Investment Securities Available-for-Sale - Gross unrealized losses and fair value (Details) $ in Thousands	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities \| security	379	66
Less than 12 months, estimated fair value	$ 827,773	$ 222,354
Less than 12 months, unrealized losses	17,926	857
12 months or greater, estimated fair value	43,889	47,739
12 months or greater, unrealized losses	573	611
Estimated fair value	871,662	270,093
Unrealized losses	$ 18,499	$ 1,468
U. S. agency securities
Debt Securities, Available-for-sale [Line Items]
Number of securities \| security	50	28
Less than 12 months, estimated fair value	$ 197,761	$ 46,412
Less than 12 months, unrealized losses	5,346	67
12 months or greater, estimated fair value	38,854	41,320
12 months or greater, unrealized losses	513	560
Estimated fair value	236,615	87,732
Unrealized losses	$ 5,859	$ 627
Residential mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Number of securities \| security	267	35
Less than 12 months, estimated fair value	$ 579,364	$ 170,178
Less than 12 months, unrealized losses	11,646	782
12 months or greater, estimated fair value	5,035	6,419
12 months or greater, unrealized losses	60	51
Estimated fair value	584,399	176,597
Unrealized losses	$ 11,706	$ 833
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Number of securities \| security	16
Less than 12 months, estimated fair value	$ 23,763
Less than 12 months, unrealized losses	481
12 months or greater, estimated fair value	0
12 months or greater, unrealized losses	0
Estimated fair value	23,763
Unrealized losses	$ 481
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Number of securities \| security	46	3
Less than 12 months, estimated fair value	$ 26,885	$ 5,764
Less than 12 months, unrealized losses	453	8
12 months or greater, estimated fair value	0	0
12 months or greater, unrealized losses	0	0
Estimated fair value	26,885	5,764
Unrealized losses	$ 453	$ 8

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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