Derivative Instruments (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) tranche shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 22, 2018 USD ($)
Derivative [Line Items]
Gain (loss) on change in credit risk valuation included in realized and unrealized gains (losses) on derivative instruments, net		$ 16.6	$ (71.1)	$ 168.4
Assets - derivative instruments		1,827.5	2,496.7
Principal borrowing outstanding		1,582.7	$ 1,370.7
Virgin Media
Derivative [Line Items]
Notional amount of derivative instruments without exchange of notional amounts at inception and maturity		3,400.0
Sumitomo Collar
Derivative [Line Items]
Aggregate market value of borrowed shares					$ 159.3
Counterparty Credit Risk
Derivative [Line Items]
Assets - derivative instruments		$ 253.2
ITV Collar
Derivative [Line Items]
Number of common stock shares owned (in shares) | shares		398,515,510
Shares needed to borrow from custody account to hedge exposure (in shares) | shares		390,000,000
Derivative asset fair value		$ 619.6
Principal borrowing outstanding		1,435.5
Lionsgate Loan
Derivative [Line Items]
Principal borrowing outstanding	$ 27.6	$ 55.3
Lionsgate Loan | Common Stock
Derivative [Line Items]
Number of common stock shares owned (in shares) | shares	833,334	1,250,000
Derivative asset fair value		$ 37.2
Lionsgate Loan | Nonvoting Common Stock
Derivative [Line Items]
Number of common stock shares owned (in shares) | shares	833,333	1,250,000
Lionsgate Forward
Derivative [Line Items]
Derivative asset fair value	$ 18.1
Number of tranches settled | tranche	25