Investment Securities Available-for-Sale - Gross unrealized losses and fair value (Details) $ in Thousands	Jun. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of securities | security	146	66
Less than 12 months, estimated fair value	$ 925,914	$ 222,354
Less than 12 months, unrealized losses	10,914	857
12 months or greater, estimated fair value	48,284	47,739
12 months or greater, unrealized losses	535	611
Estimated fair value	974,198	270,093
Unrealized losses	$ 11,449	$ 1,468
U. S. agency securities
Debt Securities, Available-for-sale [Line Items]
Number of securities | security	45	28
Less than 12 months, estimated fair value	$ 198,756	$ 46,412
Less than 12 months, unrealized losses	3,085	67
12 months or greater, estimated fair value	43,126	41,320
12 months or greater, unrealized losses	482	560
Estimated fair value	241,882	87,732
Unrealized losses	$ 3,567	$ 627
Residential mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Number of securities | security	94	35
Less than 12 months, estimated fair value	$ 693,251	$ 170,178
Less than 12 months, unrealized losses	7,500	782
12 months or greater, estimated fair value	5,158	6,419
12 months or greater, unrealized losses	53	51
Estimated fair value	698,409	176,597
Unrealized losses	$ 7,553	$ 833
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Number of securities | security	4
Less than 12 months, estimated fair value	$ 21,375
Less than 12 months, unrealized losses	239
12 months or greater, estimated fair value	0
12 months or greater, unrealized losses	0
Estimated fair value	21,375
Unrealized losses	$ 239
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Number of securities | security	3	3
Less than 12 months, estimated fair value	$ 12,532	$ 5,764
Less than 12 months, unrealized losses	90	8
12 months or greater, estimated fair value	0	0
12 months or greater, unrealized losses	0	0
Estimated fair value	12,532	5,764
Unrealized losses	$ 90	$ 8