Investments (Fair Value Realized and Unrealized Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ (22.6)	$ 59.0	$ (409.0)	$ (65.2)
ITV
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(20.5)	70.2	(450.2)	(17.6)
Lionsgate
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	13.1	(15.9)	(7.8)	(33.7)
SMAs
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(0.3)	0.0	5.6	0.0
ITI Neovision
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(2.2)	0.2	(4.1)	0.7
Other, net
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ (12.7)	$ 4.5	$ 47.5	$ (14.6)