Accumulated Other Comprehensive Income (Reclassification out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Amount reclassified from AOCI
Underwriting, selling, general and administrative expenses (see Note 4)	$ 7,695.1	$ 7,366.3	$ 7,081.9
Provision for income taxes	164.3	73.3	168.4
Net of tax	(642.5)	(276.6)	(1,361.8)
Interest expense	108.0	108.3	111.8
Income from continuing operations before income tax expense	(806.8)	(349.9)	(771.3)
Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Net of tax	25.4	30.6	(630.2)
Foreign currency translation adjustment | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Underwriting, selling, general and administrative expenses (see Note 4)	0.0	0.0	(0.8)
Provision for income taxes	0.0	0.0	0.5
Net of tax	0.0	0.0	(0.3)
Net unrealized gains (losses) on securities | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Provision for income taxes	(9.5)	(10.3)	172.7
Net of tax	35.8	38.9	(625.1)
Net realized losses on investments and fair value changes to equity securities	45.3	49.2	(797.8)
Net unrealized gains on derivative transactions | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Underwriting, selling, general and administrative expenses (see Note 4)	2.5	0.0	0.0
Provision for income taxes	0.2	0.6	0.5
Net of tax	(0.7)	(2.6)	(2.3)
Interest expense	(3.4)	(3.2)	(2.8)
Income from continuing operations before income tax expense	(0.9)	(3.2)	(2.8)
Unamortized net (losses) on Pension Plans | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Provision for income taxes	2.6	1.5	0.7
Net of tax	(9.7)	(5.7)	(2.5)
Income from continuing operations before income tax expense	(12.3)	(7.2)	(3.2)
Amortization of net loss | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Underwriting, selling, general and administrative expenses (see Note 4)	1.0	4.4	7.2
Amortization of prior service credit | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Underwriting, selling, general and administrative expenses (see Note 4)	(13.5)	(13.5)	(13.5)
Settlement loss | Reclassification out of Accumulated other Comprehensive Income
Amount reclassified from AOCI
Underwriting, selling, general and administrative expenses (see Note 4)	$ 0.2	$ 1.9	$ 3.1