Investments (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023		Dec. 31, 2022	Dec. 31, 2021
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss		$ 1.6		$ 0.0
Net Unrealized Gains Losses Recognized from Changes in Fair Value of Equity Securities Held		28.2		42.3	$ 711.0
Debt Securities, Available-for-sale, Amortized Cost		12,263.0
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		12,139.9		11,746.7
Debt Securities, Held-to-maturity, Fair Value		0.0		0.0
Fixed maturity securities, due in one year or less, amortized cost		1,604.0
Fixed maturity securities, due after one year through five years, amortized cost		6,119.3
Fixed maturity securities, due after five years through ten years, amortized cost		4,455.6
Fixed maturity securities, due after ten years, amortized cost		83.9
Fixed maturity securities, due in one year or less, estimated fair value		1,587.7
Fixed maturity securities, due after one year through five years, estimated fair value		5,999.4
Fixed maturity securities, due after five years through ten years, estimated fair value		4,467.7
Fixed maturity securities, due after ten years, estimated fair value		84.9
Bonds on deposit with Governmental Authorities		946.0
Equity Securities, FV-NI		2,660.8		3,220.9
Fixed maturity securities and equity, less than twelve months, fair value		1,487.6		9,412.0
Fixed maturity securities and equity, less than 12 months, unrealized losses		11.8		439.1
Fixed maturity securities and equity, 12 months or greater, fair value		6,005.1		1,457.4
Fixed maturity securities and equity, 12 months or greater, unrealized losses		250.2		170.9
Fixed maturity securities and equity, total, fair value		7,492.7		10,869.5
Unrealized from changes in fair value of equity securities		(123.9)		(263.4)	751.1
Gain (Loss) on Investments		(190.9)		(201.1)	758.0
Short-term investments		1,032.6		860.8
Gross investment income		598.9		469.3	440.4
Investment expenses	[1]	20.6		9.7	6.1
Net investment income		578.3		459.5	434.3
Realized gains on securities		1.2		2.6	3.4
Realized losses on securities		(181.9)		(190.2)	(1.9)
Net realized gains (losses)		(150.8)		(158.6)	598.4
OtherThanTemporaryImpairmentLossesCreditLossesOtherInvestmentsDebtSecutiriesRecognizedInEarnings		(6.2)		(123.5)
OtherThanTemporaryImpairmentLossesCreditLossesOtherInvestmentsDebtSecuritiesSaleOfSubsidiaryRecognizedInEarnings		(51.8)	[2]	(123.5)	0.0
Income taxes (credits)		(40.0)		(42.5)	159.6
Changes in unrealized investment gains (losses) on fixed maturity securities:		469.8		(833.3)	(363.8)
Deferred income taxes (credits)		98.9		(175.9)	(76.8)
Net unrealized gains (losses) on securities not included in the statements of income, net of tax		370.8		(657.3)	(287.0)
Total Common Shareholders' Equity		6,410.7		6,173.2
Debt Securities, Available-for-sale, Realized Gain (Loss)		(180.7)		(187.6)	1.5
Realized Investment Gains Losses From Sales		(15.2)		185.7	6.9
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		438.8		314.4	280.6
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		92.1		132.5	157.5
Short-term Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income		50.9		17.9	0.1
Investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	[1]	17.0		4.3	2.1
Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Available-for-sale, Amortized Cost		1,920.3		2,300.0
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		1,858.9		2,185.2
Fixed maturity securities and equity, less than twelve months, fair value		461.0		1,769.6
Fixed maturity securities and equity, less than 12 months, unrealized losses		2.7		71.0
Fixed maturity securities and equity, 12 months or greater, fair value		1,179.3		403.8
Fixed maturity securities and equity, 12 months or greater, unrealized losses		61.8		43.8
Fixed maturity securities and equity, total, fair value		1,640.4		2,173.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		3.2		0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		64.6		114.8
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Available-for-sale, Amortized Cost		774.5		896.9
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		767.6		881.5
Fixed maturity securities and equity, less than twelve months, fair value		173.1		845.6
Fixed maturity securities and equity, less than 12 months, unrealized losses		0.8		13.0
Fixed maturity securities and equity, 12 months or greater, fair value		554.7		9.8
Fixed maturity securities and equity, 12 months or greater, unrealized losses		6.2		2.5
Fixed maturity securities and equity, total, fair value		727.9		855.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		0.2		0.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		7.1		15.5
Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Available-for-sale, Amortized Cost		9,568.1		9,139.3
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		9,513.3		8,680.0
Fixed maturity securities and equity, less than twelve months, fair value		853.3		6,796.7
Fixed maturity securities and equity, less than 12 months, unrealized losses		8.2		355.0
Fixed maturity securities and equity, 12 months or greater, fair value		4,270.9		1,043.7
Fixed maturity securities and equity, 12 months or greater, unrealized losses		182.0		124.6
Fixed maturity securities and equity, total, fair value		5,124.3		7,840.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		135.5		20.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		190.3		479.6
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Debt Securities, Available-for-sale, Amortized Cost		12,263.0		12,336.3
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		12,139.9		11,746.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax		139.0		20.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		262.0		610.1
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities, adjusted cost		1,511.9		1,948.1
Equity securities, gross unrealized gains		(1,164.7)		(1,291.5)
Equity securities, gross unrealized losses		(15.7)		(18.6)
Equity Securities, FV-NI, Current		2,660.8		3,220.9
Fair Value, Inputs, Level 1 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments		1,032.6		860.8
Fair Value, Inputs, Level 1 [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		1,379.8		1,598.8
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		0.0		0.0
Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		0.0		0.0
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity Securities, FV-NI, Current		2,653.8		3,219.1
Fair Value, Inputs, Level 2 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments		0.0		0.0
Fair Value, Inputs, Level 2 [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		479.1		586.3
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		767.6		881.5
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		9,493.7		8,659.2
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity Securities, FV-NI, Current		0.0		0.0
Fair Value, Inputs, Level 3 [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Short-term investments		0.0		0.0
Fair Value, Inputs, Level 3 [Member] | Fixed Maturity Securities, U S Canadian Governments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		0.0		0.0
Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		0.0		0.0
Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities (at fair value) (amortized cost: $12,263.0 and $12,336.3)		19.5		20.8
Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity Securities, FV-NI, Current		7.0		1.7
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available-for-sale And Trading Securities, Gross Realized Gains		214.5		486.5	68.0
Available-for-sale And Trading Securities, Gross Realized Losses		(51.4)		(111.9)	(62.8)
Available-for-sale And Trading Securities, Net Realized Gains (Losses)		163.0		374.5	5.1
Other long-term investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Changes in unrealized investment gains (losses) on fixed maturity securities:		5.7		(8.5)	(2.5)
Deferred income taxes (credits)		1.1		(1.8)	(0.5)
Net changes in unrealized investment gains (losses)		4.5		(6.7)	(2.0)
Debt Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Changes in unrealized investment gains (losses) on fixed maturity securities:		464.1		(824.7)	(361.2)
Deferred income taxes (credits)		97.7		(174.1)	(76.2)
Net changes in unrealized investment gains (losses)		366.3		(650.5)	(284.9)
Other investments [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment gains (losses) - other long-term investments		$ 2.4		$ (1.2)	$ 0.2

[1] Includes interest on funds held.

[2] Includes expected loss on the pending sale of RMICC. See Note 2 for further discussion.