Investment Securities - Investment Securities by External Credit Rating (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
2020	$ 63,741
2019	513,084
2018	261,957
2017	176,068
2016	124,827
Prior	232,637
Total	1,372,314
Investment securities available-for-sale
Debt Securities, Available-for-sale [Line Items]
2020	63,741
2019	513,084
2018	250,953
2017	167,964
2016	119,431
Prior	222,588
Total	1,337,761
Investment securities available-for-sale | U.S. Treasury | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	22,091
2017	10,742
2016	0
Prior	0
Total	32,833
Investment securities available-for-sale | U.S. Treasury | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | U.S. Treasury | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | Agency | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	30,350
2018	111,128
2017	9,668
2016	16,063
Prior	68,391
Total	235,600
Investment securities available-for-sale | Agency | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | Agency | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	5,521
Prior	6,645
Total	12,166
Investment securities available-for-sale | Corporate | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | Corporate | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	19,858
2018	0
2017	0
2016	10,772
Prior	0
Total	30,630
Investment securities available-for-sale | Corporate | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	20,015
2019	42,353
2018	31,435
2017	18,080
2016	20,514
Prior	13,006
Total	145,403
Investment securities available-for-sale | Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	19,663
2019	258,095
2018	32,219
2017	39,913
2016	18,950
Prior	23,775
Total	392,615
Investment securities available-for-sale | Municipal bonds | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	1,800
Prior	2,270
Total	4,070
Investment securities available-for-sale | Municipal bonds | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	5,731
2018	0
2017	3,357
2016	0
Prior	1,324
Total	10,412
Investment securities available-for-sale | Collateralized mortgage obligations | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	9,656
Total	9,656
Investment securities available-for-sale | Collateralized mortgage obligations | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | Collateralized mortgage obligations | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	24,063
2019	156,697
2018	54,080
2017	86,204
2016	45,811
Prior	97,521
Total	464,376
Investment securities available-for-sale | Mortgage-backed securities | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities available-for-sale | Mortgage-backed securities | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities held-to-maturity
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	11,004
2017	8,104
2016	5,396
Prior	10,049
Total	34,553
Investment securities held-to-maturity | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	10,332
2017	8,104
2016	5,396
Prior	9,033
Total	32,865
Investment securities held-to-maturity | Mortgage-backed securities | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities held-to-maturity | Mortgage-backed securities | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities held-to-maturity | Other | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities held-to-maturity | Other | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Total	0
Investment securities held-to-maturity | Other | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	672
2017	0
2016	0
Prior	1,016
Total	$ 1,688