Investments (Fair Value Realized and Unrealized Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ 114.9	$ (5.5)
EdgeConneX
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	71.8	11.9
Lacework
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(67.6)	(21.4)
ITV
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	50.0	45.1
Vodafone
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	48.2	(37.4)
SMAs
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	19.3	(14.5)
Aviatrix Systems, Inc. (Aviatrix)
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(14.3)	0.0
Lionsgate
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(6.0)	34.1
Plume
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	(1.1)	(17.5)
Other, net
Net Investment Income [Line Items]
Realized and unrealized gains (losses)	$ 14.6	$ (5.8)