Investment Securities - Investment Securities by External Credit Rating (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
2020	$ 2,209,625
2019	640,139
2018	280,417
2017	258,763
2016	305,701
Prior	236,470
Total	3,931,115	$ 1,368,384
2020	0
2019	0
2018	7,185
2017	5,372
2016	4,209
Prior	6,966
Amortized Cost	23,732	37,838
2020	2,209,625
2019	640,139
2018	287,602
2017	264,135
2016	309,910
Prior	243,436
Total	3,954,847
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Total	32,533	63,555
U.S. Treasury | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	21,852
2017	10,681
2016	0
Prior	0
Total	32,533
Agency
Debt Securities, Available-for-sale [Line Items]
Total	690,386	246,358
Agency | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	359,428
2019	60,943
2018	157,942
2017	9,733
2016	20,711
Prior	81,629
Total	690,386
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Total	415,308	151,353
Corporate debt | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	60,858
2019	0
2018	0
2017	0
2016	118,631
Prior	9,145
Total	188,634
Corporate debt | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	99,999
2019	69,638
2018	5,034
2017	17,936
2016	5,449
Prior	28,618
Total	226,674
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Total	1,446,019	397,298
Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	1,022,544
2019	290,007
2018	32,798
2017	60,595
2016	15,294
Prior	23,919
Total	1,445,157
Municipal bonds | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	862
Total	862
Collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total	513,366	9,984
Collateralized mortgage obligations | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	241,971
2019	107,104
2018	29,890
2017	15,305
2016	105,641
Prior	13,455
Total	513,366
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total	833,503	499,836
Amortized Cost	22,124	36,114
Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2020	424,825
2019	112,447
2018	32,901
2017	144,513
2016	39,975
Prior	78,842
Total	833,503
2020	0
2019	0
2018	6,552
2017	5,372
2016	4,209
Prior	5,991
Amortized Cost	22,124
Other
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,608	$ 1,724
Other | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2020	0
2019	0
2018	633
2017	0
2016	0
Prior	975
Amortized Cost	$ 1,608