Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Earned premiums (affiliated net assumed $115.3 and $121.4, respectively, and $348.2 and $352.4, respectively)	$ 319.0	$ 325.8	$ 956.4	$ 968.1
Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively)	18.7	17.6	56.5	54.1
Net realized gains on investments:
Total other-than-temporary impairment losses	(0.6)	(1.1)	(3.5)	(5.4)
Other net realized investment gains	20.8	9.9	47.3	22.0
Total net realized gains on investments	20.2	8.8	43.8	16.6
Other income from affiliates	0.5	0.6	1.7	1.7
Total revenues	358.4	352.8	1,058.4	1,040.5
Losses and loss expenses (affiliated net assumed $128.9 and $100.9, respectively, and $298.5 and $281.4, respectively)	253.5	232.3	717.9	720.2
Acquisition and operating expenses (affiliated net assumed $78.9 and $73.9, respectively, and $231.0 and $217.0, respectively)	112.7	109.9	334.8	324.6
Interest expense (affiliates $0.2 and $0.2, respectively, and $0.6 and $0.6, respectively)	1.5	1.4	4.4	4.1
Other expenses	1.9	1.8	5.8	5.5
Costs and Expenses	369.6	345.4	1,062.9	1,054.4
Income before federal income taxes	(11.2)	7.4	(4.5)	(13.9)
Current Income Tax Expense (Benefit)	(0.1)	(1.5)	0.0	(1.5)
Deferred Income Tax Expense (Benefit)	(1.6)	(1.2)	0.4	(0.9)
Federal income tax expense	(1.7)	(2.7)	0.4	(2.4)
Net income	$ (9.5)	$ 10.1	$ (4.9)	$ (11.5)
Loss per common share:
Basic (usd per share)	$ (0.23)	$ 0.24	$ (0.12)	$ (0.28)
Diluted (usd per share)	(0.23)	0.24	(0.12)	(0.28)
Dividends paid per common share (usd per share)	$ 0.10	$ 0.10	$ 0.30	$ 0.30