Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,563,816	$ 2,554,332
Allowance for credit losses	0	0
Gross Unrealized Gains	130	35
Gross Unrealized Losses	(191,100)	(212,278)
Estimated Fair Value	2,372,846	2,342,089
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	419,251	389,067
Allowance for credit losses	0	0
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(66,340)	(68,705)
Estimated Fair Value	352,912	320,364
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,460	3,584
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(405)	(357)
Estimated Fair Value	3,055	3,227
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,708,890	1,804,726
Allowance for credit losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(103,082)	(115,416)
Estimated Fair Value	1,605,808	1,689,310
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	432,215	356,955
Allowance for credit losses	0	0
Gross Unrealized Gains	129	33
Gross Unrealized Losses	(21,273)	(27,800)
Estimated Fair Value	$ 411,071	$ 329,188