Investments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Other investments	$ 796.9	$ 786.1
Concentration of investment in a single investee, maximum	10.00%	10.00%
Contractual investment commitments	$ 199.4
U.S. Government
Schedule of Investments [Line Items]
Marketable securities, fixed maturities	371.4	$ 388.6
Mortgage Participations [Member]
Schedule of Investments [Line Items]
Due in current fiscal year, amortized cost	42.8
Due in year one, amortized cost	90.5
Due in year two, amortized cost	51.9
Due in year three, amortized cost	30.1
Due in year four and thereafter, amortized cost	156.1
U.S. Treasury and Government Agencies [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value	301.6	281.7
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	371.4	388.6
Limited Partnerships [Member]
Schedule of Investments [Line Items]
Other investments	415.6	387.9
Federal Home Loan Bank of Boston [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value	$ 153.0	$ 132.5