Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,535,568	$ 1,986,957
Allowance for credit losses	0	0
Gross Unrealized Gains	2,917	10,162
Gross Unrealized Losses	(20,529)	(485)
Available-for-sale investments	2,517,956	1,996,634
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	376,313	308,913
Allowance for credit losses	0	0
Gross Unrealized Gains	1,857	6,095
Gross Unrealized Losses	(7,073)	(134)
Available-for-sale investments	371,097	314,874
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,943	12,357
Allowance for credit losses	0	0
Gross Unrealized Gains	18	210
Gross Unrealized Losses	0	0
Available-for-sale investments	6,961	12,567
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,958,943	1,596,890
Allowance for credit losses	0	0
Gross Unrealized Gains	603	3,395
Gross Unrealized Losses	(11,893)	0
Available-for-sale investments	1,947,653	1,600,285
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	193,369	68,797
Allowance for credit losses	0	0
Gross Unrealized Gains	439	462
Gross Unrealized Losses	(1,563)	(351)
Available-for-sale investments	$ 192,245	$ 68,908

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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