Investments - Revenue Bonds by Type (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 591,543	$ 569,942
Fair Market Value	599,906	560,479
Lease [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,888	2,053
Fair Market Value	1,917	2,068
States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	167,213	140,181
Fair Market Value	173,658	140,534
Revenue Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,962	23,890
Fair Market Value	22,791	23,901
Revenue Bonds [Member] | Water [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,052	6,942
Fair Market Value	7,334	6,946
Revenue Bonds [Member] | Sales Tax [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,954	2,932
Fair Market Value	4,123	2,901
Revenue Bonds [Member] | College & University [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,007	2,583
Fair Market Value	3,134	2,604
Revenue Bonds [Member] | Sewer [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,190	1,392
Fair Market Value	1,225	1,398
Revenue Bonds [Member] | Other (13 sources) [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,871	7,988
Fair Market Value	$ 5,058	$ 7,984