Securities - Amortized Cost and Estimated Fair Value of Securities - Securities Available-for-sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Securities available-for-sale:
Amortized Cost	$ 230,012	$ 402,537
Gross Unrealized Gains	5,762	2,459
Gross Unrealized (Losses)		(171)
Estimated Fair Value	235,774	404,825
Agency mortgage-backed securities
Securities available-for-sale:
Amortized Cost	170,215	283,598
Gross Unrealized Gains	5,111	934
Gross Unrealized (Losses)		(171)
Estimated Fair Value	175,326	284,361
U.S. Treasury
Securities available-for-sale:
Amortized Cost	59,797	118,939
Gross Unrealized Gains	651	1,525
Estimated Fair Value	$ 60,448	$ 120,464

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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Amount, before tax, of unrealized loss 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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