Investments - Summary of Amortized Cost and Fair Values of General Obligation and Revenue Bonds (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	$ 566,930	$ 569,942
Fair Market Value	563,628	560,479
States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	143,483	140,181
Fair Market Value	145,712	140,534
General Obligation Bonds [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	120,006	116,291
Fair Market Value	121,961	116,633
General Obligation Bonds [Member] | Texas [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	38,428	36,331
Fair Market Value	38,882	36,199
General Obligation Bonds [Member] | California [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	26,741	26,928
Fair Market Value	27,465	27,357
General Obligation Bonds [Member] | Washington [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	16,003	16,036
Fair Market Value	16,306	16,062
General Obligation Bonds [Member] | Ohio [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	8,903	8,639
Fair Market Value	8,944	8,601
General Obligation Bonds [Member] | Illinois [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	7,391	6,827
Fair Market Value	7,510	6,838
General Obligation Bonds [Member] | Other [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	22,540	21,530
Fair Market Value	22,854	21,576
Revenue Bonds [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	23,477	23,890
Fair Market Value	23,751	23,901
Revenue Bonds [Member] | Water [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	7,635	6,942
Fair Market Value	7,679	6,946
Revenue Bonds [Member] | Sales Tax [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	4,888	2,932
Fair Market Value	4,992	2,901
Revenue Bonds [Member] | College & University [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	1,669	2,583
Fair Market Value	1,695	2,604
Revenue Bonds [Member] | Lease [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	1,839	2,053
Fair Market Value	1,838	2,068
Revenue Bonds [Member] | Sewer [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	1,429	1,392
Fair Market Value	1,447	1,398
Revenue Bonds [Member] | Other (13 sources) [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	6,017	7,988
Fair Market Value	6,100	7,984
Revenue Bonds [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	23,477	23,890
Fair Market Value	23,751	23,901
Revenue Bonds [Member] | Texas [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	7,507	7,526
Fair Market Value	7,572	7,506
Revenue Bonds [Member] | Washington [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	1,748	1,751
Fair Market Value	1,811	1,780
Revenue Bonds [Member] | Other [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	4,896	5,267
Fair Market Value	4,965	5,293
Revenue Bonds [Member] | Utah [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	5,356	5,364
Fair Market Value	5,389	5,353
Revenue Bonds [Member] | Indiana [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	2,635	2,641
Fair Market Value	2,692	2,654
Revenue Bonds [Member] | Virginia [Member] | States and Political Subdivisions [Member]
Amortized Cost And Estimated Fair Value Of Investment Securities Available For Sale By Type Of Bonds [Line Items]
Amortized Cost	1,335	1,341
Fair Market Value	$ 1,322	$ 1,315