Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 123,610	$ 100,717
Held-to-maturity, Amortized Cost		868,512	$ 851,903
Estimated Fair Value	869,167	832,815
Total, Amortized Cost	990,934	953,425
Total, Gross Unrealized Gains	8,375	1,466
Total, Gross Unrealized Losses	(6,532)	(21,359)
Total, Estimated Fair Value	992,777	933,532
Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	868,512	851,903
Held-to-maturity, Gross Unrealized Gains	6,983	1,303
Held-to-maturity, Gross Unrealized Losses	(6,328)	(20,391)
Estimated Fair Value	869,167	832,815
Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	238,710	205,796
Held-to-maturity, Gross Unrealized Gains	2,294	193
Held-to-maturity, Gross Unrealized Losses	(4,077)	(6,811)
Estimated Fair Value	236,927	199,178
Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	122,422	101,522
Available-for-sale, Gross Unrealized Gains	1,392	163
Available-for-sale, Gross Unrealized Losses	(204)	(968)
Available-for-sale, Estimated Fair Value	123,610	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	9,979	14,975
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(11)	(130)
Estimated Fair Value	9,968	14,845
Investment securities - U.S. government and agency obligations | Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	118,747	100,524
Available-for-sale, Gross Unrealized Gains	1,386	163
Available-for-sale, Gross Unrealized Losses	(204)	(963)
Available-for-sale, Estimated Fair Value	119,929	99,724
State and municipal obligations | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	146,500	123,987
Held-to-maturity, Gross Unrealized Gains	1,147	67
Held-to-maturity, Gross Unrealized Losses	(311)	(1,697)
Estimated Fair Value	147,336	122,357
State and municipal obligations | Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	1,952
Available-for-sale, Gross Unrealized Gains	1
Available-for-sale, Gross Unrealized Losses	0
Available-for-sale, Estimated Fair Value	1,953
Corporate debt securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	46,038
Estimated Fair Value	42,298
Corporate debt securities | Held-to-Maturity Securities | Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	82,231	66,834
Held-to-maturity, Gross Unrealized Gains	1,147	126
Held-to-maturity, Gross Unrealized Losses	(3,755)	(4,984)
Estimated Fair Value	79,623	61,976
Corporate debt securities | Debt Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	1,000
Available-for-sale, Gross Unrealized Gains	0
Available-for-sale, Gross Unrealized Losses	0
Available-for-sale, Estimated Fair Value	1,000
Mortgage-backed securities | Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	629,802	646,107
Held-to-maturity, Gross Unrealized Gains	4,689	1,110
Held-to-maturity, Gross Unrealized Losses	(2,251)	(13,580)
Estimated Fair Value	632,240	633,637
Mortgage-backed securities | Held-to-Maturity Securities | FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	232,379	237,703
Held-to-maturity, Gross Unrealized Gains	1,826	159
Held-to-maturity, Gross Unrealized Losses	(864)	(5,110)
Estimated Fair Value	233,341	232,752
Mortgage-backed securities | Held-to-Maturity Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	273,933	277,266
Held-to-maturity, Gross Unrealized Gains	2,140	753
Held-to-maturity, Gross Unrealized Losses	(789)	(6,030)
Estimated Fair Value	275,284	271,989
Mortgage-backed securities | Held-to-Maturity Securities | GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	120,071	127,611
Held-to-maturity, Gross Unrealized Gains	723	198
Held-to-maturity, Gross Unrealized Losses	(521)	(2,360)
Estimated Fair Value	120,273	125,449
Mortgage-backed securities | Held-to-Maturity Securities | SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	3,419	3,527
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(77)	(80)
Estimated Fair Value	3,342	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	723
Available-for-sale, Gross Unrealized Gains	5
Available-for-sale, Gross Unrealized Losses	0
Available-for-sale, Estimated Fair Value	$ 728