Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Fair value measurements on a recurring basis by asset class and level of input
Available-for-sale securities	$ 615,213		$ 599,941
Other investments:
Contributions made to the funds	103	$ 89
Distributions received from the funds	1,792	$ 9,157
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	615,213		599,941
Total	622,470		604,467
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	606,281		587,209
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	242,524		231,847
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	256,498		250,333
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	14,875		13,513
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	35,596		37,571
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	54,770		51,745
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	2,018		2,200
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	8,932		12,732
Trading securities	2,955
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	4,302		4,526
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	8,932		12,732
Total	11,887		12,732
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 | 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	8,932		12,732
Trading securities	2,955
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	589,423		578,563
Total	589,423		578,563
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	589,423		578,563
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	242,524		231,847
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	251,677		250,264
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	14,875		13,513
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	35,596		37,571
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	42,733		43,168
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	2,018		2,200
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
Trading 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	16,858		8,646
Total	16,858		8,646
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	16,858		8,646
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	4,821		69
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	12,037		8,577
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
Trading 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	300		3,500
Unfunded commitments	$ 300		$ 600
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