Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Earned premiums (affiliated net assumed $118.1 and $117.6, respectively, and 232.9 and 231.0, respectively)	$ 320.1	$ 322.4	$ 637.4	$ 642.3
Net investment income (affiliate $1.2 and $1.2, respectively, and 2.4 and 2.4, respectively)	19.1	19.1	37.8	36.5
Net realized gains on investments:
Total other-than-temporary impairment losses	(0.8)	(0.6)	(2.9)	(4.3)
Other net realized investment gains	16.6	7.1	26.5	12.1
Total net realized gains on investments	15.8	6.5	23.6	7.8
Other income from affiliates	0.7	0.5	1.2	1.1
Total revenues	355.7	348.5	700.0	687.7
Losses and loss expenses (affiliated net assumed $83.4 and $86.8, respectively, and 169.6 and 180.5, respectively)	230.4	262.9	464.4	487.9
Acquisition and operating expenses (affiliated net assumed $71.8 and $68.4, respectively, and 152.1 and 138.9, respectively)	109.9	107.2	222.1	214.7
Interest expense (affiliates $0.2 and $0.2, respectively, and 0.4 and 0.4, respectively)	1.5	1.4	2.9	2.7
Other expenses	2.0	2.1	3.9	3.7
Costs and Expenses	343.8	373.6	693.3	709.0
Income before federal income taxes	11.9	(25.1)	6.7	(21.3)
Current Income Tax Expense (Benefit)	0.2	0.0	0.1	0.0
Deferred Income Tax Expense (Benefit)	3.0	(0.5)	2.0	0.3
Federal income tax expense	3.2	(0.5)	2.1	0.3
Net income	$ 8.7	$ (24.6)	$ 4.6	$ (21.6)
Earnings (loss) per common share:
Basic (usd per share)	$ 0.21	$ (0.59)	$ 0.11	$ (0.52)
Diluted (usd per share)	0.21	(0.59)	0.11	(0.52)
Dividends paid per common share (usd per share)	$ 0.10	$ 0.10	$ 0.20	$ 0.20