Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Revenues
Premiums	$ 4,254.4		$ 3,980.4	$ 3,789.5
Net investment income	267.4		243.9	231.6
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	(2.7)		26.7	37.6
Net change in fair value of equity securities	(43.4)
Net other–than–temporary impairment losses on investments recognized in earnings	(4.6)		(5.6)	(27.4)
Total net realized and unrealized investment gains (losses)	(50.7)		21.1	10.2
Fees and other income	23.2		22.5	22.6
Total revenues	4,494.3		4,267.9	4,053.9
Losses and expenses
Losses and loss adjustment expenses	2,724.6		2,579.6	2,546.0
Amortization of deferred acquisition costs	891.8		840.7	803.6
Interest expense	45.1		45.2	51.4
Loss on repayment of debt	28.2			88.3
Other operating expenses	522.1		509.5	498.6
Total losses and expenses	4,211.8		3,975.0	3,987.9
Income from continuing operations before income taxes	282.5		292.9	66.0
Current	46.2		3.1	26.2
Deferred	(2.7)		73.7	(27.2)
Total income tax expense (benefit)	43.5		76.8	(1.0)
Income from continuing operations	239.0		216.1	67.0
Discontinued operations:
Net Income/gain (loss) from discontinued businesses, net of taxes	0.1		(16.8)	(1.0)
Net income	$ 391.0		$ 186.2	$ 155.1
Basic:
Income from continuing operations	$ 5.63		$ 5.08	$ 1.57
Net income per share	$ 9.21		$ 4.38	$ 3.63
Weighted average shares outstanding	42.4		42.5	42.8
Diluted:
Income from continuing operations	$ 5.56		$ 5.03	$ 1.55
Net income per share	$ 9.09		$ 4.33	$ 3.59
Weighted average shares outstanding	43.0		43.0	43.2
Chaucer [Member]
Revenues
Premiums	$ 850.0	[1]	$ 853.0	$ 838.6
Net investment income	54.9	[1]	52.0	45.7
Net realized and unrealized investment gains (losses):
Total revenues	912.4	[1]	911.7	891.4
Losses and expenses
Losses and loss adjustment expenses	515.5	[1]	549.5	419.3
Amortization of deferred acquisition costs	252.5	[1]	245.9	231.6
Interest expense	3.8	[1]	3.3	3.5
Other operating expenses	115.0	[1]	109.2	113.7
Total losses and expenses	883.0	[1]	904.6	764.6
Total income tax expense (benefit)	4.7	[1]	21.7	37.2
Discontinued operations:
Gain from sale of Chaucer business, net of taxes	131.9
Net Income/gain (loss) from discontinued businesses, net of taxes	$ 20.0	[1]	$ (13.1)	$ 89.1
Basic:
Gain from sale of business, net of taxes	$ 3.11
Net Income/gain (loss) from discontinued businesses, net of taxes	0.47		$ (0.30)	$ 2.08
Diluted:
Gain from sale of business, net of taxes	3.07
Net Income/gain (loss) from discontinued businesses, net of taxes	$ 0.46		$ (0.30)	$ 2.06
Life [Member]
Discontinued operations:
Net Income/gain (loss) from discontinued businesses, net of taxes	$ 0.1		$ (16.8)	$ (1.0)
Basic:
Net Income/gain (loss) from discontinued businesses, net of taxes			$ (0.40)	$ (0.02)
Diluted:
Net Income/gain (loss) from discontinued businesses, net of taxes			$ (0.40)	$ (0.02)

[1] 2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.