Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 719,869		$ 713,291
Other investments:
Contributions made to the funds	111	$ 103
Distributions received from the funds	3,396	$ 1,792
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	719,869		713,291
Total	724,245		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	713,877		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	5,038		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	2,004		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	255,128		253,132
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	326,169		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	14,799		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	37,905		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	70,832		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	2,002		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	5,992		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	4,376		4,412
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	5,992		5,950
Total	5,992		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		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 | 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	5,992		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	704,074		697,989
Total	704,074		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	704,074		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	5,038		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	2,004		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	255,128		253,132
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	316,366		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	14,799		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	37,905		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	70,832		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	2,002		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		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	9,803		9,352
Total	9,803		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	9,803		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		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 | Corporate debt securities
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	9,803		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		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	0		900
Unfunded commitments	$ 0		$ 300
Minimum | Fair value measurements on a recurring basis | Level 3 | Real estate
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	5 years		5 years
Maximum | Fair value measurements on a recurring basis | Level 3 | Real estate
Other investments:
Underlying assets, liquidation period from inception of fund (in years)	10 years		10 years