v3.10.0.1
Debt - Carrying Values of Debt Instruments (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2013
Debt Instrument [Line Items]      
Principal $ 8,374,400,000 $ 8,281,700,000  
Adjustments (116,700,000) (94,300,000)  
Carrying Value 8,257,700,000 8,187,400,000  
Less: current portion (41,900,000) (706,700,000)  
Long-term portion 8,215,800,000 7,480,700,000  
Uncommitted Credit Facility      
Debt Instrument [Line Items]      
Principal 44,700,000.0 0  
Adjustments 0 0  
Carrying Value 44,700,000 0  
Credit facilities: June 2019      
Debt Instrument [Line Items]      
Principal 0 130,000,000  
Adjustments 0 0  
Carrying Value 0 130,000,000  
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 80,000,000 0  
Adjustments 0 0  
Carrying Value $ 80,000,000 0  
Senior notes: May 2018      
Debt Instrument [Line Items]      
Interest Rate 3.80%    
Principal $ 0 700,000,000  
Adjustments 0 (300,000)  
Carrying Value $ 0 699,700,000  
Senior notes: September 2019      
Debt Instrument [Line Items]      
Interest Rate 5.50%    
Principal $ 650,000,000 650,000,000  
Adjustments (1,500,000) (2,100,000)  
Carrying Value $ 648,500,000 647,900,000  
Senior notes: March 2020      
Debt Instrument [Line Items]      
Interest Rate 5.00%    
Principal $ 850,000,000 850,000,000  
Adjustments (1,300,000) (1,800,000)  
Carrying Value $ 848,700,000 848,200,000  
Senior notes: November 2021      
Debt Instrument [Line Items]      
Interest Rate 5.25%    
Principal $ 600,000,000 600,000,000  
Adjustments (1,400,000) (1,500,000)  
Carrying Value $ 598,600,000 598,500,000  
Senior notes: June 2022      
Debt Instrument [Line Items]      
Interest Rate 3.55%    
Principal $ 850,000,000 850,000,000  
Adjustments (4,100,000) (4,600,000)  
Carrying Value $ 845,900,000 845,400,000  
Senior notes: May 2023      
Debt Instrument [Line Items]      
Interest Rate 4.75%   4.75%
Principal $ 550,000,000 550,000,000  
Adjustments (10,000,000) (1,000,000)  
Carrying Value $ 540,000,000 549,000,000  
Senior notes: March 2025      
Debt Instrument [Line Items]      
Interest Rate 3.20%    
Principal $ 500,000,000 500,000,000  
Adjustments (4,500,000) (4,800,000)  
Carrying Value $ 495,500,000 495,200,000  
Senior notes: June 2026      
Debt Instrument [Line Items]      
Interest Rate 2.90%    
Principal $ 500,000,000 500,000,000  
Adjustments (4,700,000) (5,000,000)  
Carrying Value $ 495,300,000 495,000,000  
Senior notes: November 2027      
Debt Instrument [Line Items]      
Interest Rate 3.375%    
Principal $ 650,000,000 650,000,000  
Adjustments (6,400,000) (7,000,000)  
Carrying Value $ 643,600,000 643,000,000  
Senior notes: May 2028      
Debt Instrument [Line Items]      
Interest Rate 3.95%    
Principal $ 800,000,000 0  
Adjustments (18,100,000) 0  
Carrying Value $ 781,900,000 0  
Senior notes: March 2035      
Debt Instrument [Line Items]      
Interest Rate 6.086%    
Principal $ 181,900,000 181,900,000  
Adjustments (14,700,000) (14,900,000)  
Carrying Value $ 167,200,000 167,000,000  
Senior notes: March 2040      
Debt Instrument [Line Items]      
Interest Rate 6.20%    
Principal $ 399,900,000 399,900,000  
Adjustments (3,900,000) (3,900,000)  
Carrying Value $ 396,000,000 396,000,000  
Senior notes: May 2041      
Debt Instrument [Line Items]      
Interest Rate 5.70%    
Principal $ 385,700,000 385,700,000  
Adjustments (5,400,000) (5,500,000)  
Carrying Value $ 380,300,000 380,200,000  
Debentures: May 2021      
Debt Instrument [Line Items]      
Interest Rate 9.25%    
Principal $ 35,300,000 35,300,000  
Adjustments (800,000) (1,000,000)  
Carrying Value $ 34,500,000 34,300,000  
Debentures: September 2035      
Debt Instrument [Line Items]      
Interest Rate 7.40%    
Principal $ 148,100,000 148,100,000  
Adjustments (34,100,000) (34,500,000)  
Carrying Value 114,000,000 113,600,000  
Tax-exempt: 2019 - 2044      
Debt Instrument [Line Items]      
Principal 1,042,400,000 1,042,400,000  
Adjustments (5,800,000) (6,400,000)  
Carrying Value $ 1,036,600,000 1,036,000,000  
Tax-exempt: 2019 - 2044 | Minimum      
Debt Instrument [Line Items]      
Interest Rate 1.45%    
Tax-exempt: 2019 - 2044 | Maximum      
Debt Instrument [Line Items]      
Interest Rate 5.25%    
Capital leases: 2017 - 2046      
Debt Instrument [Line Items]      
Principal $ 106,400,000 108,400,000  
Adjustments 0 0  
Carrying Value $ 106,400,000 $ 108,400,000  
Capital leases: 2017 - 2046 | Minimum      
Debt Instrument [Line Items]      
Interest Rate 3.273%    
Capital leases: 2017 - 2046 | Maximum      
Debt Instrument [Line Items]      
Interest Rate 12.203%