Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 607,867	$ 485,756
Allowance for credit losses	0	0
Gross Unrealized Gains	9,762	2,675
Gross Unrealized Losses	(3,047)	(762)
Estimated Fair Value	614,582	487,669
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	206,640	215,888
Allowance for credit losses	0	0
Gross Unrealized Gains	6,400	1,895
Gross Unrealized Losses	0	(658)
Estimated Fair Value	213,040	217,125
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,931	19,474
Allowance for credit losses	0	0
Gross Unrealized Gains	156	145
Gross Unrealized Losses	(74)	(83)
Estimated Fair Value	16,013	19,536
U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	306,491	250,394
Allowance for credit losses	0	0
Gross Unrealized Gains	3,206	635
Gross Unrealized Losses	(8)	(21)
Estimated Fair Value	309,689	$ 251,008
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	78,805
Allowance for credit losses	0
Gross Unrealized Gains	0
Gross Unrealized Losses	(2,965)
Estimated Fair Value	$ 75,840

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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