Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Earned premiums (affiliated net assumed $472.0, $437.6 and $212.4, respectively)	$ 1,291.9	$ 1,270.5	$ 1,074.1
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	74.7	71.7	74.7
Net realized gain on investments:
Total other-than-temporary impairment losses	(6.8)	(7.9)	(2.5)
Portion of loss recognized in other comprehensive income	0.0	0.0	0.0
Other net realized investment gains	43.3	32.2	23.2
Total net realized gain on investments	36.5	24.3	20.7
Other income (affiliates $2.3, $2.1 and $1.9, respectively)	2.3	2.1	3.2
Total revenues	1,405.4	1,368.6	1,172.7
Losses and loss expenses (affiliated net assumed $383.8, $293.3 and $250.8, respectively)	942.4	862.8	771.3
Acquisition and operating expenses (affiliated net assumed $285.8, $248.6 and $156.9, respectively)	430.4	426.8	361.9
Interest expense (affiliates $0.8, $0.7 and $0.7, respectively)	5.5	5.4	5.4
Other expenses	7.9	6.3	7.3
Total expenses	1,386.2	1,301.3	1,145.9
Income before federal income taxes	19.2	67.3	26.8
Current	(1.7)	2.9	0.1
Deferred	(0.1)	13.2	(80.7)
Total federal income tax (benefit) expense	(1.8)	16.1	(80.6)
Net income	$ 21.0	$ 51.2	$ 107.4
Earnings per common share:
Basic (usd per share)	$ 0.50	$ 1.25	$ 2.63
Diluted (usd per share)	0.50	1.23	2.60
Dividends paid per common share (usd per share)	$ 0.40	$ 0.40	$ 0.40