Investments - Revenue Bonds by Type (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 600,832	$ 592,378
Fair Market Value	620,154	600,799
Lease [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,945	1,949
Fair Market Value	1,988	1,995
Electricity [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,419	1,421
Fair Market Value	1,478	1,521
States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	195,565	181,900
Fair Market Value	203,166	188,265
Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,916	25,861
Fair Market Value	28,829	26,678
Revenue Bonds [Member] | Water [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,499	7,515
Fair Market Value	7,774	7,775
Revenue Bonds [Member] | College & University [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,993	1,997
Fair Market Value	2,031	2,019
Revenue Bonds [Member] | Sales Tax [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,752	4,760
Fair Market Value	4,854	4,811
Revenue Bonds [Member] | Sewer [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,639	3,596
Fair Market Value	4,751	3,678
Revenue Bonds [Member] | Other Revenue Source [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,669	4,623
Fair Market Value	$ 5,953	$ 4,879