Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 758,713	$ 713,291
Other investments:
Contributions made to the funds	410	578	$ 928
Distributions received from the funds	10,768	16,113	$ 26,735
Fair value measurements on a recurring basis
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	758,713	713,291
Total	763,529	717,703
Fair value measurements on a recurring basis | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	745,961	707,341
Fair value measurements on a recurring basis | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,734	5,031
Fair value measurements on a recurring basis | Government sponsored entities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		2,026
Fair value measurements on a recurring basis | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	259,264	253,132
Fair value measurements on a recurring basis | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	503
Fair value measurements on a recurring basis | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	346,523	322,948
Fair value measurements on a recurring basis | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	25,571	16,102
Fair value measurements on a recurring basis | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	32,804	36,849
Fair value measurements on a recurring basis | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	58,034	69,253
Fair value measurements on a recurring basis | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,528	2,000
Fair value measurements on a recurring basis | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	12,752
Fair value measurements on a recurring basis | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		5,950
Fair value measurements on a recurring basis | Real estate
Consolidated fair value measurements on a recurring basis by asset class and level of input
Other investments	4,816	4,412
Fair value measurements on a recurring basis | Level 1
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2,015	5,950
Total	2,015	5,950
Fair value measurements on a recurring basis | Level 1 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Government sponsored entities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		0
Fair value measurements on a recurring basis | Level 1 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 1 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 1 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2,015
Fair value measurements on a recurring basis | Level 1 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		5,950
Fair value measurements on a recurring basis | Level 2
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	746,619	697,989
Total	746,619	697,989
Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	735,882	697,989
Fair value measurements on a recurring basis | Level 2 | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,734	5,031
Fair value measurements on a recurring basis | Level 2 | Government sponsored entities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		2,026
Fair value measurements on a recurring basis | Level 2 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	259,264	253,132
Fair value measurements on a recurring basis | Level 2 | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	503
Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	338,644	313,596
Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	25,571	16,102
Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	32,804	36,849
Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	55,834	69,253
Fair value measurements on a recurring basis | Level 2 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,528	2,000
Fair value measurements on a recurring basis | Level 2 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10,737
Fair value measurements on a recurring basis | Level 2 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		0
Fair value measurements on a recurring basis | Level 3
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10,079	9,352
Total	10,079	9,352
Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	10,079	9,352
Fair value measurements on a recurring basis | Level 3 | U.S. treasury
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 3 | Government sponsored entities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		0
Fair value measurements on a recurring basis | Level 3 | States & political subdivisions
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 3 | Foreign government securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Corporate debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,879	9,352
Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 3 | Commercial mortgage-backed securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 3 | Collateralized debt obligations
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	2,200	0
Fair value measurements on a recurring basis | Level 3 | Other debt securities
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0	0
Fair value measurements on a recurring basis | Level 3 | Nonredeemable preferred stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	0
Fair value measurements on a recurring basis | Level 3 | Common stock
Consolidated fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities		0
Fair value measurements on a recurring basis | Level 3 | Real estate
Other investments:
Contributions made to the funds	0	0
Distributions received from the funds	500	900
Unfunded commitments	$ 0	$ 300
Fair value measurements on a recurring basis | Level 3 | Real estate | Minimum
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	5 years	5 years
Fair value measurements on a recurring basis | Level 3 | Real estate | Maximum
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	10 years	10 years