Investment Securities Available-for-Sale - Expected maturities for residential mortgage backed securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost [Abstract]
Amortized Cost	$ 1,674,264	$ 1,129,057
Estimated Fair Value [Abstract]
Allowance for Credit Losses	(132)	(167)
Estimated Fair Value	1,680,832	1,150,885
U. S. agency securities
Amortized Cost [Abstract]
One year or less	159,698	53,916
After one year through five years	122,900	110,083
Five years through ten years	76,741	17,087
Amortized Cost	359,339	181,087
Estimated Fair Value [Abstract]
One year or less	158,781	53,906
After one year through five years	122,872	110,777
Five years through ten years	75,658	17,240
Residential mortgage backed securities
Amortized Cost [Abstract]
Residential mortgage backed securities	1,143,931	811,328
Amortized Cost	1,143,931	811,328
Estimated Fair Value [Abstract]
Residential mortgage backed securities	1,146,322	825,001
Municipal bonds
Amortized Cost [Abstract]
One year or less	2,764	4,329
After one year through five years	26,121	26,622
Five years through ten years	66,559	69,309
After ten years	0	2,000
Amortized Cost	95,444	102,259
Estimated Fair Value [Abstract]
One year or less	2,796	4,348
After one year through five years	27,751	28,272
Five years through ten years	69,732	73,389
After ten years	0	2,121
Corporate bonds
Amortized Cost [Abstract]
One year or less	6,314	5,218
After one year through five years	48,958	22,189
Five years through ten years	20,278	0
After ten years	0	6,976
Amortized Cost	75,550	34,383
Estimated Fair Value [Abstract]
One year or less	6,542	5,220
After one year through five years	49,744	23,267
Five years through ten years	20,766	0
After ten years	$ 0	$ 7,511