Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,113,257	$ 2,563,984
Allowance for credit losses	0	0
Gross Unrealized Gains	8,688	1,306
Gross Unrealized Losses	(99,340)	(153,668)
Estimated Fair Value	2,022,605	2,411,622
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	515,923	468,204
Allowance for credit losses	0	0
Gross Unrealized Gains	1,664	703
Gross Unrealized Losses	(54,875)	(62,480)
Estimated Fair Value	462,712	406,427
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,849	3,408
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(318)	(279)
Estimated Fair Value	2,531	3,129
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	997,530	1,645,609
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(31,323)	(66,870)
Estimated Fair Value	966,207	1,578,739
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	596,955	446,763
Allowance for credit losses	0	0
Gross Unrealized Gains	7,024	603
Gross Unrealized Losses	(12,824)	(24,039)
Estimated Fair Value	$ 591,155	$ 423,327