Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	$ 56.5	$ 33.5
Net of tax	(236.4)	(113.6)
Interest expense	26.8	27.0
Underwriting, selling, general and administrative expenses	1,937.3	1,823.2
Income before income tax expense	(292.9)	(147.1)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	6.2	(0.3)
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized losses on investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on investments and fair value changes to equity securities	14.7	4.0
Provision for income taxes	(3.1)	(0.8)
Net of tax	11.6	3.2
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on derivative transactions
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	0.8	0.3
Net of tax	(3.1)	(1.0)
Interest expense	(0.7)	(1.3)
Underwriting, selling, general and administrative expenses	(3.2)	0.0
Income before income tax expense	(3.9)	(1.3)
Reclassification out of Accumulated Other Comprehensive Income | Unamortized net losses on Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	0.7	0.6
Net of tax	(2.3)	(2.5)
Income before income tax expense	(3.0)	(3.1)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Underwriting, selling, general and administrative expenses	0.3	0.3
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Underwriting, selling, general and administrative expenses	$ (3.3)	$ (3.4)