Investment Securities - Investment Securities by External Credit Rating (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
2020	$ 1,754,222
2019	659,563
2018	285,117
2017	288,452
2016	369,089
Prior	272,268
Total	3,628,711
Investment securities available-for-sale:
Debt Securities, Available-for-sale [Line Items]
2020	1,754,222
2019	659,563
2018	277,198
2017	281,544
2016	364,476
Prior	263,728
Total	3,600,731
Investment securities available-for-sale: | U.S. Treasury | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	22,006
2017	10,733
2016	0
Prior	0
Total	32,739
Investment securities available-for-sale: | Agency | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	284,262
2019	44,884
2018	159,319
2017	9,869
2016	21,246
Prior	83,465
Total	603,045
Investment securities available-for-sale: | Corporate | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	59,797
2019	19,922
2018	0
2017	0
2016	119,221
Prior	9,256
Total	208,196
Investment securities available-for-sale: | Corporate | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	80,417
2019	41,866
2018	5,074
2017	18,029
2016	8,801
Prior	35,153
Total	189,340
Investment securities available-for-sale: | Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	904,523
2019	284,555
2018	32,497
2017	50,942
2016	15,238
Prior	25,296
Total	1,313,051
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	0
Prior	857
Total	857
Investment securities available-for-sale: | Collateralized mortgage obligations | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	119,689
2019	86,130
2018	14,500
2017	3,975
2016	114,460
Prior	19,526
Total	358,280
Investment securities available-for-sale: | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	305,534
2019	182,206
2018	43,802
2017	187,996
2016	85,510
Prior	90,175
Total	895,223
Investment securities held-to-maturity:
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	7,919
2017	6,908
2016	4,613
Prior	8,540
Total	27,980
Investment securities held-to-maturity: | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	7,291
2017	6,908
2016	4,613
Prior	7,545
Total	26,357
Investment securities held-to-maturity: | Other | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	628
2017	0
2016	0
Prior	995
Total	$ 1,623