Investments - Revenue and General Obligation Bonds by Location (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
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
General Obligation Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	167,649	156,039
Fair Market Value	174,337	161,587
General Obligation Bonds [Member] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	65,619	59,439
Fair Market Value	68,182	61,519
General Obligation Bonds [Member] | California [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,445	23,392
Fair Market Value	24,579	24,313
General Obligation Bonds [Member] | Washington [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,873	23,882
Fair Market Value	24,996	25,030
General Obligation Bonds [Member] | Other [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,712	49,326
Fair Market Value	56,580	50,725
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] | Texas [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,068	6,035
Fair Market Value	7,320	6,298
Revenue Bonds [Member] | Utah [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,787	1,737
Fair Market Value	2,927	1,856
Revenue Bonds [Member] | Indiana [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	364	365
Fair Market Value	379	380
Revenue Bonds [Member] | Other [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,697	17,724
Fair Market Value	$ 18,203	$ 18,144