Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,423,183	$ 2,563,984
Allowance for credit losses	0	0
Gross Unrealized Gains	2,738	1,306
Gross Unrealized Losses	(154,813)	(153,668)
Estimated Fair Value	2,271,108	2,411,622
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	485,026	468,204
Allowance for credit losses	0	0
Gross Unrealized Gains	353	703
Gross Unrealized Losses	(71,171)	(62,480)
Estimated Fair Value	414,208	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	(292)	(279)
Estimated Fair Value	2,909	3,129
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,396,341	1,645,609
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(63,613)	(66,870)
Estimated Fair Value	1,332,728	1,578,739
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	538,615	446,763
Allowance for credit losses	0	0
Gross Unrealized Gains	2,385	603
Gross Unrealized Losses	(19,737)	(24,039)
Estimated Fair Value	$ 521,263	$ 423,327