Consolidated Statements of Income and Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Net premiums earned	$ 1,023,401,000	$ 930,890,000	$ 828,330,000
Investment income, net of investment expenses	106,822,000	100,781,000	104,609,000
Net realized investment gains (losses)
Other-than-temporary impairment charges	0	(1,300,000)	0
All other net realized gains (includes reclassifications for net unrealized gains on available-for-sale securities of $4,520 in 2016; $4,513 in 2015; and $5,085 in 2014 previously included in accumulated other comprehensive income)	6,103,000	4,146,000	7,270,000
Total net realized investment gains	6,103,000	2,846,000	7,270,000
Other income	621,000	401,000	1,685,000
Total revenues	1,136,947,000	1,034,918,000	941,894,000
Benefits, losses and expenses
Losses and loss settlement expenses	683,798,000	549,088,000	536,243,000
Increase in liability for future policy benefits	59,969,000	50,945,000	36,623,000
Amortization of deferred policy acquisition costs	211,013,000	186,817,000	167,449,000
Other underwriting expenses (includes reclassifications for employee benefit costs of $5,486 in 2016; $7,468 in 2015; and $3,072 in 2014 previously included in accumulated other comprehensive income)	103,421,000	102,937,000	94,871,000
Interest on policyholders' accounts	20,079,000	23,680,000	30,245,000
Total benefits, losses and expenses	1,078,280,000	913,467,000	865,431,000
Income before income taxes	58,667,000	121,451,000	76,463,000
Federal income tax expense (includes reclassifications of $338 in 2016; $1,034 in 2015; and ($704) in 2014 previously included in accumulated other comprehensive income)	8,763,000	32,325,000	17,326,000
Net income	49,904,000	89,126,000	59,137,000
Other comprehensive income (loss)
Change in net unrealized appreciation on investments	13,017,000	(28,185,000)	55,888,000
Change in liability for underfunded employee benefit plans	30,045,000	8,714,000	(47,685,000)
Other comprehensive income (loss), before tax and reclassification adjustments	43,062,000	(19,471,000)	8,203,000
Income tax effect	(15,072,000)	6,814,000	(2,871,000)
Other comprehensive income (loss), after tax, before reclassification adjustments	27,990,000	(12,657,000)	5,332,000
Reclassification adjustment for net realized gains included in income	(4,520,000)	(4,513,000)	(5,085,000)
Reclassification adjustment for employee benefit costs included in expense	5,486,000	7,468,000	3,072,000
Total reclassification adjustments, before tax	966,000	2,955,000	(2,013,000)
Income tax effect	(338,000)	(1,034,000)	704,000
Total reclassification adjustments, after tax	628,000	1,921,000	(1,309,000)
Comprehensive income	$ 78,522,000	$ 78,390,000	$ 63,160,000
Weighted average common shares outstanding (in shares)	25,335,706	25,047,405	25,230,854
Basic earnings per common share (in usd per share)	$ 1.97	$ 3.56	$ 2.34
Diluted earnings per common share (in usd per share)	$ 1.93	$ 3.53	$ 2.32