Investments - Revenue Bonds by Type (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,050,172	$ 957,459
Fair Market Value	1,025,032	973,314
Tax revenue [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,205
Fair Market Value	1,986
States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	294,077	290,729
Fair Market Value	286,380	304,268
Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	38,259	42,697
Fair Market Value	36,863	44,586
Revenue Bonds [Member] | Water [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,145	15,534
Fair Market Value	13,581	16,220
Revenue Bonds [Member] | Sewer [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,037	3,932
Fair Market Value	4,922	4,165
Revenue Bonds [Member] | Sales Tax [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,106	5,514
Fair Market Value	2,035	5,842
Revenue Bonds [Member] | Lease [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,321	6,556
Fair Market Value	3,263	6,718
Revenue Bonds [Member] | Other Revenue Source [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,649	11,161
Fair Market Value	$ 13,062	$ 11,641