Investments (Fair Value Realized and Unrealized Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ (557.3)	$ (323.5)	$ 820.6
Vodafone
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(362.4)	0.0	0.0
Lacework
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(148.6)	(26.3)	223.9
EdgeConneX
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	122.3	43.4	28.9
Plume Design, Inc. (Plume)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(77.8)	(55.4)	219.5
ITV
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(40.5)	(233.9)	15.3
Lionsgate
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	32.9	(69.2)	33.9
SMAs
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(26.4)	(49.1)	(10.1)
Aviatrix Systems, Inc. (Aviatrix)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(22.7)	0.0	65.4
Televisa Univision
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(9.9)	23.1	301.6
Pax8
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	1.3	79.3	0.0
Skillz Inc. (Skillz)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	0.0	(34.7)	(100.4)
TiBiT Communications, Inc. (TiBiT)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	0.0	26.4	0.0
Other, net
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(25.5)	(27.1)	42.6
Gain on sale of investment	$ 8.0	$ 15.7	$ 12.9