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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,841,143	$ 2,042,473
Allowance for credit losses	0	0
Gross Unrealized Gains	14,581	8,838
Gross Unrealized Losses	(79,940)	(118,085)
Estimated Fair Value	1,775,784	1,933,226
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	550,645	516,358
Allowance for credit losses	0	0
Gross Unrealized Gains	1,796	205
Gross Unrealized Losses	(56,821)	(73,235)
Estimated Fair Value	495,620	443,328
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,490	2,849
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(350)	(490)
Estimated Fair Value	2,140	2,359
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	648,861	948,009
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(14,283)	(32,265)
Estimated Fair Value	634,578	915,744
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	639,147	575,257
Allowance for credit losses	0	0
Gross Unrealized Gains	12,785	8,633
Gross Unrealized Losses	(8,486)	(12,095)
Estimated Fair Value	$ 643,446	$ 571,795

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