Investments - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,116,279	$ 1,986,957
Allowance for credit losses	0	0
Gross Unrealized Gains	6,922	10,162
Gross Unrealized Losses	(7,316)	(485)
Estimated Fair Value	2,115,885	1,996,634
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	348,080	308,913
Allowance for credit losses	0	0
Gross Unrealized Gains	3,492	6,095
Gross Unrealized Losses	(6,545)	(134)
Estimated Fair Value	345,027	314,874
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,375	12,357
Allowance for credit losses	0	0
Gross Unrealized Gains	80	210
Gross Unrealized Losses	(15)	0
Estimated Fair Value	8,440	12,567
U.S. government-sponsored enterprises and Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,560,848	1,596,890
Allowance for credit losses	0	0
Gross Unrealized Gains	2,581	3,395
Gross Unrealized Losses	(146)	0
Estimated Fair Value	1,563,283	1,600,285
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	198,976	68,797
Allowance for credit losses	0	0
Gross Unrealized Gains	769	462
Gross Unrealized Losses	(610)	(351)
Estimated Fair Value	$ 199,135	$ 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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