Investments (Fair Value Realized and Unrealized Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ (109.4)	$ (22.6)	$ 373.3	$ (409.0)
Univision Holdings Inc. (Univision)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	5.9	0.0	161.3	0.0
Plume Design, Inc. (Plume)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	78.7	0.0	133.8	29.6
Skillz Inc. (Skillz)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(104.6)	(0.1)	(83.3)	45.3
Aviatrix Systems, Inc. (Aviatrix)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	42.6	0.0	65.4	0.0
Lacework Inc. (Lacework)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	0.0	0.0	48.8	0.0
EdgeConneX Inc. (EdgeConneX)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	2.7	0.0	20.5	0.0
Lions Gate Entertainment Corp (Lionsgate)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(39.6)	13.1	18.0	(7.8)
ITV
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(118.6)	(20.5)	(9.1)	(450.2)
Other
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ 23.5	$ (15.1)	$ 17.9	$ (25.9)