v3.10.0.1
Debt - Narrative (Details)
1 Months Ended 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
extension
May 31, 2018
USD ($)
Jun. 30, 2018
USD ($)
extension
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
extension
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2013
Debt Instrument [Line Items]                  
Balance availability under Credit Facilities $ 1,745,300,000   $ 1,745,300,000   $ 1,745,300,000   $ 1,639,100,000    
Line of credit facility amount outstanding 80,000,000   80,000,000   80,000,000   130,000,000    
Letters of credit utilizing availability under Credit Facilities 406,500,000   406,500,000   406,500,000   462,700,000    
Long-term debt 8,257,700,000   8,257,700,000   8,257,700,000   8,187,400,000    
Principal amount 8,374,400,000   8,374,400,000   $ 8,374,400,000   8,281,700,000    
Percentage of tax exempt financings         100.00%        
Net interest income related to net swap settlements     500,000 $ 1,300,000 $ 1,400,000 $ 2,600,000      
Gains (losses) on the change in fair value of the hedged senior notes     2,500,000 (1,400,000) 9,200,000 200,000      
Gains (losses) on the related interest rate swaps     (2,700,000) 1,600,000 (9,500,000) 200,000      
Interest Rate Locks                  
Debt Instrument [Line Items]                  
Terminated derivative, notional amount   $ 600,000,000              
Notional value of swap agreements $ 450,000,000   $ 450,000,000   $ 450,000,000        
Derivative, interest rate floor 1.90%   1.90%   1.90%        
Derivative, interest rate cap 2.95%   2.95%   2.95%        
Accumulated and other comprehensive income (loss), net of tax $ (12,200,000)   $ (12,200,000) (3,400,000) $ (12,200,000) (3,400,000)      
Effective portion of interest rate locks amortized as an adjustment to interest expense     300,000 $ 700,000 800,000 $ 1,300,000      
Interest Rate Locks | Fair Value, Measurements, Recurring                  
Debt Instrument [Line Items]                  
Interest rate swaps - other assets 23,900,000   23,900,000   23,900,000   19,100,000    
Interest Rate Locks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2                  
Debt Instrument [Line Items]                  
Interest rate swaps - other assets 23,900,000   23,900,000   23,900,000   19,100,000    
Interest Rate Swap                  
Debt Instrument [Line Items]                  
Notional value of swap agreements 300,000,000   300,000,000   300,000,000        
Interest rate swaps at fair value (1,500,000)   (1,500,000)   (1,500,000)   8,000,000    
Expected amortization of yield adjustment on senior notes         (600,000)        
Interest Rate Swap | Fair Value, Measurements, Recurring                  
Debt Instrument [Line Items]                  
Interest rate swaps - other assets             8,000,000    
Interest Rate Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2                  
Debt Instrument [Line Items]                  
Interest rate swaps - other assets             8,000,000    
Line Of Credit Due May 2021                  
Debt Instrument [Line Items]                  
Maximum borrowing capacity               $ 1,000,000,000  
Line Of Credit Due June 2019                  
Debt Instrument [Line Items]                  
Maximum borrowing capacity               $ 1,250,000,000.00  
Long-term debt 0   0   0   130,000,000    
Principal amount 0   0   0   130,000,000    
Uncommitted Revolver                  
Debt Instrument [Line Items]                  
Long-term debt 44,700,000   44,700,000   44,700,000   0    
Principal amount 44,700,000.0   44,700,000.0   44,700,000.0   0    
Senior notes: May 2028                  
Debt Instrument [Line Items]                  
Long-term debt 781,900,000   781,900,000   781,900,000   0    
Principal amount $ 800,000,000   $ 800,000,000   $ 800,000,000   0    
Interest rate 3.95%   3.95%   3.95%        
Senior notes: May 2028 | Interest Rate Locks                  
Debt Instrument [Line Items]                  
Proceeds from retirement of certain hedging relationships, net of fees paid to issue senior notes   $ 31,100,000.0              
Senior notes: May 2018                  
Debt Instrument [Line Items]                  
Long-term debt $ 0   $ 0   $ 0   699,700,000    
Principal amount $ 0   $ 0   $ 0   700,000,000    
Interest rate 3.80%   3.80%   3.80%        
Tax Exempt Bonds And Other Tax Exempt Financings                  
Debt Instrument [Line Items]                  
Long-term debt $ 1,036,600,000   $ 1,036,600,000   $ 1,036,600,000   1,036,000,000    
Principal amount 1,042,400,000   1,042,400,000   1,042,400,000   1,042,400,000    
Capital Lease Obligations                  
Debt Instrument [Line Items]                  
Principal amount 106,400,000   106,400,000   106,400,000   108,400,000    
Capital lease liabilities 106,400,000   106,400,000   106,400,000   108,400,000    
Senior Notes $550.0 million 4.750% Due May 2023                  
Debt Instrument [Line Items]                  
Long-term debt 540,000,000   540,000,000   540,000,000   549,000,000    
Principal amount $ 550,000,000   $ 550,000,000   $ 550,000,000   550,000,000    
Interest rate 4.75%   4.75%   4.75%       4.75%
Senior Notes $500.0 million 3.20% Due March 2025                  
Debt Instrument [Line Items]                  
Long-term debt $ 495,500,000   $ 495,500,000   $ 495,500,000   495,200,000    
Principal amount $ 500,000,000   $ 500,000,000   $ 500,000,000   500,000,000    
Interest rate 3.20%   3.20%   3.20%        
Senior Notes $500.0 million 3.20% Due March 2025 | Interest Rate Locks                  
Debt Instrument [Line Items]                  
Accumulated and other comprehensive income (loss), net of tax $ 11,600,000   $ 11,600,000   $ 11,600,000   $ (11,800,000)    
Line of Credit | Revolving Credit Facility                  
Debt Instrument [Line Items]                  
Maximum borrowing capacity $ 2,250,000,000.00   $ 2,250,000,000.00   $ 2,250,000,000.00        
Number of available extensions | extension 2   2   2        
Extension period 1 year                
Facility feature, increase limit $ 1,000,000,000.0   $ 1,000,000,000.0   $ 1,000,000,000.0