Investment Securities - Investment Securities by External Credit Rating (Details) $ in Thousands	Mar. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
2021	$ 213,825
2020	2,202,563
2019	630,449
2018	250,083
2017	164,354
Prior	417,994
Total	3,879,268
Investment securities available-for-sale:
Debt Securities, Available-for-sale [Line Items]
2021	213,825
2020	2,202,563
2019	630,449
2018	243,644
2017	159,168
Prior	407,688
Total	3,857,337
Investment securities available-for-sale: | U.S. Treasury | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	21,510
2017	10,553
Prior	0
Total	32,063
Investment securities available-for-sale: | Agency | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	352,414
2019	73,090
2018	151,364
2017	9,075
Prior	77,216
Total	663,159
Investment securities available-for-sale: | Corporate | A1 - A3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	56,760
2019	0
2018	0
2017	0
Prior	93,295
Total	150,055
Investment securities available-for-sale: | Corporate | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	98,707
2019	69,231
2018	0
2017	17,919
Prior	21,922
Total	207,779
Investment securities available-for-sale: | Municipal bonds | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	11,286
2020	994,512
2019	285,174
2018	32,304
2017	59,458
Prior	35,689
Total	1,418,423
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	2,313
Total	2,313
Investment securities available-for-sale: | Municipal bonds | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Prior	1,326
Total	1,326
Investment securities available-for-sale: | Collateralized mortgage obligations | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	17,334
2020	241,843
2019	93,697
2018	25,311
2017	14,634
Prior	116,152
Total	508,971
Investment securities available-for-sale: | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	185,205
2020	458,327
2019	109,257
2018	13,155
2017	47,529
Prior	59,775
Total	873,248
Investment securities held-to-maturity:
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	6,439
2017	5,186
Prior	10,306
Total	21,931
Investment securities held-to-maturity: | Mortgage-backed securities | Aaa - Aa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	5,822
2017	5,186
Prior	9,345
Total	20,353
Investment securities held-to-maturity: | Other | Baa1 - Baa3
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	617
2017	0
Prior	961
Total	$ 1,578