Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other									$ 4.9	$ (2.7)		$ 26.7
Net other–than–temporary impairment losses on investments recognized in earnings									(2.0)	(4.6)		(5.6)
Income from continuing operations before income taxes									522.1	282.5		292.9
Loss adjustment expenses and other operating expenses									(2,865.5)	(2,724.6)		(2,579.6)
Tax benefit									(93.1)	(43.5)		(76.8)
Income from continuing operations	$ 110.2	$ 111.2	$ 85.0	$ 122.6	$ 2.1	$ 104.0	$ 82.4	$ 50.5	429.0	239.0		216.1
Gain on sale of Chaucer business									(1.2)	131.9
Net of tax	$ 109.8	$ 118.9	$ 74.0	$ 122.4	$ 123.6	$ 100.4	$ 99.3	$ 67.7	425.1	391.0		186.2
Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses										(515.5)	[1]	(549.5)
Income (loss) from discontinued businesses, net of taxes									1.6	20.0	[1]	(13.1)
Gain from sale of Chaucer business, net of taxes									(1.2)	131.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									7.2	(62.0)		20.3
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other									5.0	(3.2)		26.6
Net other–than–temporary impairment losses on investments recognized in earnings									(2.0)	(2.6)		(3.6)
Income from continuing operations before income taxes									3.0	(5.8)		23.0
Tax benefit									14.1	10.3		4.2
Income from continuing operations									17.1	4.5		27.2
Gain from sale of Chaucer business, net of taxes									(0.1)	0.1
Net of tax									16.9	(14.7)		29.7
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member] | Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of Chaucer business									(0.1)	(19.1)
Income (loss) from discontinued businesses, net of taxes										(0.2)		2.5
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined Benefit Pension and Postretirement Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses									(11.4)	(9.5)		(12.4)
Tax benefit									2.4	1.9		4.3
Income from continuing operations									(9.0)	(7.6)		(8.1)
Gain on sale of Chaucer business										(16.4)
Income (loss) from discontinued businesses, net of taxes										(0.1)		(1.3)
Net of tax									(9.0)	(24.1)		$ (9.4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Currency Translation Obligation Recognized With Sale [Member] | Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of Chaucer business									$ (0.7)	$ (23.2)

[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.