Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		$ 325.4	$ 316.3	$ 297.8
Losses, Claims and Settlement Expenses		9,929.5	9,471.2	9,237.6
Unearned Premiums		2,224.7	2,104.9	1,971.5
Other Policyholders' Benefits and Funds		194.4	198.6	204.7
Premium Revenue		5,498.3	5,253.4	5,080.2
Net Investment Income		450.7	431.8	409.4
Benefits, Claims, Losses and Settlement Expenses		2,572.7	2,460.7	2,478.8
Amortization of Deferred Policy Acquisition Costs		532.2	489.6	454.0
Other Operating Expenses		2,786.3	2,633.2	2,604.7
Premiums Written		5,533.2	5,354.7	5,210.1
Reinsurance Recoverables Policy And Claim Reserves		3,755.3	3,428.6	3,311.3
General Insurance Segment [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		324.0	314.1	294.3
Losses, Claims and Settlement Expenses		6,021.3	5,766.1	5,471.5
Unearned Premiums		1,733.0	1,696.4	1,597.3
Other Policyholders' Benefits and Funds		129.1	130.8	130.2
Premium Revenue		3,432.4	3,277.1	3,110.8
Net Investment Income		356.4	341.0	318.9
Benefits, Claims, Losses and Settlement Expenses		2,464.6	2,365.8	2,234.4
Amortization of Deferred Policy Acquisition Costs		531.5	488.4	449.9
Other Operating Expenses		554.3	521.2	506.8
Premiums Written		3,469.0	3,380.4	3,245.2
Title Insurance Group [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs		0.0	0.0	0.0
Losses, Claims and Settlement Expenses		530.9	533.4	559.7
Unearned Premiums		0.0	0.0	0.0
Other Policyholders' Benefits and Funds		6.0	6.3	6.8
Premium Revenue		1,993.2	1,885.6	1,827.6
Net Investment Income		41.4	38.8	37.3
Benefits, Claims, Losses and Settlement Expenses		67.4	48.3	20.8
Amortization of Deferred Policy Acquisition Costs		0.0	0.0	0.0
Other Operating Expenses		2,233.0	2,107.7	2,067.0
Premiums Written		1,993.2	1,885.6	1,827.6
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		118.9	154.5	271.7
Unearned Premiums		1.3	3.0	5.1
Other Policyholders' Benefits and Funds		0.0	0.0	0.0
Premium Revenue		59.2	75.9	122.9
Net Investment Income		17.6	20.1	21.7
Benefits, Claims, Losses and Settlement Expenses		31.7	29.9	197.8
Amortization of Deferred Policy Acquisition Costs		0.0	0.0	0.0
Other Operating Expenses		14.8	16.3	20.3
Premiums Written		57.5	73.9	118.4
Corporate & Other [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs	[1]	1.4	2.2	3.4
Losses, Claims and Settlement Expenses	[1]	8.4	10.8	13.5
Unearned Premiums	[1]	0.0	0.0	0.0
Other Policyholders' Benefits and Funds	[1]	44.0	44.6	46.4
Premium Revenue	[1]	13.4	14.6	18.8
Net Investment Income	[2]	35.1	31.7	31.4
Benefits, Claims, Losses and Settlement Expenses	[2]	8.8	16.7	25.7
Amortization of Deferred Policy Acquisition Costs	[2]	0.7	1.2	4.1
Other Operating Expenses	[2]	(15.9)	(12.0)	10.3
Premiums Written	[2]	13.3	14.6	18.8
Reinsurance Recoverable Including Reinsurance Premium Paid [Member]
Supplementary Insurance Information, by Segment
Deferred Policy Acquisition Costs	[3]	0.0	0.0	0.0
Losses, Claims and Settlement Expenses	[3]	3,249.7	3,006.3	2,921.1
Unearned Premiums	[3]	490.4	405.4	369.0
Other Policyholders' Benefits and Funds	[3]	15.1	16.8	21.1
Premium Revenue	[3]	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] 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.

[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] 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 $3.7 billion, $3.4 billion, and $3.3 billion at December 31, 2019, 2018 and 2017, respectively. This accounting treatment does not have any effect on the Company's results of operations.

[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 $3.7 billion, $3.4 billion, and $3.3 billion at December 31, 2019, 2018 and 2017, respectively. This accounting treatment does not have any effect on the Company's results of operations.