Investment Securities - Investment Securities by External Credit Rating (Details) $ in Thousands	Jun. 30, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
2021	$ 1,097,599
2020	2,113,159
2019	632,831
2018	177,013
2017	148,068
Debt Securities, Originated Five or More Years before Latest Fiscal Year	337,710
Debt Securities, Total	4,506,380
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
2021	1,097,599
2020	2,113,159
2019	632,831
2018	171,376
2017	143,587
Debt Securities, Originated Five or More Years before Latest Fiscal Year	328,895
Debt Securities, Total	4,487,447
Available-for-sale Securities [Member] | U.S. Treasury | Moody's, Aa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	33,148
2020	26,266
2019	20,481
2018	21,408
2017	10,505
Debt Securities, Originated Five or More Years before Latest Fiscal Year	0
Debt Securities, Total	111,808
Available-for-sale Securities [Member] | Agency | Moody's, Aa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	9,572
2020	349,992
2019	55,601
2018	83,752
2017	0
Debt Securities, Originated Five or More Years before Latest Fiscal Year	56,250
Debt Securities, Total	555,167
Available-for-sale Securities [Member] | Corporate | Moody's, A3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	58,502
2019	0
2018	0
2017	0
Debt Securities, Originated Five or More Years before Latest Fiscal Year	60,573
Debt Securities, Total	119,075
Available-for-sale Securities [Member] | Corporate | Moody's, Baa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	36,553
2020	100,494
2019	70,721
2018	0
2017	17,925
Debt Securities, Originated Five or More Years before Latest Fiscal Year	13,614
Debt Securities, Total	239,307
Available-for-sale Securities [Member] | Municipal bonds | Moody's, Aa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	40,975
2020	907,667
2019	297,852
2018	32,830
2017	60,376
Debt Securities, Originated Five or More Years before Latest Fiscal Year	36,124
Debt Securities, Total	1,375,824
Available-for-sale Securities [Member] | Municipal bonds | Moody's, A3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Debt Securities, Originated Five or More Years before Latest Fiscal Year	2,309
Debt Securities, Total	2,309
Available-for-sale Securities [Member] | Municipal bonds | Moody's, Baa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	0
2017	0
Debt Securities, Originated Five or More Years before Latest Fiscal Year	1,340
Debt Securities, Total	1,340
Available-for-sale Securities [Member] | Collateralized mortgage obligations | Moody's, Aa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	142,912
2020	232,492
2019	91,915
2018	20,786
2017	14,443
Debt Securities, Originated Five or More Years before Latest Fiscal Year	112,190
Debt Securities, Total	614,738
Available-for-sale Securities [Member] | Mortgage-backed securities | Moody's, Aa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	834,439
2020	437,746
2019	96,261
2018	12,600
2017	40,338
Debt Securities, Originated Five or More Years before Latest Fiscal Year	46,495
Debt Securities, Total	1,467,879
Held-to-maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	5,637
2017	4,481
Debt Securities, Originated Five or More Years before Latest Fiscal Year	8,815
Debt Securities, Total	18,933
Held-to-maturity Securities [Member] | Mortgage-backed securities | Moody's, Aa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	5,029
2017	4,481
Debt Securities, Originated Five or More Years before Latest Fiscal Year	7,868
Debt Securities, Total	17,378
Held-to-maturity Securities [Member] | Other | Moody's, Baa3 Rating [Member]
Debt Securities, Available-for-sale [Line Items]
2021	0
2020	0
2019	0
2018	608
2017	0
Debt Securities, Originated Five or More Years before Latest Fiscal Year	947
Debt Securities, Total	$ 1,555