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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,013,021	$ 485,756
Allowance for credit losses	0	0
Gross Unrealized Gains	10,129	2,675
Gross Unrealized Losses	(2,202)	(762)
Estimated Fair Value	2,020,948	487,669
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	312,549	215,888
Allowance for credit losses	0	0
Gross Unrealized Gains	5,832	1,895
Gross Unrealized Losses	(378)	(658)
Estimated Fair Value	318,003	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	224	145
Gross Unrealized Losses	0	(83)
Estimated Fair Value	16,155	19,536
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,608,837	250,394
Allowance for credit losses	0	0
Gross Unrealized Gains	4,073	635
Gross Unrealized Losses	(6)	(21)
Estimated Fair Value	1,612,904	$ 251,008
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	75,704
Allowance for credit losses	0
Gross Unrealized Gains	0
Gross Unrealized Losses	(1,818)
Estimated Fair Value	$ 73,886

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