Investment Securities - Investment Securities by External Credit Rating (Details) $ in Thousands	Sep. 30, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
2021	$ 1,754,910
2020	1,992,857
2019	598,053
2018	166,053
2017	122,107
Prior	246,422
Total	4,880,402
Investment securities available-for-sale
Debt Securities, Available-for-sale [Line Items]
2021	1,754,910
2020	1,838,458
2019	598,053
2018	161,247
2017	118,340
Prior	238,807
Total	4,709,815
Investment securities available-for-sale | U.S. Treasury | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	32,973
2020	14,634
2019	0
2018	21,274
2017	10,461
Prior	0
Total	79,342
Investment securities available-for-sale | Agency | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	24,393
2020	324,241
2019	44,487
2018	76,415
2017	0
Prior	46,390
Total	515,926
Investment securities available-for-sale | Agency | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	5,715
2017	0
Prior	8,452
Total	14,167
Investment securities available-for-sale | Corporate | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2021	79,252
2020	57,799
2019	0
2018	0
2017	17,864
Prior	10,029
Total	164,944
Investment securities available-for-sale | Corporate | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2021	36,473
2020	100,614
2019	70,704
2018	0
2017	0
Prior	13,656
Total	221,447
Investment securities available-for-sale | Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	118,941
2020	725,524
2019	294,323
2018	32,429
2017	50,536
Prior	33,264
Total	1,255,017
Investment securities available-for-sale | Municipal bonds | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	849
Total	849
Investment securities available-for-sale | Collateralized mortgage obligations | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	212,687
2020	224,252
2019	86,604
2018	18,523
2017	14,199
Prior	105,966
Total	662,231
Investment securities available-for-sale | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	1,250,191
2020	391,394
2019	101,935
2018	6,891
2017	25,280
Prior	20,201
Total	1,795,892
Investment securities held-to-maturity
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	154,399
2019	0
2018	4,806
2017	3,767
Prior	7,615
Total	170,587
Investment securities held-to-maturity | Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	154,399
2019	0
2018	0
2017	0
Prior	0
Total	154,399
Investment securities held-to-maturity | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	4,206
2017	3,767
Prior	6,683
Total	14,656
Investment securities held-to-maturity | Other | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	600
2017	0
Prior	932
Total	$ 1,532