Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,440,204	$ 2,535,568
Allowance for credit losses	0	0
Gross Unrealized Gains	74	2,917
Gross Unrealized Losses	(98,727)	(20,529)
Estimated Fair Value	2,341,551	2,517,956
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	400,647	376,313
Allowance for credit losses	0	0
Gross Unrealized Gains	74	1,857
Gross Unrealized Losses	(29,641)	(7,073)
Estimated Fair Value	371,080	371,097
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,926	6,943
Allowance for credit losses	0	0
Gross Unrealized Gains	0	18
Gross Unrealized Losses	(77)	0
Estimated Fair Value	4,849	6,961
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,799,080	1,958,943
Allowance for credit losses	0	0
Gross Unrealized Gains	0	603
Gross Unrealized Losses	(63,830)	(11,893)
Estimated Fair Value	1,735,250	1,947,653
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	235,551	193,369
Allowance for credit losses	0	0
Gross Unrealized Gains	0	439
Gross Unrealized Losses	(5,179)	(1,563)
Estimated Fair Value	$ 230,372	$ 192,245

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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