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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,487,749	$ 2,554,332
Allowance for credit losses	0	0
Gross Unrealized Gains	396	35
Gross Unrealized Losses	(177,083)	(212,278)
Estimated Fair Value	2,311,062	2,342,089
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	387,158	389,067
Allowance for credit losses	0	0
Gross Unrealized Gains	62	2
Gross Unrealized Losses	(61,886)	(68,705)
Estimated Fair Value	325,334	320,364
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,460	3,584
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(294)	(357)
Estimated Fair Value	3,166	3,227
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,758,032	1,804,726
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(92,548)	(115,416)
Estimated Fair Value	1,665,484	1,689,310
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	339,099	356,955
Allowance for credit losses	0	0
Gross Unrealized Gains	334	33
Gross Unrealized Losses	(22,355)	(27,800)
Estimated Fair Value	$ 317,078	$ 329,188

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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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