Investments (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Schedule of Investments [Line Items]
Derivative assets	$ 3,273,000,000.0		$ 3,273,000,000.0		$ 2,496,700,000
Principal borrowings outstanding	26,977,300,000		26,977,300,000		29,315,300,000
ITV Collar Loan
Schedule of Investments [Line Items]
Derivative assets	697,100,000		697,100,000
Principal borrowings outstanding	1,330,200,000		1,330,200,000		1,379,600,000
Lionsgate Loan
Schedule of Investments [Line Items]
Derivative assets	39,200,000		39,200,000
Principal borrowings outstanding	$ 55,300,000		$ 55,300,000
VodafoneZiggo JV
Schedule of Investments [Line Items]
Percent of interest income earned on loan included in investment	100.00%		100.00%
Percent of remaining results of operations included in investment	50.00%		50.00%
Revenue	$ 1,096,900,000	$ 1,138,600,000	$ 3,275,300,000	$ 3,468,500,000
JV Services
Schedule of Investments [Line Items]
Revenue	66,100,000	$ 44,500,000	156,100,000	132,800,000
VodafoneZiggo JV Loan | VodafoneZiggo JV
Schedule of Investments [Line Items]
Due from related party	872,200,000		$ 872,200,000		916,100,000
Receivable interest rate			5.55%
Accrued interests settled in cash			$ 37,800,000
Equity Method Investee | VodafoneZiggo JV
Schedule of Investments [Line Items]
Transfer of assets			15,500,000	$ 12,300,000
Due from related party	$ 37,600,000		$ 37,600,000		$ 24,400,000