Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,070,350	$ 1,986,957
Allowance for credit losses	0	0
Gross Unrealized Gains	6,311	10,162
Gross Unrealized Losses	(4,352)	(485)
Estimated Fair Value	2,072,309	1,996,634
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	373,019	308,913
Allowance for credit losses	0	0
Gross Unrealized Gains	2,892	6,095
Gross Unrealized Losses	(3,762)	(134)
Estimated Fair Value	372,149	314,874
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,375	12,357
Allowance for credit losses	0	0
Gross Unrealized Gains	107	210
Gross Unrealized Losses	0	0
Estimated Fair Value	8,482	12,567
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,471,892	1,596,890
Allowance for credit losses	0	0
Gross Unrealized Gains	2,003	3,395
Gross Unrealized Losses	(264)	0
Estimated Fair Value	1,473,631	1,600,285
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	217,064	68,797
Allowance for credit losses	0	0
Gross Unrealized Gains	1,309	462
Gross Unrealized Losses	(326)	(351)
Estimated Fair Value	$ 218,047	$ 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 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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