Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 749,266		$ 713,291
Other investments:
Contributions made to the funds	330	$ 449
Distributions received from the funds	7,986	$ 12,404
Fair value measurements on a recurring basis
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	749,266		713,291
Total	753,604		717,703
Fair value measurements on a recurring basis | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	749,266		707,341
Fair value measurements on a recurring basis | U.S. treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,834		5,031
Fair value measurements on a recurring basis | Government sponsored entities
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	261,966		253,132
Fair value measurements on a recurring basis | Foreign government securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	507
Fair value measurements on a recurring basis | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	346,372		322,948
Fair value measurements on a recurring basis | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	25,719		16,102
Fair value measurements on a recurring basis | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	34,331		36,849
Fair value measurements on a recurring basis | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	62,381		69,253
Fair value measurements on a recurring basis | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	6,156		2,000
Fair value measurements on a recurring basis | Common stock
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
Fair value measurements on a recurring basis by asset class and level of input
Other investments	4,338		4,412
Other investments:
Contributions made to the funds	0		0
Distributions received from the funds	300		900
Unfunded commitments	$ 0		$ 300
Fair value measurements on a recurring basis | 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 | Real estate | Maximum
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	10 years		10 years
Fair value measurements on a recurring basis | Level 1
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 0		$ 5,950
Total	0		5,950
Fair value measurements on a recurring basis | Level 1 | Fixed maturities
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
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
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
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
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
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
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
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
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
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 | Common stock
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	741,351		697,989
Total	741,351		697,989
Fair value measurements on a recurring basis | Level 2 | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	741,351		697,989
Fair value measurements on a recurring basis | Level 2 | U.S. treasury
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	11,834		5,031
Fair value measurements on a recurring basis | Level 2 | Government sponsored entities
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	261,966		253,132
Fair value measurements on a recurring basis | Level 2 | Foreign government securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	507
Fair value measurements on a recurring basis | Level 2 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	338,457		313,596
Fair value measurements on a recurring basis | Level 2 | Residential mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	25,719		16,102
Fair value measurements on a recurring basis | Level 2 | Commercial mortgage-backed securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	34,331		36,849
Fair value measurements on a recurring basis | Level 2 | Collateralized debt obligations
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	62,381		69,253
Fair value measurements on a recurring basis | Level 2 | Other debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	6,156		2,000
Fair value measurements on a recurring basis | Level 2 | Common stock
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,915		9,352
Total	7,915		9,352
Fair value measurements on a recurring basis | Level 3 | Fixed maturities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,915		9,352
Fair value measurements on a recurring basis | Level 3 | U.S. treasury
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
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
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
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
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	7,915		9,352
Fair value measurements on a recurring basis | Level 3 | Residential mortgage-backed securities
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
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
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 | Other debt securities
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 | Common stock
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities			$ 0