Investments (Schedule of Other Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Other investments	$ 796.9	$ 786.1
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	371.4	388.6
Allowance for credit losses	(10.0)	(3.2)
Mortgage Loans on Real Estate [Member] | South Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	102.4	103.2
Mortgage Loans on Real Estate [Member] | Pacific [Member]
Schedule of Investments [Line Items]
Other investments	92.0	92.5
Mortgage Loans on Real Estate [Member] | Mid-Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	57.0	58.1
Mortgage Loans on Real Estate [Member] | New England [Member]
Schedule of Investments [Line Items]
Other investments	41.0	47.8
Mortgage Loans on Real Estate [Member] | West South Central [Member]
Schedule of Investments [Line Items]
Other investments	32.4	33.0
Mortgage Loans on Real Estate [Member] | East North Central [Member]
Schedule of Investments [Line Items]
Other investments	19.8	21.0
Mortgage Loans on Real Estate [Member] | Mountain [Member]
Schedule of Investments [Line Items]
Other investments	14.6	13.6
Mortgage Loans on Real Estate [Member] | Other Locations [Member]
Schedule of Investments [Line Items]
Other investments	22.2	22.6
Mortgage Loans on Real Estate [Member] | Office [Member]
Schedule of Investments [Line Items]
Other investments	141.9	149.3
Mortgage Loans on Real Estate [Member] | Apartments [Member]
Schedule of Investments [Line Items]
Other investments	104.3	103.9
Mortgage Loans on Real Estate [Member] | Retail [Member]
Schedule of Investments [Line Items]
Other investments	50.8	52.7
Mortgage Loans on Real Estate [Member] | Hotel [Member]
Schedule of Investments [Line Items]
Other investments	48.0	48.9
Mortgage Loans on Real Estate [Member] | Industrial [Member]
Schedule of Investments [Line Items]
Other investments	$ 36.4	$ 37.0