Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other									$ 17.9	$ 4.9	$ (2.7)
Income from continuing operations before income taxes									444.8	522.1	282.5
Loss adjustment expenses and other operating expenses									(2,845.2)	(2,865.5)	(2,724.6)
Tax benefit									(82.8)	(93.1)	(43.5)
Income from continuing operations	$ 165.9	$ 119.5	$ 115.3	$ (38.7)	$ 110.2	$ 111.2	$ 85.0	$ 122.6	362.0	429.0	239.0
Gain on sale of Chaucer business										(1.2)	131.9
Net of tax	$ 164.6	$ 118.9	$ 115.2	$ (40.0)	$ 109.8	$ 118.9	$ 74.0	$ 122.4	358.7	425.1	391.0
Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses											(515.5)
Income (loss) from discontinued businesses, net of taxes									0.4	1.6	20.0
Gain from sale of Chaucer business										(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									1.2	7.2	(62.0)
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									13.4	5.0	(3.2)
Impairment losses on investments									(17.6)	(2.0)	(2.6)
Income from continuing operations before income taxes									(4.2)	3.0	(5.8)
Tax benefit									10.2	14.1	10.3
Income from continuing operations									6.0	17.1	4.5
Gain from sale of Chaucer business									(0.1)	(0.1)	0.1
Net of tax									5.9	16.9	(14.7)
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)
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									(6.0)	(11.4)	(9.5)
Tax benefit									1.3	2.4	1.9
Income from continuing operations									(4.7)	(9.0)	(7.6)
Gain on sale of Chaucer business											(16.4)
Income (loss) from discontinued businesses, net of taxes											(0.1)
Net of tax									$ (4.7)	(9.0)	(24.1)
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)