Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	$ 504.7	$ 464.4
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	1.1	1.1
Other investments	3.5	3.5
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	168.5	1,020.7
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	168.5	1,020.7
Total financial instruments, excluding held-for-sale	594.2	1,439.6
Total financial instruments	597.0	1,442.9
Other investments	425.7	418.9
Debt	694.2	825.0
Estimated Fair Value [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	168.5	1,020.7
Total financial instruments, excluding held-for-sale	168.5	1,020.7
Total financial instruments	171.3	1,024.0
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial instruments, excluding held-for-sale	1.8	8.7
Total financial instruments	1.8	8.7
Other investments	1.8	8.7
Debt	694.2	825.0
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial instruments, excluding held-for-sale	423.9	410.2
Total financial instruments	423.9	410.2
Other investments	423.9	410.2
Estimated Fair Value [Member] | Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	2.8	3.3
Estimated Fair Value [Member] | Chaucer [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	$ 2.8	$ 3.3