Schedule II. Condensed Financial Statements of Parent Company (Condensed Statement of Income and Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Investment income									$ 51,190	$ 55,284	$ 46,559
Total revenues	$ 273,355	$ 269,617	$ 258,487	$ 251,278	$ 264,537	$ 255,625	$ 244,866	$ 231,334	1,052,737	996,362	899,271
Expenses
Net income									51,023	49,904	89,126
Other comprehensive income (loss)
Change in unrealized appreciation on investments held by subsidiary									72,251	13,017	(28,185)
Change in liability for underfunded employee benefit plans									(26,122)	30,045	8,714
Other comprehensive income (loss), before tax and reclassification adjustments									46,129	43,062	(19,471)
Income tax effect									(17,540)	(15,072)	6,814
Other comprehensive income (loss), after tax, before reclassification adjustments									28,589	27,990	(12,657)
Reclassification adjustment for net realized gains included in income									(6,390)	(4,520)	(4,513)
Reclassification adjustment for employee benefit costs included in expense									5,408	5,486	7,468
Total reclassification adjustments, before tax									(982)	966	2,955
Income tax effect									344	(338)	(1,034)
Total reclassification adjustments, after tax									(638)	628	1,921
Comprehensive income									78,974	78,522	78,390
United Fire Group, Inc.
Revenues
Investment income									133	66	25
Total revenues									133	66	25
Expenses
Other operating expenses									103	73	116
Total expenses									103	73	116
Income (loss) before income taxes and equity in net income of subsidiary									30	(7)	(91)
Federal income tax benefit									(1,060)	(3)	(42)
Net income (loss) before equity in net income of subsidiary									1,090	(4)	(49)
Equity in net income of subsidiary									49,933	49,908	89,175
Net income									51,023	49,904	89,126
Other comprehensive income (loss)
Change in unrealized appreciation on investments held by subsidiary									72,251	13,017	(28,185)
Change in liability for underfunded employee benefit plans									(26,122)	30,045	8,714
Other comprehensive income (loss), before tax and reclassification adjustments									46,129	43,062	(19,471)
Income tax effect									(17,540)	(15,072)	6,814
Other comprehensive income (loss), after tax, before reclassification adjustments									28,589	27,990	(12,657)
Reclassification adjustment for net realized gains included in income									(6,390)	(4,520)	(4,513)
Reclassification adjustment for employee benefit costs included in expense									5,408	5,486	7,468
Total reclassification adjustments, before tax									(982)	966	2,955
Income tax effect									344	(338)	(1,034)
Total reclassification adjustments, after tax									(638)	628	1,921
Comprehensive income									$ 78,974	$ 78,522	$ 78,390