Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost		$ 990,582	$ 772,060
Held-to-maturity, Estimated Fair Value		761,660
Total, Amortized Cost	1,077,960	854,438
Total, Gross Unrealized Gains	1,906	1,967
Total, Gross Unrealized Losses	(22,353)	(13,164)
Total, Estimated Fair Value	1,057,513	843,241
Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	990,582	772,060
Held-to-maturity, Gross Unrealized Gains	1,906	1,967
Held-to-maturity, Gross Unrealized Losses	(21,089)	(12,367)
Held-to-maturity, Estimated Fair Value	971,399	761,660
Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	249,792	240,950
Held-to-maturity, Gross Unrealized Gains	652	531
Held-to-maturity, Gross Unrealized Losses	(6,667)	(5,502)
Held-to-maturity, Estimated Fair Value	243,777	235,979
Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	87,378
Held-to-maturity, Gross Unrealized Gains	0
Held-to-maturity, Gross Unrealized Losses	(1,264)
Held-to-maturity, Estimated Fair Value	86,114
U.S. Agency Obligations | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	14,970	14,968
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(154)	(65)
Held-to-maturity, Estimated Fair Value	14,816	14,903
U.S. Agency Obligations | Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	85,861	82,378
Available-for-sale, Gross Unrealized Gains	0	0
Available-for-sale, Gross Unrealized Losses	(1,261)	(797)
Available-for-sale, Estimated Fair Value	84,600	81,581
Mortgage-Backed Securities | Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	740,790	531,110
Held-to-maturity, Gross Unrealized Gains	1,254	1,436
Held-to-maturity, Gross Unrealized Losses	(14,422)	(6,865)
Held-to-maturity, Estimated Fair Value	727,622	525,681
Mortgage-Backed Securities | Held-to-Maturity Securities | FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	277,110	186,921
Held-to-maturity, Gross Unrealized Gains	73	151
Held-to-maturity, Gross Unrealized Losses	(5,789)	(2,937)
Held-to-maturity, Estimated Fair Value	271,394	184,135
Mortgage-Backed Securities | Held-to-Maturity Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	317,419	263,103
Held-to-maturity, Gross Unrealized Gains	1,028	1,193
Held-to-maturity, Gross Unrealized Losses	(6,679)	(3,000)
Held-to-maturity, Estimated Fair Value	311,768	261,296
Mortgage-Backed Securities | Held-to-Maturity Securities | GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	141,218	75,243
Held-to-maturity, Gross Unrealized Gains	153	64
Held-to-maturity, Gross Unrealized Losses	(1,952)	(928)
Held-to-maturity, Estimated Fair Value	139,419	74,379
Mortgage-Backed Securities | Held-to-Maturity Securities | SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	5,043	5,843
Held-to-maturity, Gross Unrealized Gains	0	28
Held-to-maturity, Gross Unrealized Losses	(2)	0
Held-to-maturity, Estimated Fair Value	5,041	5,871
Mortgage-Backed Securities | Debt Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	1,517
Held-to-maturity, Gross Unrealized Gains	0
Held-to-maturity, Gross Unrealized Losses	(3)
Held-to-maturity, Estimated Fair Value	1,514
State and Municipal Obligations | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	149,055	149,958
Held-to-maturity, Gross Unrealized Gains	2	219
Held-to-maturity, Gross Unrealized Losses	(2,636)	(1,475)
Held-to-maturity, Estimated Fair Value	146,421	148,702
Corporate Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	59,034
Held-to-maturity, Estimated Fair Value	55,252
Corporate Debt Securities | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	85,767	76,024
Held-to-maturity, Gross Unrealized Gains	650	312
Held-to-maturity, Gross Unrealized Losses	(3,877)	(3,962)
Held-to-maturity, Estimated Fair Value	$ 82,540	$ 72,374