Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Premiums	$ 1,071.7	$ 1,004.4	$ 3,172.4	$ 2,957.0
Net investment income	66.4	62.6	198.0	181.3
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	(0.2)	14.6	(0.1)	25.0
Net change in fair value of equity securities	23.6		6.7
Net other–than–temporary impairment losses on investments recognized in earnings	(0.4)	(1.3)	(2.8)	(4.5)
Total net realized and unrealized investment gains	23.0	13.3	3.8	20.5
Fees and other income	5.5	5.6	17.3	16.6
Total revenues	1,166.6	1,085.9	3,391.5	3,175.4
Losses and expenses
Losses and loss adjustment expenses	676.4	656.5	2,018.5	1,938.2
Amortization of deferred acquisition costs	224.4	211.4	664.7	625.6
Interest expense	11.2	11.3	33.9	33.9
Other operating expenses	131.9	124.0	393.0	380.1
Total losses and expenses	1,043.9	1,003.2	3,110.1	2,977.8
Income from continuing operations before income taxes	122.7	82.7	281.4	197.6
Income tax expense:
Current	16.2	16.8	27.1	8.6
Deferred	2.5	6.4	17.4	44.8
Total income tax expense	18.7	23.2	44.5	53.4
Income from continuing operations	104.0	59.5	236.9	144.2
Income (loss) from discontinued operations	(3.6)		30.5
Net income	$ 100.4	$ 11.1	$ 267.4	$ 134.7
Basic:
Income from continuing operations	$ 2.45	$ 1.40	$ 5.57	$ 3.39
Net income per share	$ 2.36	$ 0.26	$ 6.29	$ 3.17
Weighted average shares outstanding	42.5	42.4	42.5	42.5
Diluted:
Income from continuing operations	$ 2.41	$ 1.38	$ 5.50	$ 3.36
Net income per share	$ 2.33	$ 0.26	$ 6.21	$ 3.14
Weighted average shares outstanding	43.1	42.9	43.1	42.9
Chaucer [Member]
Income tax expense:
Income (loss) from discontinued operations		$ (47.2)		$ (8.3)
Basic:
Income (loss) from discontinued operations	$ (0.09)	$ (1.11)	$ 0.72	$ (0.19)
Diluted:
Income (loss) from discontinued operations	$ (0.08)	$ (1.10)	$ 0.71	$ (0.19)
Life [Member]
Income tax expense:
Income (loss) from discontinued operations		$ (1.2)		$ (1.2)
Basic:
Income (loss) from discontinued operations		$ (0.03)		$ (0.03)
Diluted:
Income (loss) from discontinued operations		$ (0.02)		$ (0.03)