Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Earned premiums (affiliated net assumed $437.6, $212.4 and $200.0, respectively)	$ 1,270.5	$ 1,074.1	$ 1,055.0
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	71.7	74.7	72.8
Net realized gain on investments:
Total other-than-temporary impairment losses	(7.9)	(2.5)	(4.0)
Portion of loss recognized in other comprehensive income	0.0	0.0	0.0
Other net realized investment gains	32.2	23.2	27.2
Total net realized gain on investments	24.3	20.7	23.2
Other income (affiliates $2.1, $1.9 and $2.0, respectively)	2.1	3.2	2.0
Total revenues	1,368.6	1,172.7	1,153.0
Losses and loss expenses (affiliated net assumed $293.3, $250.8 and $162.5, respectively)	862.8	771.3	719.8
Acquisition and operating expenses (affiliated net assumed $313.0, $156.9 and $172.7, respectively)	426.8	361.9	354.8
Interest expense (affiliates $0.7, $0.7 and $0.7, respectively)	5.4	5.4	8.5
Other expenses	6.3	7.3	8.6
Total expenses	1,301.3	1,145.9	1,091.7
Income before federal income taxes	67.3	26.8	61.3
Current	2.9	0.1	0.5
Deferred	13.2	(80.7)	0.0
Total federal income tax expense (benefit)	16.1	(80.6)	0.5
Net income	$ 51.2	$ 107.4	$ 60.8
Earnings per common share:
Basic (usd per share)	$ 1.25	$ 2.63	$ 1.50
Diluted (usd per share)	1.23	2.60	1.49
Dividends paid per common share (usd per share)	$ 0.4	$ 0.4	$ 0.40