Investments (Fair Value Realized and Unrealized Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ 141.6	$ (128.3)	$ (386.4)	$ (124.2)
ITV
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	42.2	(111.8)	(429.7)	(87.8)
Lionsgate
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	8.5	(17.2)	(20.9)	(17.8)
SMAs
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	16.6	0.0	5.9	0.0
ITI Neovision
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	6.1	5.3	(1.9)	0.5
Other
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ 68.2	$ (4.6)	$ 60.2	$ (19.1)