Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	$ 149,683	$ 149,640
Available-for-sale, Gross Unrealized Gains	4,059	1,413
Available-for-sale, Gross Unrealized Losses	(4)	(93)
Available-for-sale, Estimated Fair Value	153,738	150,960
Amortized Cost	920,687	772,975
Held-to-maturity, Amortized Cost	920,687
Held-to-maturity, Gross Unrealized Gains	19,053	8,224
Held-to-maturity, Gross Unrealized Losses	(11,158)	(3,909)
Estimated Fair Value	928,582	777,290
Allowance for Credit Losses	(2,529)	0
Total, Amortized Cost	1,070,370	922,615
Total, Gross Unrealized Gains	23,112	9,637
Total, Gross Unrealized Losses	(11,162)	(4,002)
Total, Estimated Fair Value	1,082,320	928,250
Investment securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	149,286	149,145
Available-for-sale, Gross Unrealized Gains	4,051	1,408
Available-for-sale, Gross Unrealized Losses	(4)	(93)
Available-for-sale, Estimated Fair Value	153,333	150,460
Amortized Cost		208,961
Held-to-maturity, Amortized Cost	320,682
Held-to-maturity, Gross Unrealized Gains	3,578	2,384
Held-to-maturity, Gross Unrealized Losses	(8,993)	(2,629)
Estimated Fair Value	315,267	208,716
Allowance for Credit Losses	(1,686)
U.S. government and agency obligations \| Investment securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	149,261	149,120
Available-for-sale, Gross Unrealized Gains	4,051	1,408
Available-for-sale, Gross Unrealized Losses	(4)	(93)
Available-for-sale, Estimated Fair Value	153,308	150,435
Amortized Cost		4,984
Held-to-maturity, Amortized Cost	6,238
Held-to-maturity, Gross Unrealized Gains	54	14
Held-to-maturity, Gross Unrealized Losses	0	0
Estimated Fair Value	6,292	4,998
Allowance for Credit Losses	0
State and municipal obligations \| Investment securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	25	25
Available-for-sale, Gross Unrealized Gains	0	0
Available-for-sale, Gross Unrealized Losses	0	0
Available-for-sale, Estimated Fair Value	25	25
Amortized Cost		124,430
Held-to-maturity, Amortized Cost	232,167
Held-to-maturity, Gross Unrealized Gains	3,014	1,537
Held-to-maturity, Gross Unrealized Losses	(425)	(208)
Estimated Fair Value	234,756	125,759
Allowance for Credit Losses	(35)
Corporate debt securities
Securities Financing Transaction [Line Items]
Amortized Cost	38,332
Estimated Fair Value	31,292
Corporate debt securities \| Investment securities
Securities Financing Transaction [Line Items]
Amortized Cost		79,547
Held-to-maturity, Amortized Cost	82,277
Held-to-maturity, Gross Unrealized Gains	510	833
Held-to-maturity, Gross Unrealized Losses	(8,568)	(2,421)
Estimated Fair Value	74,219	77,959
Allowance for Credit Losses	(1,651)
Mortgage-backed securities
Securities Financing Transaction [Line Items]
Amortized Cost		564,014
Held-to-maturity, Amortized Cost	600,005
Held-to-maturity, Gross Unrealized Gains	15,475	5,840
Held-to-maturity, Gross Unrealized Losses	(2,165)	(1,280)
Estimated Fair Value	613,315	568,574
Allowance for Credit Losses	(843)	0
Mortgage-backed securities \| FHLMC
Securities Financing Transaction [Line Items]
Amortized Cost		206,985
Held-to-maturity, Amortized Cost	198,263
Held-to-maturity, Gross Unrealized Gains	5,835	2,221
Held-to-maturity, Gross Unrealized Losses	(30)	(524)
Estimated Fair Value	204,068	208,682
Allowance for Credit Losses	0
Mortgage-backed securities \| FNMA
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	397	495
Available-for-sale, Gross Unrealized Gains	8	5
Available-for-sale, Gross Unrealized Losses	0	0
Available-for-sale, Estimated Fair Value	405	500
Amortized Cost		244,428
Held-to-maturity, Amortized Cost	240,839
Held-to-maturity, Gross Unrealized Gains	7,604	2,680
Held-to-maturity, Gross Unrealized Losses	(165)	(493)
Estimated Fair Value	248,278	246,615
Allowance for Credit Losses	0
Mortgage-backed securities \| GNMA
Securities Financing Transaction [Line Items]
Amortized Cost		110,661
Held-to-maturity, Amortized Cost	104,891
Held-to-maturity, Gross Unrealized Gains	2,027	939
Held-to-maturity, Gross Unrealized Losses	(416)	(212)
Estimated Fair Value	106,502	111,388
Allowance for Credit Losses	0
Mortgage-backed securities \| SBA
Securities Financing Transaction [Line Items]
Amortized Cost		1,940
Held-to-maturity, Amortized Cost	6,108
Held-to-maturity, Gross Unrealized Gains	9	0
Held-to-maturity, Gross Unrealized Losses	(46)	(51)
Estimated Fair Value	6,071	$ 1,889
Allowance for Credit Losses	0
Mortgage-backed securities \| CMO
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	49,904
Held-to-maturity, Gross Unrealized Gains	0
Held-to-maturity, Gross Unrealized Losses	(1,508)
Estimated Fair Value	48,396
Allowance for Credit Losses	$ (843)

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