Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		$ 328.0	$ 325.4	[1]	$ 316.3	[1]
Losses, Claims and Settlement Expenses		10,671.0	9,929.5	[1]	9,471.2	[1]
Unearned Premiums		2,397.1	2,224.7	[1]	2,104.9	[1]
Other Policyholders' Benefits and Funds		195.9	194.4	[1]	198.6	[1]
Premium Revenue		6,345.8	5,919.9	[1]	5,651.1	[1]
Net Investment Income		438.9	450.7	[1]	431.8	[1]
Benefits, Claims, Losses and Settlement Expenses		2,491.4	2,572.7	[1]	2,460.7	[1]
Amortization of Deferred Policy Acquisition Costs		501.5	532.2	[1]	489.6	[1]
Other Operating Expenses		3,484.5	3,033.1	[1]	2,870.2	[1]
Premiums Written		6,382.1	5,954.8	[1]	5,752.4	[1]
Reinsurance Recoverables Policy And Claim Reserves		4,295.1	3,755.3		3,428.6
General Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		327.2	324.0		314.1
Losses, Claims and Settlement Expenses		6,328.0	6,021.3		5,766.1
Unearned Premiums		1,765.2	1,733.0		1,696.4
Other Policyholders' Benefits and Funds		133.8	129.1		130.8
Premium Revenue		3,394.2	3,432.4		3,277.1
Net Investment Income		352.2	356.4		341.0
Benefits, Claims, Losses and Settlement Expenses		2,372.0	2,464.6		2,365.8
Amortization of Deferred Policy Acquisition Costs		500.9	531.5		488.4
Other Operating Expenses		564.0	554.3		521.2
Premiums Written		3,431.3	3,469.0		3,380.4
Title Insurance Group [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		0.0	0.0	[1]	0.0	[1]
Losses, Claims and Settlement Expenses		556.1	530.9	[1]	533.4	[1]
Unearned Premiums		0.0	0.0	[1]	0.0	[1]
Other Policyholders' Benefits and Funds		6.5	6.0	[1]	6.3	[1]
Premium Revenue		2,894.4	2,414.8	[1]	2,283.3	[1]
Net Investment Income		42.0	41.4	[1]	38.8	[1]
Benefits, Claims, Losses and Settlement Expenses		75.3	67.4	[1]	48.3	[1]
Amortization of Deferred Policy Acquisition Costs		0.0	0.0	[1]	0.0	[1]
Other Operating Expenses		2,909.9	2,479.8	[1]	2,344.7	[1]
Premiums Written		2,894.4	2,414.8	[1]	2,283.3	[1]
RFIG Run-off Business [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		0.0	0.0		0.0
Losses, Claims and Settlement Expenses		127.6	118.9		154.5
Unearned Premiums		0.4	1.3		3.0
Other Policyholders' Benefits and Funds		0.0	0.0		0.0
Premium Revenue		45.1	59.2		75.9
Net Investment Income		15.2	17.6		20.1
Benefits, Claims, Losses and Settlement Expenses		36.9	31.7		29.9
Amortization of Deferred Policy Acquisition Costs		0.0	0.0		0.0
Other Operating Expenses		13.6	14.8		16.3
Premiums Written		44.3	57.5		73.9
Corporate & Other [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs	[2]	0.8	1.4		2.2
Losses, Claims and Settlement Expenses	[2]	8.6	8.4		10.8
Unearned Premiums	[2]	0.0	0.0		0.0
Other Policyholders' Benefits and Funds	[2]	42.4	44.0		44.6
Premium Revenue	[2]	12.0	13.4		14.6
Net Investment Income	[3]	29.4	35.1		31.7
Benefits, Claims, Losses and Settlement Expenses	[3]	7.1	8.8		16.7
Amortization of Deferred Policy Acquisition Costs	[3]	0.5	0.7		1.2
Other Operating Expenses	[3]	(3.1)	(15.9)		(12.0)
Premiums Written	[3]	12.0	13.3		14.6
Reinsurance Recoverable Including Reinsurance Premium Paid [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs	[4]	0.0	0.0		0.0
Losses, Claims and Settlement Expenses	[4]	3,650.5	3,249.7		3,006.3
Unearned Premiums	[4]	631.4	490.4		405.4
Other Policyholders' Benefits and Funds	[4]	13.1	15.1		16.8
Premium Revenue	[4]	0.0	0.0		0.0
Net Investment Income	[4]	0.0	0.0		0.0
Benefits, Claims, Losses and Settlement 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] Certain reclassifications were made to net premiums earned, title, escrow, and other fees, and underwriting, acquisition, and other expenses in 2019 and 2018 to conform to the presentation adopted in 2020.

[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] 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 $4.2 billion, $3.7 billion, and $3.4 billion at December 31, 2020, 2019 and 2018, respectively. This accounting treatment does not have any effect on the Company's results of operations.