Investments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Other investments	$ 793.2	$ 733.2
Concentration of investment in a single investee, maximum	10.00%	10.00%
Contractual investment commitments	$ 205.3
U.S. Government-Sponsored Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities	459.3	$ 432.3
Mortgage Participations [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities	459.3	432.3
U.S. Treasury and Government Agencies [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value	315.2	308.7
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	467.6	441.2
Due in year two, amortized cost	57.5
Due in year three, amortized cost	27.7
Due in year four, amortized cost	16.5
Due in year five, amortized cost	77.1
Due in year six and thereafter, amortized cost	288.8
Limited Partnerships [Member]
Schedule of Investments [Line Items]
Other investments	312.5	278.5
Federal Home Loan Bank of Boston [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value	$ 103.9	$ 94.0