Investments (Schedule of Other Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Other investments	$ 786.1	$ 767.4
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	388.6	434.0
Allowance for credit losses	(3.2)	(7.1)
Mortgage Loans on Real Estate [Member] | South Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	103.2	108.8
Mortgage Loans on Real Estate [Member] | Pacific [Member]
Schedule of Investments [Line Items]
Other investments	92.5	93.0
Mortgage Loans on Real Estate [Member] | Mid-Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	58.1	58.9
Mortgage Loans on Real Estate [Member] | New England [Member]
Schedule of Investments [Line Items]
Other investments	47.8	58.2
Mortgage Loans on Real Estate [Member] | West South Central [Member]
Schedule of Investments [Line Items]
Other investments	33.0	48.6
Mortgage Loans on Real Estate [Member] | East North Central [Member]
Schedule of Investments [Line Items]
Other investments	21.0	27.7
Mortgage Loans on Real Estate [Member] | Mountain [Member]
Schedule of Investments [Line Items]
Other investments	13.6	18.5
Mortgage Loans on Real Estate [Member] | Other Locations [Member]
Schedule of Investments [Line Items]
Other investments	22.6	27.4
Mortgage Loans on Real Estate [Member] | Office [Member]
Schedule of Investments [Line Items]
Other investments	149.3	154.8
Mortgage Loans on Real Estate [Member] | Apartments [Member]
Schedule of Investments [Line Items]
Other investments	103.9	125.5
Mortgage Loans on Real Estate [Member] | Retail [Member]
Schedule of Investments [Line Items]
Other investments	52.7	61.4
Mortgage Loans on Real Estate [Member] | Hotel [Member]
Schedule of Investments [Line Items]
Other investments	48.9	62.0
Mortgage Loans on Real Estate [Member] | Industrial [Member]
Schedule of Investments [Line Items]
Other investments	$ 37.0	$ 37.4