Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023		Dec. 31, 2022	Dec. 31, 2021
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		$ 417.8		$ 382.5	$ 350.4
Loss and Loss Adjustment Expenses Reserves		12,538.2		12,221.5	11,425.5
Unearned Premiums		3,042.6		2,787.8	2,559.4
Other Policyholders' Benefits and Funds		150.4		182.2	190.6
Premium Revenue		6,445.9		7,342.1	7,559.8
Net Investment Income		578.3		459.5	434.3
Loss and Loss Adjustment Expenses		2,596.6		2,440.2	2,420.9
Amortization of Deferred Policy Acquisition Costs		680.0		591.4	502.2
Other Operating Expenses		3,234.1		4,194.5	4,496.3
Premiums Written		6,682.9		7,511.6	7,685.0
Reinsurance Recoverables Policy And Loss Reserves		5,776.0		5,468.5	4,825.1
Disposal Group, Not Discontinued Operations
Supplementary Insurance Information, by Segment
Loss and Loss Adjustment Expenses Reserves		54.9
Unearned Premiums		0.1
General Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		417.6		382.2	349.9
Loss and Loss Adjustment Expenses Reserves		6,955.2		6,824.8	6,587.0
Unearned Premiums		2,253.1		2,028.5	1,870.7
Other Policyholders' Benefits and Funds		109.3		137.1	133.6
Premium Revenue		4,119.2		3,808.6	3,555.5
Net Investment Income		462.7		358.0	342.4
Loss and Loss Adjustment Expenses		2,553.3		2,364.6	2,303.1
Amortization of Deferred Policy Acquisition Costs		680.0		591.2	501.9
Other Operating Expenses		723.0		669.5	648.0
Premiums Written		4,356.3		3,978.2	3,680.9
Title Insurance Group [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		0.0		0.0	0.0
Loss and Loss Adjustment Expenses Reserves		598.5		612.8	594.2
Unearned Premiums		0.0		0.0	0.0
Other Policyholders' Benefits and Funds		1.5		3.3	6.2
Premium Revenue		2,300.9		3,500.6	3,960.5
Net Investment Income		57.0		47.9	43.8
Loss and Loss Adjustment Expenses		48.7		89.1	112.9
Amortization of Deferred Policy Acquisition Costs		0.0		0.0	0.0
Other Operating Expenses		2,438.2		3,484.6	3,820.6
Premiums Written		2,300.9		3,500.6	3,960.5
RFIG Run-off [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		0.0	[1]	0.0	0.0
Loss and Loss Adjustment Expenses Reserves		0.0	[1]	77.9	111.2
Unearned Premiums		0.0	[1]	0.1	0.2
Other Policyholders' Benefits and Funds		0.0	[1]	0.0	0.0
Premium Revenue		16.4	[1]	23.2	32.6
Net Investment Income		6.3	[1]	6.7	11.4
Loss and Loss Adjustment Expenses		(11.0)	[1]	(17.5)	(1.7)
Amortization of Deferred Policy Acquisition Costs		0.0	[1]	0.0	0.0
Other Operating Expenses		12.5	[1]	12.3	13.0
Premiums Written		16.4	[1]	23.1	32.4
Corporate & Other [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs	[2]	0.2		0.2	0.5
Loss and Loss Adjustment Expenses Reserves	[2]	6.6		6.3	7.6
Unearned Premiums	[2]	0.0		0.0	0.0
Other Policyholders' Benefits and Funds	[2]	30.8		31.8	39.4
Premium Revenue	[2]	9.1		9.6	11.0
Net Investment Income	[3]	52.2		46.8	36.5
Loss and Loss Adjustment Expenses	[3]	5.5		4.0	6.5
Amortization of Deferred Policy Acquisition Costs	[3]	0.0		0.2	0.3
Other Operating Expenses	[3]	60.1		28.0	14.5
Premiums Written	[3]	9.1		9.6	10.9
Reinsurance Recoverable [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs	[4]	0.0		0.0	0.0
Loss and Loss Adjustment Expenses Reserves	[4]	4,977.7		4,699.5	4,125.3
Unearned Premiums	[4]	789.5		759.1	688.4
Other Policyholders' Benefits and Funds	[4]	8.6		9.8	11.3
Premium Revenue	[4]	0.0		0.0	0.0
Net Investment Income	[4]	0.0		0.0	0.0
Loss and Loss Adjustment Expenses	[4]	0.0		0.0	0.0
Amortization of Deferred Policy Acquisition Costs	[4]	0.0		0.0	0.0
Other Operating Expenses	[4]	0.0		0.0	0.0
Premiums Written	[4]	$ 0.0		$ 0.0	$ 0.0

[1] RFIG Run-off loss and loss adjustment expense reserves of $54.9 and unearned premiums of $0.1 have been classified as held-for-sale as of December 31, 2023. See Note 2 in the Notes to Consolidated Financial Statements included in the Annual Report on Form 10-K for further discussion.

[2] Includes amounts for a small life and accident insurance business as well as those of the parent holding company, its internal corporate services subsidiaries and consolidation elimination adjustments.

[3] (b)    Includes amounts for a small life and accident insurance business as well as those of the parent holding company, its internal corporate services subsidiaries and consolidation elimination adjustments.

[4] In accordance with GAAP, reinsured losses and unearned premiums are to be reported as assets. Assets and liabilities were, as a result, increased by corresponding amounts of approximately $5.7 billion, $5.4 billion, and $4.8 billion at December 31, 2023, 2022, and 2021, respectively. This accounting treatment does not have any effect on the Company's results of operations.