Investment Securities Available-for-Sale - Expected maturities for residential mortgage backed securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost [Abstract]
Allowance for Credit Losses	$ (156)	$ 0
Amortized Cost	956,803	839,192
Estimated Fair Value [Abstract]
Estimated Fair Value	977,570
U. S. agency securities
Amortized Cost [Abstract]
One year or less	38,000	96,332
After one year through five years	81,181	76,121
Five years through ten years	11,132	7,775
Amortized Cost	130,313	180,228
Estimated Fair Value [Abstract]
One year or less	38,111	96,226
After one year through five years	82,057	75,821
Five years through ten years	11,224	7,747
Residential mortgage backed securities
Amortized Cost [Abstract]
Residential mortgage backed securities	702,655	541,490
Amortized Cost	702,655	541,490
Estimated Fair Value [Abstract]
Residential mortgage backed securities	716,643	543,852
Municipal bonds
Amortized Cost [Abstract]
One year or less	4,834	5,897
After one year through five years	26,677	21,416
Five years through ten years	56,781	42,589
After ten years	2,000	2,000
Amortized Cost	90,292	71,902
Estimated Fair Value [Abstract]
One year or less	4,868	5,969
After one year through five years	27,912	21,953
Five years through ten years	59,771	44,015
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	2,072	1,994
Corporate bonds
Amortized Cost [Abstract]
One year or less	5,214	502
After one year through five years	21,156	8,528
After ten years	6,975	1,500
Amortized Cost	33,345	10,530
Estimated Fair Value [Abstract]
One year or less	5,257	508
After one year through five years	22,124	8,725
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	7,489	1,500
U.S. Treasury
Amortized Cost [Abstract]
Residential mortgage backed securities		34,844
Amortized Cost		34,844
Estimated Fair Value [Abstract]
Residential mortgage backed securities		34,855
Other equity investments
Amortized Cost [Abstract]
Residential mortgage backed securities	198	198
Amortized Cost	198	198
Estimated Fair Value [Abstract]
Residential mortgage backed securities	$ 198	198
Estimated Fair Value		$ 843,363

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