Discontinued Operations (Summary of Discontinued Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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
Total revenues	$ 1,102.8	$ 1,166.6	$ 1,133.4	$ 1,091.5	$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	4,494.3		4,267.9	4,053.9
Losses and operating expenses
Losses and LAE									2,724.6		2,579.6	2,546.0
Amortization of deferred acquisition costs									891.8		840.7	803.6
Other operating expenses									522.1		509.5	498.6
Total losses and expenses									4,211.8		3,975.0	3,987.9
Other items:
Interest expense									(45.1)		(45.2)	(51.4)
Net realized and unrealized investment gains (losses)									(50.7)		21.1	10.2
Income tax expense									(43.5)		(76.8)	1.0
Income (loss) from Chaucer business, net of taxes									0.1		(16.8)	(1.0)
Chaucer [Member]
Revenues
Premiums									850.0	[1]	853.0	838.6
Net investment income									54.9	[1]	52.0	45.7
Other income									7.5	[1]	6.7	7.1
Total revenues									912.4	[1]	911.7	891.4
Losses and operating expenses
Losses and LAE									515.5	[1]	549.5	419.3
Amortization of deferred acquisition costs									252.5	[1]	245.9	231.6
Other operating expenses									115.0	[1]	109.2	113.7
Total losses and expenses									883.0	[1]	904.6	764.6
Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income)									29.4	[1]	7.1	126.8
Other items:
Interest expense									(3.8)	[1]	(3.3)	(3.5)
Net realized and unrealized investment gains (losses)									(1.3)	[1]	2.6	(1.6)
Other income									0.4	[1]	2.2	4.6
Income from Chaucer business before income taxes									24.7	[1]	8.6	126.3
Income tax expense									(4.7)	[1]	(21.7)	(37.2)
Income (loss) from Chaucer business, net of taxes									$ 20.0	[1]	$ (13.1)	$ 89.1

[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.