Investment Securities - Investment Securities by External Credit Rating (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
2020	$ 317,975
2019	632,236
2018	440,549
2017	321,626
2016	385,358
Prior	270,879
Total	2,368,623
Investment securities available-for-sale:
Debt Securities, Available-for-sale [Line Items]
2020	317,975
2019	632,236
2018	430,348
2017	314,139
2016	380,002
Prior	261,366
Total	2,336,066
Investment securities available-for-sale: | U.S. Treasury | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	22,103
2017	10,767
2016	0
Prior	0
Total	32,870
Investment securities available-for-sale: | Agency | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	28,924
2019	44,590
2018	159,345
2017	9,759
2016	21,556
Prior	75,816
Total	339,990
Investment securities available-for-sale: | Corporate | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	19,856
2018	0
2017	0
2016	119,687
Prior	9,344
Total	148,887
Investment securities available-for-sale: | Corporate | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	19,652
2019	41,571
2018	5,100
2017	18,131
2016	8,857
Prior	31,801
Total	125,112
Investment securities available-for-sale: | Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	67,718
2019	264,432
2018	32,497
2017	50,889
2016	15,176
Prior	30,623
Total	461,335
Investment securities available-for-sale: | Collateralized mortgage obligations | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	37,610
2019	44,082
2018	162,056
2017	4,208
2016	119,932
Prior	16,851
Total	384,739
Investment securities available-for-sale: | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	164,071
2019	217,705
2018	49,247
2017	220,385
2016	94,794
Prior	96,931
Total	843,133
Investment securities held-to-maturity:
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	10,201
2017	7,487
2016	5,356
Prior	9,513
Total	32,557
Investment securities held-to-maturity: | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	9,538
2017	7,487
2016	5,356
Prior	8,511
Total	30,892
Investment securities held-to-maturity: | Other | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	663
2017	0
2016	0
Prior	1,002
Total	$ 1,665