Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,506,087	$ 1,986,957
Allowance for credit losses	0	0
Gross Unrealized Gains	6,171	10,162
Gross Unrealized Losses	(6,886)	(485)
Estimated Fair Value	2,505,372	1,996,634
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	368,545	308,913
Allowance for credit losses	0	0
Gross Unrealized Gains	2,636	6,095
Gross Unrealized Losses	(5,408)	(134)
Estimated Fair Value	365,773	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	108	210
Gross Unrealized Losses	0	0
Estimated Fair Value	7,051	12,567
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,925,622	1,596,890
Allowance for credit losses	0	0
Gross Unrealized Gains	1,952	3,395
Gross Unrealized Losses	(1,125)	0
Estimated Fair Value	1,926,449	1,600,285
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	204,977	68,797
Allowance for credit losses	0	0
Gross Unrealized Gains	1,475	462
Gross Unrealized Losses	(353)	(351)
Estimated Fair Value	$ 206,099	$ 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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