Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Earned premiums (affiliated net assumed $71.9 and $115.3, respectively, and $251.1 and $349.0, respectively)	$ 353.2	$ 319.6	$ 1,024.4	$ 930.0
Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively)	17.9	19.3	54.5	60.4
Net realized gains on investments:
Total net realized gains on investments	20.0	(5.0)	(39.3)	50.2
Other income from affiliates	0.5	0.6	1.6	1.6
Total revenues	391.6	334.5	1,041.2	1,042.2
Losses and loss expenses (affiliated net assumed $34.7 and $128.9, respectively, and $105.4 and $298.8, respectively)	251.6	208.4	762.2	637.6
Acquisition and operating expenses (affiliated net ceded $26.8 and affiliated net assumed $78.9, respectively, and $52.3 and $231.0, respectively)	123.0	109.7	357.4	325.2
Interest expense (affiliates $0.2 and $0.2, respectively, and $0.7 and $0.6, respectively)	1.1	1.2	3.6	3.7
Other Expenses	1.1	2.5	5.6	9.0
Costs and Expenses	376.8	321.8	1,128.8	975.5
Income before federal income taxes	14.8	12.7	(87.6)	66.7
Current Income Tax Expense (Benefit)			(0.4)	(0.4)
Deferred Income Tax Expense (Benefit)	3.2	1.2	(18.5)	12.4
Federal income tax expense	3.2	1.2	(18.9)	12.0
Net income	$ 11.6	$ 11.5	$ (68.7)	$ 54.7
(Loss) earnings per common share:
Basic (usd per share)	$ 0.26	$ 0.26	$ (1.57)	$ 1.26
Diluted (usd per share)	$ 0.26	$ 0.25	$ (1.57)	$ 1.25