Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Earned premiums (affiliated net assumed $71.9 and $115.3, respectively, and $251.1 and $349.0, respectively)	$ 330.5	$ 302.7
Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively)	18.9	19.4
Net realized gains on investments:
Total net realized gains on investments	(135.2)	44.9
Other income from affiliates	0.6	0.6
Total revenues	214.8	367.6
Losses and loss expenses (affiliated net assumed $34.7 and $128.9, respectively, and $105.4 and $298.8, respectively)	239.4	194.3
Acquisition and operating expenses (affiliated net ceded $26.8 and affiliated net assumed $78.9, respectively, and $52.3 and $231.0, respectively)	114.9	107.6
Interest expense (affiliates $0.2 and $0.2, respectively, and $0.7 and $0.6, respectively)	1.2	1.2
Other Expenses	3.3	2.9
Costs and Expenses	358.8	306.0
Income before federal income taxes	(144.0)	61.6
Current Income Tax Expense (Benefit)	0.0	(0.4)
Deferred Income Tax Expense (Benefit)	(29.4)	12.6
Federal income tax expense	(29.4)	12.2
Net income	$ (114.6)	$ 49.4
(Loss) earnings per common share:
Basic (usd per share)	$ (2.62)	$ 1.14
Diluted (usd per share)	(2.62)	1.12
Dividends paid per common share (usd per share)	$ 0.10	$ 0.10