Investments (Schedule of Other Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Other investments	$ 793.2	$ 733.2
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	467.6	441.2
Valuation allowance	(7.9)	(1.3)
Mortgage Loans on Real Estate [Member] | Pacific [Member]
Schedule of Investments [Line Items]
Other investments	117.0	102.3
Mortgage Loans on Real Estate [Member] | South Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	108.3	95.9
Mortgage Loans on Real Estate [Member] | West South Central [Member]
Schedule of Investments [Line Items]
Other investments	70.8	71.2
Mortgage Loans on Real Estate [Member] | Mid-Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	73.8	53.3
Mortgage Loans on Real Estate [Member] | New England [Member]
Schedule of Investments [Line Items]
Other investments	43.5	42.5
Mortgage Loans on Real Estate [Member] | East North Central [Member]
Schedule of Investments [Line Items]
Other investments	27.8	27.8
Mortgage Loans on Real Estate [Member] | Mountain [Member]
Schedule of Investments [Line Items]
Other investments	12.5	12.5
Mortgage Loans on Real Estate [Member] | Other Locations [Member]
Schedule of Investments [Line Items]
Other investments	21.8	37.0
Mortgage Loans on Real Estate [Member] | Office [Member]
Schedule of Investments [Line Items]
Other investments	160.4	138.2
Mortgage Loans on Real Estate [Member] | Apartments [Member]
Schedule of Investments [Line Items]
Other investments	128.3	125.1
Mortgage Loans on Real Estate [Member] | Retail [Member]
Schedule of Investments [Line Items]
Other investments	68.5	66.5
Mortgage Loans on Real Estate [Member] | Hotel [Member]
Schedule of Investments [Line Items]
Other investments	62.7	62.1
Mortgage Loans on Real Estate [Member] | Industrial [Member]
Schedule of Investments [Line Items]
Other investments	$ 55.6	$ 50.6