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Debt (Details) - USD ($)	Dec. 31, 2018	May 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal	$ 8,445,200,000		$ 8,281,700,000
Adjustments	(107,700,000)		(94,300,000)
Carrying Value	8,337,500,000		8,187,400,000
Less: current portion	(690,700,000)		(706,700,000)
Long-term portion	7,646,800,000		7,480,700,000
Credit facilities: Uncommitted facility
Debt Instrument [Line Items]
Principal	33,400,000		0
Adjustments	0		0
Carrying Value	33,400,000		0
Credit facilities: June 2019
Debt Instrument [Line Items]
Principal	0		130,000,000.0
Adjustments	0		0
Carrying Value	0		130,000,000.0
Credit facilities: May 2021
Debt Instrument [Line Items]
Principal	0		0
Adjustments	0		0
Carrying Value	0		0
Credit facilities: June 2023
Debt Instrument [Line Items]
Principal	159,000,000.0		0
Adjustments	0		0
Carrying Value	$ 159,000,000.0		0
Senior notes: May 2018
Debt Instrument [Line Items]
Interest Rate	3.80%	3.80%
Principal	$ 0	$ 700,000,000.0	700,000,000.0
Adjustments	0		(300,000)
Carrying Value	$ 0		699,700,000
Senior notes: September 2019
Debt Instrument [Line Items]
Interest Rate	5.50%
Principal	$ 650,000,000.0		650,000,000.0
Adjustments	(900,000)		(2,100,000)
Carrying Value	$ 649,100,000		647,900,000
Senior notes: March 2020
Debt Instrument [Line Items]
Interest Rate	5.00%
Principal	$ 850,000,000.0		850,000,000.0
Adjustments	(1,000,000.0)		(1,800,000)
Carrying Value	$ 849,000,000.0		848,200,000
Senior notes: November 2021
Debt Instrument [Line Items]
Interest Rate	5.25%
Principal	$ 600,000,000.0		600,000,000.0
Adjustments	(1,200,000)		(1,500,000)
Carrying Value	$ 598,800,000		598,500,000
Senior notes: June 2022
Debt Instrument [Line Items]
Interest Rate	3.55%
Principal	$ 850,000,000.0		850,000,000.0
Adjustments	(3,600,000)		(4,600,000)
Carrying Value	$ 846,400,000		845,400,000
Senior notes: May 2023
Debt Instrument [Line Items]
Interest Rate	4.75%
Principal	$ 550,000,000.0		550,000,000.0
Adjustments	(5,500,000)		(1,000,000.0)
Carrying Value	$ 544,500,000		549,000,000.0
Senior notes: March 2025
Debt Instrument [Line Items]
Interest Rate	3.20%
Principal	$ 500,000,000.0		500,000,000.0
Adjustments	(4,300,000)		(4,800,000)
Carrying Value	$ 495,700,000		495,200,000
Senior notes: June 2026
Debt Instrument [Line Items]
Interest Rate	2.90%
Principal	$ 500,000,000.0		500,000,000.0
Adjustments	(4,400,000)		(5,000,000.0)
Carrying Value	$ 495,600,000		$ 495,000,000.0
Senior notes: November 2027
Debt Instrument [Line Items]
Interest Rate	3.375%		3.375%
Principal	$ 650,000,000.0		$ 650,000,000.0
Adjustments	(5,900,000)		(7,000,000.0)
Carrying Value	$ 644,100,000		643,000,000.0
Senior notes: May 2028
Debt Instrument [Line Items]
Interest Rate	3.95%	3.95%
Principal	$ 800,000,000.0	$ 800,000,000.0	0
Adjustments	(17,300,000)		0
Carrying Value	$ 782,700,000		0
Senior notes: March 2035
Debt Instrument [Line Items]
Interest Rate	6.086%			6.09%
Principal	$ 181,900,000		181,900,000
Adjustments	(14,400,000)		(14,900,000)
Carrying Value	$ 167,500,000		167,000,000.0
Senior notes: March 2040
Debt Instrument [Line Items]
Interest Rate	6.20%			6.20%
Principal	$ 399,900,000		399,900,000
Adjustments	(3,800,000)		(3,900,000)
Carrying Value	$ 396,100,000		396,000,000.0
Senior notes: May 2041
Debt Instrument [Line Items]
Interest Rate	5.70%			5.70%
Principal	$ 385,700,000		385,700,000
Adjustments	(5,300,000)		(5,500,000)
Carrying Value	$ 380,400,000		380,200,000
Debentures: May 2021
Debt Instrument [Line Items]
Interest Rate	9.25%
Principal	$ 35,300,000		35,300,000
Adjustments	(700,000)		(1,000,000.0)
Carrying Value	$ 34,600,000		34,300,000
Debentures: September 2035
Debt Instrument [Line Items]
Interest Rate	7.40%			7.40%
Principal	$ 148,100,000		148,100,000
Adjustments	(33,800,000)		(34,500,000)
Carrying Value	114,300,000		113,600,000
Tax-exempt: 2019 - 2044
Debt Instrument [Line Items]
Principal	1,042,400,000		1,042,400,000
Adjustments	(5,600,000)		(6,400,000)
Carrying Value	$ 1,036,800,000		1,036,000,000.0
Tax-exempt: 2019 - 2044 \| Minimum
Debt Instrument [Line Items]
Interest Rate	1.97%
Tax-exempt: 2019 - 2044 \| Maximum
Debt Instrument [Line Items]
Interest Rate	2.50%
Capital leases: 2019 - 2046
Debt Instrument [Line Items]
Principal	$ 109,500,000		108,400,000
Adjustments	0		0
Carrying Value	$ 109,500,000		$ 108,400,000
Capital leases: 2019 - 2046 \| Minimum
Debt Instrument [Line Items]
Interest Rate	3.273%
Capital leases: 2019 - 2046 \| Maximum
Debt Instrument [Line Items]
Interest Rate	12.203%