Debt (Senior Notes and Debentures) (Details) - USD ($)	8 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 31, 2018
Debt Instrument [Line Items]
Principal amount	$ 8,445,200,000	$ 8,445,200,000	$ 8,281,700,000
Combined aggregate principal debentures authorized for purchase				$ 575,400,000
Payment of premiums due on debt instruments				148,100,000
Early tender consideration for purchase of senior notes				$ 28,700,000
Senior notes: May 2028
Debt Instrument [Line Items]
Principal amount	$ 800,000,000.0	$ 800,000,000.0	0		$ 800,000,000.0
Interest rate	3.95%	3.95%			3.95%
Senior notes: May 2018
Debt Instrument [Line Items]
Principal amount	$ 0	$ 0	700,000,000.0		$ 700,000,000.0
Interest rate	3.80%	3.80%			3.80%
Senior notes: November 2027
Debt Instrument [Line Items]
Principal amount	$ 650,000,000.0	$ 650,000,000.0	$ 650,000,000.0
Interest rate	3.375%	3.375%	3.375%
Senior notes: March 2040
Debt Instrument [Line Items]
Principal amount	$ 399,900,000	$ 399,900,000	$ 399,900,000
Interest rate	6.20%	6.20%		6.20%
Senior notes: May 2041
Debt Instrument [Line Items]
Principal amount	$ 385,700,000	$ 385,700,000	385,700,000
Interest rate	5.70%	5.70%		5.70%
Debentures: September 2035
Debt Instrument [Line Items]
Principal amount	$ 148,100,000	$ 148,100,000	148,100,000
Interest rate	7.40%	7.40%		7.40%
Senior notes: March 2035
Debt Instrument [Line Items]
Principal amount	$ 181,900,000	$ 181,900,000	181,900,000
Interest rate	6.086%	6.086%		6.09%
Senior notes: 2026
Debt Instrument [Line Items]
Principal amount				$ 500,000,000.0
Interest rate				2.90%
Interest Rate Locks
Debt Instrument [Line Items]
Amount of ineffectiveness on cash flow hedges		$ 0	$ 0	$ 0
Terminated derivative, notional amount	$ 600,000,000.0
Interest Rate Locks | Senior notes: May 2028
Debt Instrument [Line Items]
Amount of ineffectiveness on cash flow hedges	0
Proceeds from issuance and retirement senior notes and certain hedging relationships	$ 31,100,000