Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 122,558	$ 100,717
Held-to-maturity, Amortized Cost		905,496	$ 851,903
Estimated Fair Value	896,812	832,815
Total, Amortized Cost	1,028,041	953,425
Total, Gross Unrealized Gains	3,742	1,466
Total, Gross Unrealized Losses	(12,413)	(21,359)
Total, Estimated Fair Value	1,019,370	933,532
Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	905,496	851,903
Held-to-maturity, Gross Unrealized Gains	3,097	1,303
Held-to-maturity, Gross Unrealized Losses	(11,781)	(20,391)
Estimated Fair Value	896,812	832,815
Held-to-Maturity Securities \| Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	245,263	205,796
Held-to-maturity, Gross Unrealized Gains	1,069	193
Held-to-maturity, Gross Unrealized Losses	(4,208)	(6,811)
Estimated Fair Value	242,124	199,178
Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	122,545	101,522
Available-for-sale, Gross Unrealized Gains	645	163
Available-for-sale, Gross Unrealized Losses	(632)	(968)
Available-for-sale, Estimated Fair Value	122,558	100,717
Investment securities - U.S. government and agency obligations \| Held-to-Maturity Securities \| Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	14,977	14,975
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(78)	(130)
Estimated Fair Value	14,899	14,845
Investment securities - U.S. government and agency obligations \| Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	118,632	100,524
Available-for-sale, Gross Unrealized Gains	642	163
Available-for-sale, Gross Unrealized Losses	(630)	(963)
Available-for-sale, Estimated Fair Value	118,644	99,724
State and municipal obligations \| Held-to-Maturity Securities \| Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	147,499	123,987
Held-to-maturity, Gross Unrealized Gains	517	67
Held-to-maturity, Gross Unrealized Losses	(641)	(1,697)
Estimated Fair Value	147,375	122,357
State and municipal obligations \| Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	2,055
Available-for-sale, Gross Unrealized Gains	1
Available-for-sale, Gross Unrealized Losses	0
Available-for-sale, Estimated Fair Value	2,056
Corporate debt securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	48,011
Estimated Fair Value	44,541
Corporate debt securities \| Held-to-Maturity Securities \| Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	82,787	66,834
Held-to-maturity, Gross Unrealized Gains	552	126
Held-to-maturity, Gross Unrealized Losses	(3,489)	(4,984)
Estimated Fair Value	79,850	61,976
Corporate debt securities \| Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	999
Available-for-sale, Gross Unrealized Gains	0
Available-for-sale, Gross Unrealized Losses	0
Available-for-sale, Estimated Fair Value	999
Mortgage-backed securities \| Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	660,233	646,107
Held-to-maturity, Gross Unrealized Gains	2,028	1,110
Held-to-maturity, Gross Unrealized Losses	(7,573)	(13,580)
Estimated Fair Value	654,688	633,637
Mortgage-backed securities \| Held-to-Maturity Securities \| FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	245,662	237,703
Held-to-maturity, Gross Unrealized Gains	688	159
Held-to-maturity, Gross Unrealized Losses	(2,738)	(5,110)
Estimated Fair Value	243,612	232,752
Mortgage-backed securities \| Held-to-Maturity Securities \| FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	288,289	277,266
Held-to-maturity, Gross Unrealized Gains	1,086	753
Held-to-maturity, Gross Unrealized Losses	(2,963)	(6,030)
Estimated Fair Value	286,412	271,989
Mortgage-backed securities \| Held-to-Maturity Securities \| GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	122,837	127,611
Held-to-maturity, Gross Unrealized Gains	254	198
Held-to-maturity, Gross Unrealized Losses	(1,794)	(2,360)
Estimated Fair Value	121,297	125,449
Mortgage-backed securities \| Held-to-Maturity Securities \| SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	3,445	3,527
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(78)	(80)
Estimated Fair Value	3,367	3,447
Mortgage-backed securities \| Investment Securities \| FNMA
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost		998
Available-for-sale, Gross Unrealized Gains		0
Available-for-sale, Gross Unrealized Losses		(5)
Available-for-sale, Estimated Fair Value		$ 993
Mortgage-backed securities \| Debt Securities \| FNMA
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	859
Available-for-sale, Gross Unrealized Gains	2
Available-for-sale, Gross Unrealized Losses	(2)
Available-for-sale, Estimated Fair Value	$ 859

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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