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Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,428,037	$ 2,563,984
Allowance for credit losses	0	0
Gross Unrealized Gains	3,928	1,306
Gross Unrealized Losses	(148,703)	(153,668)
Estimated Fair Value	2,283,262	2,411,622
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	509,861	468,204
Allowance for credit losses	0	0
Gross Unrealized Gains	206	703
Gross Unrealized Losses	(73,956)	(62,480)
Estimated Fair Value	436,111	406,427
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,201	3,408
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(320)	(279)
Estimated Fair Value	2,881	3,129
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,296,942	1,645,609
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(55,211)	(66,870)
Estimated Fair Value	1,241,731	1,578,739
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	618,033	446,763
Allowance for credit losses	0	0
Gross Unrealized Gains	3,722	603
Gross Unrealized Losses	(19,216)	(24,039)
Estimated Fair Value	$ 602,539	$ 423,327

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