Investments - Revenue Bonds by Type (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 576,363	$ 592,378
Fair Market Value	599,333	600,799
States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	209,760	181,900
Fair Market Value	221,998	188,265
Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,837	25,861
Fair Market Value	30,248	26,678
Revenue Bonds [Member] | Water [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,363	7,515
Fair Market Value	10,949	7,775
Revenue Bonds [Member] | Sewer [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,219	4,760
Fair Market Value	5,408	4,811
Revenue Bonds [Member] | Sales Tax [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,572	1,949
Fair Market Value	3,743	1,995
Revenue Bonds [Member] | Lease [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,773	3,596
Fair Market Value	2,864	3,678
Revenue Bonds [Member] | Charter School Aid [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,565	1,232
Fair Market Value	1,689	1,290
Revenue Bonds [Member] | Other Revenue Source [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,345	6,809
Fair Market Value	$ 5,595	$ 7,129