Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available for sale:
Amortized Cost	$ 1,986,957	$ 485,756
Gross Unrealized Gains	10,162	2,675
Gross Unrealized Losses	(485)	(762)
Estimated Fair Value	1,996,634	487,669
Mortgage-backed securities
Available for sale:
Amortized Cost	308,913	215,888
Gross Unrealized Gains	6,095	1,895
Gross Unrealized Losses	(134)	(658)
Estimated Fair Value	314,874	217,125
Utah Housing Corporation bonds
Available for sale:
Amortized Cost	12,357	19,474
Gross Unrealized Gains	210	145
Gross Unrealized Losses	0	(83)
Estimated Fair Value	12,567	19,536
U.S. government-sponsored enterprises and Treasuries
Available for sale:
Amortized Cost	1,596,890	250,394
Gross Unrealized Gains	3,395	635
Gross Unrealized Losses	0	(21)
Estimated Fair Value	1,600,285	$ 251,008
Other securities
Available for sale:
Amortized Cost	68,797
Gross Unrealized Gains	462
Gross Unrealized Losses	(351)
Estimated Fair Value	$ 68,908

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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Amount of 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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