Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Earned premiums (affiliated net assumed $458.0, $472.0 and $437.6, respectively)	$ 1,275.1	$ 1,291.9	$ 1,270.5
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	78.8	74.7	71.7
Net realized gain on investments:
Total other-than-temporary impairment losses	(3.5)	(6.8)	(7.9)
Portion of loss recognized in other comprehensive income	0.0	0.0	0.0
Other net realized investment gains	68.6	43.3	32.2
Total net realized gain on investments	65.1	36.5	24.3
Other income (affiliates $2.3, $2.3 and $2.1, respectively)	2.3	2.3	2.1
Total revenues	1,421.3	1,405.4	1,368.6
Losses and loss expenses (affiliated net assumed $379.6, $383.8 and $293.3, respectively)	918.3	942.4	862.8
Acquisition and operating expenses (affiliated net assumed $270.7, $285.8 and $248.6, respectively)	455.8	430.4	426.8
Interest expense (affiliates $0.8, $0.8 and $0.7, respectively)	5.9	5.5	5.4
Other expenses	7.9	7.9	6.3
Total expenses	1,387.9	1,386.2	1,301.3
Income before federal income taxes	33.4	19.2	67.3
Current	0.4	(1.7)	2.9
Deferred	43.7	(0.1)	13.2
Total federal income tax expense (benefit)	44.1	(1.8)	16.1
Net (loss) income	$ (10.7)	$ 21.0	$ 51.2
(Loss) earnings per common share:
Basic (usd per share)	$ (0.25)	$ 0.50	$ 1.25
Diluted (usd per share)	(0.25)	0.50	1.23
Dividends paid per common share (usd per share)	$ 0.40	$ 0.40	$ 0.40