Investments - Revenue and General Obligation Bonds by Location (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
General Obligation Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	180,923	156,039
Fair Market Value	191,750	161,587
General Obligation Bonds [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	76,494	59,439
Fair Market Value	81,128	61,519
General Obligation Bonds [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,447	23,882
Fair Market Value	29,067	25,030
General Obligation Bonds [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,330	23,392
Fair Market Value	25,023	24,313
General Obligation Bonds [Member] | Other [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,652	49,326
Fair Market Value	56,532	50,725
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] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,053	6,035
Fair Market Value	7,452	6,298
Revenue Bonds [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	364	365
Fair Market Value	382	380
Revenue Bonds [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,256	1,737
Fair Market Value	2,377	1,856
Revenue Bonds [Member] | Other [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,164	17,724
Fair Market Value	$ 20,037	$ 18,144