Schedule 2 - Parent Company Financial Statements - Statements of Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Cash Flows from Operating Activities:
Net Loss			$ (272.3)	$ (286.6)	[1]	$ (123.7)	[1]
Adjustments to Reconcile Net Loss to Net Cash (Used in) Provided by Operating Activities:
Cash Dividends from Subsidiaries			640.9	311.7		347.0
Income (Loss) from Change in Fair Value of Equity and Convertible Securities			(4.7)	79.9	[1]	(114.6)	[1]
Net Realized Gain on Sale of Investments			18.6	(4.3)	[1]	(64.8)	[1]
Impairment Losses			1.1	25.8	[1]	11.0	[1]
Loss from Early Extinguishment of Debt			0.0	3.7	[2]	0.0	[1]
Other Assets and Liabilities			(33.0)	(11.2)	[2]	89.1	[2]
Net Cash (Used in) Provided by Operating Activities			(134.2)	(210.3)	[2]	350.7	[2]
Cash Flows from Investing Activities:
Proceeds from the Sales, Calls and Maturities of Fixed Maturities			673.0	1,295.5	[2]	1,388.9	[2]
Fixed Maturities			(447.4)	(1,815.8)	[2]	(1,825.4)	[2]
Equity Securities			(44.4)	(58.9)	[2]	(124.3)	[2]
Net Purchases of Short-term Investments			(238.4)	6.1	[2]	687.2	[2]
Equity Securities			149.0	536.0	[2]	316.6	[2]
Acquisition of Business			0.0	0.0	[2]	(316.6)	[2]
Other Investments			(19.8)	(13.0)	[2]	(104.9)	[2]
Net Cash Provided by (Used in) Investing Activities			107.9	(108.4)	[2]	(118.2)	[2]
Cash Flows from Financing Activities:
Proceeds from Issuance of 5.875% Fixed-Rate Reset Junior Subordinated Debentures Due 2062			0.0	145.6		0.0
Repayments of Long-term Debt			0.0	(280.0)		(50.0)
Dividends Paid			(80.1)	(79.7)		(80.6)
Proceeds from Shares Issued under Employee Stock Purchase Plan			4.3	4.9		5.4
Common Stock Repurchases			0.0	0.0		(161.7)
Other			1.9	0.6		3.7
Net Cash (Used in) Provided by Financing Activities			(122.0)	382.9		(290.4)
Net (decrease) increase in cash			(148.3)	64.2		(57.9)
Cash, Beginning of Year	[3]		212.4
Cash, End of Year		$ 64.1	64.1	212.4	[3]
Senior Notes, 3.800 Percent Due February 23, 2032
Cash Flows from Financing Activities:
Proceeds from Issuance of 3.800% Senior Notes due February 23, 2032			0.0	396.3		0.0
Issue Fess			0.0	(1.2)		0.0
5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Cash Flows from Financing Activities:
Issue Fess			0.0	(0.9)		0.0
Parent Company
Cash Flows from Operating Activities:
Net Loss			(272.3)	(286.6)		(123.7)
Adjustments to Reconcile Net Loss to Net Cash (Used in) Provided by Operating Activities:
Equity in Net Income (Loss) of Subsidiaries			162.5	247.1		109.2
Cash Dividends from Subsidiaries			320.8	25.3		170.3
Income (Loss) from Change in Fair Value of Equity and Convertible Securities			1.5	14.8		(10.0)
Net Realized Gain on Sale of Investments			11.9	(3.0)		(10.6)
Settlement Costs Related to Defined Benefit Pension Plan		70.2		0.0		0.0
Impairment Losses		0.4	0.4	0.2		0.0
Other Assets and Liabilities			(30.6)	(48.9)		(35.3)
Net Cash (Used in) Provided by Operating Activities			264.4	(51.1)		99.9
Cash Flows from Investing Activities:
Capital Contributed to Subsidiaries			(181.5)	(537.8)		(36.5)
Proceeds from the Sales, Calls and Maturities of Fixed Maturities			50.8	0.1		181.3
Fixed Maturities			0.0	(40.3)		0.0
Equity Securities			(2.1)	(5.6)		(48.7)
Net Purchases of Short-term Investments			(112.2)	138.9		411.3
Equity Securities			14.8	71.9		28.5
Acquisition of Business			0.0	0.0		(370.9)
Other Investments			(23.2)	(3.1)		0.0
Net Cash Provided by (Used in) Investing Activities			(253.4)	(375.9)		165.0
Cash Flows from Financing Activities:
Repayments of Long-term Debt			0.0	0.0		(50.0)
Dividends Paid			(79.6)	(79.7)		(80.6)
Proceeds from Shares Issued under Employee Stock Purchase Plan			4.3	4.9		5.4
Common Stock Repurchases			0.0	0.0		(161.7)
Other			(0.5)	0.6		3.7
Net Cash (Used in) Provided by Financing Activities			(75.8)	465.6		(283.2)
Net (decrease) increase in cash			(64.8)	38.6		(18.3)
Cash, Beginning of Year			66.3	27.7		46.0
Cash, End of Year		$ 1.5	1.5	66.3		27.7
Parent Company | Senior Notes, 3.800 Percent Due February 23, 2032
Cash Flows from Financing Activities:
Proceeds from Issuance of 3.800% Senior Notes due February 23, 2032			0.0	396.3		0.0
Issue Fess			0.0	(1.2)		0.0
Parent Company | 5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Cash Flows from Financing Activities:
Issue Fess			0.0	(0.9)		0.0
Proceeds from Issuance of 5.875% Fixed-Rate Reset Junior Subordinated Debentures Due 2062			0.0	145.6		0.0
Parent Company | Senior Notes, 2.400 Percent Due September 30, 2030
Cash Flows from Financing Activities:
Proceeds from Issuance of 3.800% Senior Notes due February 23, 2032			0.0	0.0		0.0
Issue Fess			$ 0.0	$ 0.0		$ 0.0

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.

[2] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.

[3] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.