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Long-term Debt (Details) - USD ($)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Borrowings      
Unamortized discount, debt issuance costs, and other $ (13,100,000)   $ (13,800,000)
Long-term debt 1,586,900,000   1,586,200,000
Borrowings under the facility 0 $ 263,000,000.0  
Letters of credit outstanding 25,900,000   26,300,000
3.75% senior notes due 2027      
Borrowings      
Long-term debt, carrying amount $ 600,000,000.0   $ 600,000,000.0
Interest rate (as a percent) 3.75%   3.75%
3.75% senior notes due 2027 | Significant Observable Inputs (Level 2)      
Borrowings      
Fair value of notes $ 571,900,000   $ 607,100,000
3.5% senior notes due 2024      
Borrowings      
Long-term debt, carrying amount $ 400,000,000.0   $ 400,000,000.0
Interest rate (as a percent) 3.50%   3.50%
3.5% senior notes due 2024 | Significant Observable Inputs (Level 2)      
Borrowings      
Fair value of notes $ 386,400,000   $ 403,700,000
3.75% senior notes due 2022      
Borrowings      
Long-term debt, carrying amount $ 350,000,000.0   $ 350,000,000.0
Interest rate (as a percent) 3.75%   3.75%
3.75% senior notes due 2022 | Significant Observable Inputs (Level 2)      
Borrowings      
Fair value of notes $ 349,600,000   $ 358,900,000
5.125% senior notes due 2020      
Borrowings      
Long-term debt, carrying amount $ 250,000,000.0   $ 250,000,000.0
Interest rate (as a percent) 5.125%   5.125%
5.125% senior notes due 2020 | Significant Observable Inputs (Level 2)      
Borrowings      
Fair value of notes $ 258,500,000   $ 264,800,000
Revolving credit facility      
Borrowings      
Borrowings under the facility 0    
Revolving credit facility 0   0
Availability under revolving line of credit 1,000,000,000.0   $ 1,000,000,000.0
Letter of credit | Credit Agreement      
Borrowings      
Maximum borrowing capacity 80,000,000.0    
Availability under revolving line of credit $ 55,900,000