Securities - Amortized Cost and Estimated Fair Value of Securities - Securities Available-for-sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Securities available-for-sale:
Amortized Cost	$ 364,160	$ 402,537
Gross Unrealized Gains	9,410	2,459
Gross Unrealized (Losses)		(171)
Allowances for Credit Losses		404,825
Estimated Fair Value	373,570	404,825
Agency mortgage-backed securities
Securities available-for-sale:
Amortized Cost	244,968	283,598
Gross Unrealized Gains	6,868	934
Gross Unrealized (Losses)		(171)
Allowances for Credit Losses		284,361
Estimated Fair Value	251,836	284,361
U.S. Treasury
Securities available-for-sale:
Amortized Cost	119,192	118,939
Gross Unrealized Gains	2,542	1,525
Allowances for Credit Losses		120,464
Estimated Fair Value	$ 121,734	$ 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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