Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,563,984	$ 2,554,332
Allowance for credit losses	0	0
Gross Unrealized Gains	1,306	35
Gross Unrealized Losses	(153,668)	(212,278)
Available-for-sale investments	2,411,622	2,342,089
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	468,204	389,067
Allowance for credit losses	0	0
Gross Unrealized Gains	703	2
Gross Unrealized Losses	(62,480)	(68,705)
Available-for-sale investments	406,427	320,364
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,408	3,584
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(279)	(357)
Available-for-sale investments	3,129	3,227
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,645,609	1,804,726
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(66,870)	(115,416)
Available-for-sale investments	1,578,739	1,689,310
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	446,763	356,955
Allowance for credit losses	0	0
Gross Unrealized Gains	603	33
Gross Unrealized Losses	(24,039)	(27,800)
Available-for-sale investments	$ 423,327	$ 329,188