Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Debt and Equity Securities, Gain (Loss)	$ 17.8	$ 20.8	$ 18.2	$ 47.3
Earned premiums (affiliated net assumed $71.9 and $115.3, respectively, and $251.1 and $349.0, respectively)	306.8	319.0	929.2	957.2
Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively)	20.8	18.7	62.2	56.5
Net realized gains on investments:
Total other-than-temporary impairment losses	0.0	(0.6)	0.0	(3.5)
Total net realized gains on investments	17.8	20.2	18.2	43.8
Other income from affiliates	0.6	0.5	1.8	1.7
Total revenues	346.0	358.4	1,011.4	1,059.2
Losses and loss expenses (affiliated net assumed $34.7 and $128.9, respectively, and $105.4 and $298.8, respectively)	191.0	253.5	621.1	718.2
Acquisition and operating expenses (affiliated net ceded $26.8 and affiliated net assumed $78.9, respectively, and $52.3 and $231.0, respectively)	110.9	112.7	333.0	334.1
Interest expense (affiliates $0.2 and $0.2, respectively, and $0.7 and $0.6, respectively)	1.2	1.5	4.5	4.4
Other expenses	1.9	1.9	7.4	5.8
Costs and Expenses	305.0	369.6	966.0	1,062.5
Income before federal income taxes	41.0	(11.2)	45.4	(3.3)
Current Income Tax Expense (Benefit)	(0.1)	(0.1)	(1.1)	0.0
Deferred Income Tax Expense (Benefit)	7.7	(1.6)	9.2	0.8
Federal income tax expense	7.6	(1.7)	8.1	0.8
Net income	$ 33.4	$ (9.5)	$ 37.3	$ (4.1)
Earnings (loss) per common share:
Basic (usd per share)	$ 0.78	$ (0.23)	$ 0.87	$ (0.10)
Diluted (usd per share)	0.76	(0.23)	0.86	(0.10)
Dividends paid per common share (usd per share)	$ 0.10	$ 0.10	$ 0.30	$ 0.30