Operating Segment and Related Information - Long-term investments, Property and Equipment and Assets (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Investments	$ 4,056.8	$ 4,392.1
Total Assets	6,339.1	6,958.3
Operating Segments | Construction
Revenues from External Customers and Long-Lived Assets [Line Items]
Investments	0.9	0.9
Total Assets	557.3	530.4
Operating Segments | Insurance
Revenues from External Customers and Long-Lived Assets [Line Items]
Investments	4,090.0	4,423.0
Total Assets	5,264.4	5,611.9
Operating Segments | Life Science [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Investments	21.0	22.0
Total Assets	24.8	28.4
Operating Segments | Broadcasting
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Assets	257.7	257.9
Operating Segments | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Investments	42.9	43.1
Total Assets	44.2	366.3
Operating Segments | Energy
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Assets	144.1	142.8
Operating Segments | Telecommunications
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Assets	123.4	89.3
Corporate
Revenues from External Customers and Long-Lived Assets [Line Items]
Total Assets	20.7	27.2
Intersegment Eliminations
Revenues from External Customers and Long-Lived Assets [Line Items]
Investments	(98.0)	(96.9)
Total Assets	$ (97.5)	$ (95.9)