Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost		$ 888,848	$ 772,060
Held-to-maturity, Estimated Fair Value	$ 864,173	761,660
Total, Amortized Cost	990,451	854,438
Total, Gross Unrealized Gains	1,897	1,967
Total, Gross Unrealized Losses	(28,160)	(13,164)
Total, Estimated Fair Value	964,188	843,241
Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	888,848	772,060
Held-to-maturity, Gross Unrealized Gains	1,897	1,967
Held-to-maturity, Gross Unrealized Losses	(26,572)	(12,367)
Held-to-maturity, Estimated Fair Value	864,173	761,660
Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	214,781	240,950
Held-to-maturity, Gross Unrealized Gains	921	531
Held-to-maturity, Gross Unrealized Losses	(6,112)	(5,502)
Held-to-maturity, Estimated Fair Value	209,590	235,979
Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	101,603
Held-to-maturity, Gross Unrealized Gains	0
Held-to-maturity, Gross Unrealized Losses	(1,588)
Held-to-maturity, Estimated Fair Value	100,015
U.S. Agency Obligations | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	14,974	14,968
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(192)	(65)
Held-to-maturity, Estimated Fair Value	14,782	14,903
U.S. Agency Obligations | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	100,450	82,378
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(1,577)	(797)
Held-to-maturity, Estimated Fair Value	98,873	81,581
Mortgage-Backed Securities | Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	674,067	531,110
Held-to-maturity, Gross Unrealized Gains	976	1,436
Held-to-maturity, Gross Unrealized Losses	(20,460)	(6,865)
Held-to-maturity, Estimated Fair Value	654,583	525,681
Mortgage-Backed Securities | Held-to-Maturity Securities | FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	249,884	186,921
Held-to-maturity, Gross Unrealized Gains	45	151
Held-to-maturity, Gross Unrealized Losses	(8,126)	(2,937)
Held-to-maturity, Estimated Fair Value	241,803	184,135
Mortgage-Backed Securities | Held-to-Maturity Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	289,210	263,103
Held-to-maturity, Gross Unrealized Gains	755	1,193
Held-to-maturity, Gross Unrealized Losses	(9,382)	(3,000)
Held-to-maturity, Estimated Fair Value	280,583	261,296
Mortgage-Backed Securities | Held-to-Maturity Securities | GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	130,847	75,243
Held-to-maturity, Gross Unrealized Gains	176	64
Held-to-maturity, Gross Unrealized Losses	(2,858)	(928)
Held-to-maturity, Estimated Fair Value	128,165	74,379
Mortgage-Backed Securities | Held-to-Maturity Securities | SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	4,126	5,843
Held-to-maturity, Gross Unrealized Gains	0	28
Held-to-maturity, Gross Unrealized Losses	(94)	0
Held-to-maturity, Estimated Fair Value	4,032	5,871
Mortgage-Backed Securities | Debt Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	1,153
Held-to-maturity, Gross Unrealized Gains	0
Held-to-maturity, Gross Unrealized Losses	(11)
Held-to-maturity, Estimated Fair Value	1,142
State and Municipal Obligations | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	134,988	149,958
Held-to-maturity, Gross Unrealized Gains	0	219
Held-to-maturity, Gross Unrealized Losses	(2,885)	(1,475)
Held-to-maturity, Estimated Fair Value	132,103	148,702
Corporate Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	44,550
Held-to-maturity, Estimated Fair Value	41,583
Corporate Debt Securities | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	64,819	76,024
Held-to-maturity, Gross Unrealized Gains	921	312
Held-to-maturity, Gross Unrealized Losses	(3,035)	(3,962)
Held-to-maturity, Estimated Fair Value	$ 62,705	$ 72,374