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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,081,236	$ 485,756
Allowance for credit losses	0	0
Gross Unrealized Gains	9,924	2,675
Gross Unrealized Losses	(453)	(762)
Estimated Fair Value	2,090,707	487,669
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	308,757	215,888
Allowance for credit losses	0	0
Gross Unrealized Gains	5,670	1,895
Gross Unrealized Losses	(100)	(658)
Estimated Fair Value	314,327	217,125
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,357	19,474
Allowance for credit losses	0	0
Gross Unrealized Gains	170	145
Gross Unrealized Losses	0	(83)
Estimated Fair Value	12,527	19,536
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,687,856	250,394
Allowance for credit losses	0	0
Gross Unrealized Gains	3,688	635
Gross Unrealized Losses	(10)	(21)
Estimated Fair Value	1,691,534	$ 251,008
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,266
Allowance for credit losses	0
Gross Unrealized Gains	396
Gross Unrealized Losses	(343)
Estimated Fair Value	$ 72,319

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