Investment Securities Available-for-Sale - Amortized cost and estimated fair value of securities available-for-sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,129,057	$ 838,994
Gross Unrealized Gains	23,463	7,206
Gross Unrealized Losses	(1,468)	(3,035)
Allowance for credit losses	(167)	0
Estimated Fair Value	1,150,885	843,165
U. S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	181,087	180,228
Gross Unrealized Gains	1,461	621
Gross Unrealized Losses	(627)	(1,055)
Estimated Fair Value	181,921	179,794
Residential mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	811,328	541,490
Gross Unrealized Gains	14,506	4,337
Gross Unrealized Losses	(833)	(1,975)
Estimated Fair Value	825,001	543,852
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	102,259	71,902
Gross Unrealized Gains	5,872	2,034
Gross Unrealized Losses	0	(5)
Allowance for credit losses	(18)
Estimated Fair Value	108,113	73,931
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,383	10,530
Gross Unrealized Gains	1,624	203
Gross Unrealized Losses	(8)
Allowance for credit losses	(149)
Estimated Fair Value	$ 35,850	10,733
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		34,844
Gross Unrealized Gains		11
Estimated Fair Value		$ 34,855

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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