,254
Total securities, 12 months or longer, Unrealized Losses	(20,427)	(9,358)
Total securities, Estimated Fair Value	743,155	711,675
Total securities, Unrealized Losses	(21,359)	(13,164)
Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Available-for-sale, Less than 12 Months, Estimated Fair Value	1,978
Debt Securites, Available-for-sale, Less than 12 months, Unrealized Losses	(8)
Debt Securites, Available-for-sale, 12 Months or Longer, Estimated Fair Value	66,438
Debt Securites, Available-for-sale, 12 Months or Longer, Unrealized Losses	(960)
Debt Securites, Available-for-sale, Total, Estimated Fair Value	68,416
Debt Securites, Available-for-sale, Total, Unrealized Losses	(968)
Investment Securities | U.S. Agency Obligations
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Available-for-sale, Less than 12 Months, Estimated Fair Value	985	69,375
Debt Securites, Available-for-sale, Less than 12 months, Unrealized Losses	(3)	(496)
Debt Securites, Available-for-sale, 12 Months or Longer, Estimated Fair Value	66,438	12,206
Debt Securites, Available-for-sale, 12 Months or Longer, Unrealized Losses	(960)	(301)
Debt Securites, Available-for-sale, Total, Estimated Fair Value	67,423	81,581
Debt Securites, Available-for-sale, Total, Unrealized Losses	(963)	(797)
Investment Securities | Other investments
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Available-for-sale, Less than 12 Months, Estimated Fair Value	993
Debt Securites, Available-for-sale, Less than 12 months, Unrealized Losses	(5)
Debt Securites, Available-for-sale, 12 Months or Longer, Estimated Fair Value	0
Debt Securites, Available-for-sale, 12 Months or Longer, Unrealized Losses	0
Debt Securites, Available-for-sale, Total, Estimated Fair Value	993
Debt Securites, Available-for-sale, Total, Unrealized Losses	(5)
Held-to-Maturity Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	118,203	381,046
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(924)	(3,310)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	556,536	249,048
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(19,467)	(9,057)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	674,739	630,094
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(20,391)	(12,367)
Held-to-Maturity Securities | Mortgage-Backed Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	112,877	257,225
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(899)	(2,062)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	392,559	178,579
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(12,681)	(4,803)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	505,436	435,804
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(13,580)	(6,865)
Held-to-Maturity Securities | FHLMC | Mortgage-Backed Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	46,615	98,138
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(159)	(781)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	147,763	68,238
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(4,951)	(2,156)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	194,378	166,376
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(5,110)	(2,937)
Held-to-Maturity Securities | Mortgage-backed securities - FNMA | Mortgage-Backed Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	27,594	132,982
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(125)	(1,058)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	185,328	65,060
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(5,905)	(1,942)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	212,922	198,042
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(6,030)	(3,000)
Held-to-Maturity Securities | GNMA | Mortgage-Backed Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	35,221	26,105
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(535)	(223)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	59,468	45,281
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(1,825)	(705)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	94,689	71,386
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(2,360)	(928)
Held-to-Maturity Securities | SBA | Mortgage-Backed Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	3,447
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(80)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	0
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	0
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	3,447
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(80)	0
Held-to-Maturity Securities | Investment Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	5,326	123,821
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(25)	(1,248)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	163,977	70,469
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(6,786)	(4,254)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	169,303	194,290
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(6,811)	(5,502)
Held-to-Maturity Securities | Investment Securities | U.S. Agency Obligations
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	0	14,903
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	0	(65)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	14,845	0
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(130)	0
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	14,845	14,903
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(130)	(65)
Held-to-Maturity Securities | Investment Securities | State and Municipal Obligations
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	2,856	104,883
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(4)	(1,153)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	106,073	14,363
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(1,693)	(322)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	108,929	119,246
Debt Securites, Held-to-maturity, Total, Unrealized Losses	(1,697)	(1,475)
Held-to-Maturity Securities | Investment Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Debt Securites, Held-to-maturity, Less than 12 months, Estimated Fair Value	2,470	4,035
Debt Securites, Held-to-maturity, Less than 12 months, Unrealized Losses	(21)	(30)
Debt Securites, Held-to-maturity, 12 months or longer, Estimated Fair Value	43,059	56,106
Debt Securites, Held-to-maturity, 12 months or longer, Unrealized Losses	(4,963)	(3,932)
Debt Securites, Held-to-maturity, Total, Estimated Fair Value	45,529	60,141
Debt Securites, Held-to-maturity, Total, Unrealized Losses	$ (4,984)	$ (3,962)