Investments (Narrative) (Details) - USD ($)		12 Months Ended
	Jan. 02, 2018	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities		$ 7,400,000	$ 11,000,000
Cash collateral maintained		102.00%
Equity securities, at fair value		$ 464,400,000	576,200,000
Equity securities, cost			433,700,000
Other investments		661,500,000	$ 562,700,000
Due in one year or less, Amortized Cost		$ 207,400,000
Concentration of investment in a single investee, maximum		10.00%	10.00%
Contractual investment commitments		$ 162,900,000
U.S. Government-Sponsored Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities		396,700,000	$ 396,700,000
Mortgage Participations [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities		356,800,000	356,800,000
Federal Home Loan Bank of Boston [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value		234,900,000	225,700,000
Security Lending Program [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value		4,800,000	23,400,000
US Treasury and Government Agencies [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value		299,900,000	286,900,000
Equity Securities [Member] | Retained Earnings [Member]
Schedule of Investments [Line Items]
Cumulative effect on retained earnings, before tax	$ 142,500,000
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments		405,700,000	365,800,000
Due in one year or less, Amortized Cost		0
Due in year two, Amortized Cost		10,000,000
Due in year three, Amortized Cost		57,400,000
Due in year four, Amortized Cost		28,100,000
Due in year five, Amortized Cost		17,000,000
Due in year six and thereafter, Amortized Cost			293,200,000
Limited Partnership [Member]
Schedule of Investments [Line Items]
Other investments		$ 235,700,000	$ 186,400,000