Securities - Estimated Fair Value and Unrealized Loss for Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 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	$ 429,651	$ 450,421
Total securities, Less than 12 months, Unrealized Losses	(8,820)	(3,806)
Total securities, 12 months or longer, Estimated Fair Value	431,471	261,254
Total securities, 12 months or longer, Unrealized Losses	(19,340)	(9,358)
Total securities, Estimated Fair Value	861,122	711,675
Total securities, Unrealized Losses	(28,160)	(13,164)
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	371,257	381,046
Held-to-maturity, Less than 12 months, Unrealized Losses	(8,134)	(3,310)
Held-to-maturity, 12 months or longer, Estimated Fair Value	389,850	249,048
Held-to-maturity, 12 months or longer, Unrealized Losses	(18,438)	(9,057)
Held-to-maturity, Total, Estimated Fair Value	761,107	630,094
Held-to-maturity, Total, Unrealized Losses	(26,572)	(12,367)
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	67,517	123,821
Held-to-maturity, Less than 12 months, Unrealized Losses	(941)	(1,248)
Held-to-maturity, 12 months or longer, Estimated Fair Value	114,030	70,469
Held-to-maturity, 12 months or longer, Unrealized Losses	(5,171)	(4,254)
Held-to-maturity, Total, Estimated Fair Value	181,547	194,290
Held-to-maturity, Total, Unrealized Losses	(6,112)	(5,502)
Held-to-Maturity Securities | Investment Securities | Other investments
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	4,903	4,035
Held-to-maturity, Less than 12 months, Unrealized Losses	(68)	(30)
Held-to-maturity, 12 months or longer, Estimated Fair Value	41,583	56,106
Held-to-maturity, 12 months or longer, Unrealized Losses	(2,967)	(3,932)
Held-to-maturity, Total, Estimated Fair Value	46,486	60,141
Held-to-maturity, Total, Unrealized Losses	(3,035)	(3,962)
U.S. 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		14,903
Held-to-maturity, Less than 12 months, Unrealized Losses		(65)
Held-to-maturity, 12 months or longer, Estimated Fair Value		0
Held-to-maturity, 12 months or longer, Unrealized Losses		0
Held-to-maturity, Total, Estimated Fair Value		14,903
Held-to-maturity, Total, Unrealized Losses		(65)
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	303,740	257,225
Held-to-maturity, Less than 12 months, Unrealized Losses	(7,193)	(2,062)
Held-to-maturity, 12 months or longer, Estimated Fair Value	275,820	178,579
Held-to-maturity, 12 months or longer, Unrealized Losses	(13,267)	(4,803)
Held-to-maturity, Total, Estimated Fair Value	579,560	435,804
Held-to-maturity, Total, Unrealized Losses	(20,460)	(6,865)
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	125,973	98,138
Held-to-maturity, Less than 12 months, Unrealized Losses	(2,463)	(781)
Held-to-maturity, 12 months or longer, Estimated Fair Value	112,290	68,238
Held-to-maturity, 12 months or longer, Unrealized Losses	(5,663)	(2,156)
Held-to-maturity, Total, Estimated Fair Value	238,263	166,376
Held-to-maturity, Total, Unrealized Losses	(8,126)	(2,937)
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	120,922	132,982
Held-to-maturity, Less than 12 months, Unrealized Losses	(3,594)	(1,058)
Held-to-maturity, 12 months or longer, Estimated Fair Value	112,293	65,060
Held-to-maturity, 12 months or longer, Unrealized Losses	(5,788)	(1,942)
Held-to-maturity, Total, Estimated Fair Value	233,215	198,042
Held-to-maturity, Total, Unrealized Losses	(9,382)	(3,000)
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	52,813	26,105
Held-to-maturity, Less than 12 months, Unrealized Losses	(1,042)	(223)
Held-to-maturity, 12 months or longer, Estimated Fair Value	51,237	45,281
Held-to-maturity, 12 months or longer, Unrealized Losses	(1,816)	(705)
Held-to-maturity, Total, Estimated Fair Value	104,050	71,386
Held-to-maturity, Total, Unrealized Losses	(2,858)	(928)
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	4,032
Held-to-maturity, Less than 12 months, Unrealized Losses	(94)
Held-to-maturity, 12 months or longer, Estimated Fair Value	0
Held-to-maturity, 12 months or longer, Unrealized Losses	0
Held-to-maturity, Total, Estimated Fair Value	4,032
Held-to-maturity, Total, Unrealized Losses	(94)	0
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	9,835
Held-to-maturity, Less than 12 months, Unrealized Losses	(129)
Held-to-maturity, 12 months or longer, Estimated Fair Value	4,947
Held-to-maturity, 12 months or longer, Unrealized Losses	(63)
Held-to-maturity, Total, Estimated Fair Value	14,782
Held-to-maturity, Total, Unrealized Losses	(192)
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	52,779	104,883
Held-to-maturity, Less than 12 months, Unrealized Losses	(744)	(1,153)
Held-to-maturity, 12 months or longer, Estimated Fair Value	67,500	14,363
Held-to-maturity, 12 months or longer, Unrealized Losses	(2,141)	(322)
Held-to-maturity, Total, Estimated Fair Value	120,279	119,246
Held-to-maturity, Total, Unrealized Losses	(2,885)	(1,475)
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	58,394
Held-to-maturity, Less than 12 months, Unrealized Losses	(686)
Held-to-maturity, 12 months or longer, Estimated Fair Value	41,621
Held-to-maturity, 12 months or longer, Unrealized Losses	(902)
Held-to-maturity, Total, Estimated Fair Value	100,015
Held-to-maturity, Total, Unrealized Losses	(1,588)
Investment Securities | U.S. Agency Obligations
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Held-to-maturity, Less than 12 months, Estimated Fair Value	57,252	69,375
Held-to-maturity, Less than 12 months, Unrealized Losses	(675)	(496)
Held-to-maturity, 12 months or longer, Estimated Fair Value	41,621	12,206
Held-to-maturity, 12 months or longer, Unrealized Losses	(902)	(301)
Held-to-maturity, Total, Estimated Fair Value	98,873	81,581
Held-to-maturity, Total, Unrealized Losses	(1,577)	$ (797)
Investment Securities | 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	1,142
Held-to-maturity, Less than 12 months, Unrealized Losses	(11)
Held-to-maturity, 12 months or longer, Estimated Fair Value	0
Held-to-maturity, 12 months or longer, Unrealized Losses	0
Held-to-maturity, Total, Estimated Fair Value	1,142
Held-to-maturity, Total, Unrealized Losses	$ (11)