Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,956,353	$ 1,812,408
Allowance for credit losses	0	0
Gross Unrealized Gains	5,199	17,389
Gross Unrealized Losses	(114,443)	(71,727)
Estimated Fair Value	2,847,109	1,758,070
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	598,069	545,550
Allowance for credit losses	0	0
Gross Unrealized Gains	9	3,045
Gross Unrealized Losses	(86,181)	(54,371)
Estimated Fair Value	511,897	494,224
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,207	2,490
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(346)	(334)
Estimated Fair Value	1,861	2,156
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,639,028	649,087
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13,967)	(9,581)
Estimated Fair Value	1,625,061	639,506
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	717,049	615,281
Allowance for credit losses	0	0
Gross Unrealized Gains	5,190	14,344
Gross Unrealized Losses	(13,949)	(7,441)
Estimated Fair Value	$ 708,290	$ 622,184

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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