Condensed Consolidated Statements of Income (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Earned premiums (affiliated net assumed $105.4 and $47.1, respectively)	$ 315.3	$ 262.5
Net investment income (affiliate $1.2 and $1.2, respectively)	15.4	17.6
Net realized gains on investments:
Total other-than-temporary impairment losses	(0.5)	(0.8)
Portion of loss recognized in other comprehensive income	0	0
Other net realized investment gains	4.3	11.5
Total net realized gains on investments	3.8	10.7
Other income from affiliates	0.4	0.5
Total revenues	334.9	291.3
Losses and loss expenses (affiliated net assumed $74.1 and $30.4, respectively)	195.0	171.8
Acquisition and operating expenses	103.4	88.5
Interest expense (affiliates $0.2 and $0.2, respectively)	1.3	1.3
Other expenses	2.0	2.0
Total expenses	301.7	263.6
Income before federal income taxes	33.2	27.7
Federal income tax expense	8.5	0.6
Net income	$ 24.7	$ 27.1
Earnings per common share:
Basic (usd per share)	$ 0.60	$ 0.67
Diluted (usd per share)	$ 0.60	$ 0.66
Dividends paid per common share (usd per share)	$ 0.1	$ 0.1