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Long-Term Debt and Commercial Paper (Senior Unsecured Notes and Credit Agreement) (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jan. 31, 2021
Feb. 29, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
May 21, 2020
Debt Instrument [Line Items]            
Letters of credit, amount outstanding     $ 39.7      
Senior Notes at Five Point Five Percent Due 2020 [Member]            
Debt Instrument [Line Items]            
Repayment of senior notes     350.0 $ 0.0 $ 0.0  
Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity under revolving credit facility     1,800.0      
Additional borrowing capacity under accordion feature of revolving credit facility     500.0      
Revolving credit facilities letter of credit sublimit     200.0      
Additional borrowing capacity under revolving credit facility     $ 1,800.0      
Leverage ratio, minimum threshold, current credit spread     2.0      
Leverage ratio, maximum threshold, current credit spread     3.25      
Leverage ratio, minimum threshold, increase in credit spread     3.25      
Impact on credit spread from increase in leverage ratio     0.125%      
Senior Notes [Member] | Senior Notes at Five Point Five Percent Due 2020 [Member]            
Debt Instrument [Line Items]            
Repayment of senior notes   $ 350.0        
Senior notes     $ 0.0 $ 350.0    
Percentage interest on debt instrument       5.50%    
Senior Notes [Member] | Senior Notes at Three Point Three Five Percent Due 2021 [Member]            
Debt Instrument [Line Items]            
Senior notes     $ 300.0 $ 300.0    
Percentage interest on debt instrument     3.35%      
Senior Notes [Member] | Senior Notes at Three Point Five Percent Due 2024 [Member]            
Debt Instrument [Line Items]            
Senior notes     $ 450.0 450.0    
Percentage interest on debt instrument     3.50%      
Senior Notes [Member] | Senior Notes at Four Point Five Percent Due 2025 [Member]            
Debt Instrument [Line Items]            
Senior notes     $ 450.0 450.0    
Percentage interest on debt instrument     4.50%      
Senior Notes [Member] | Senior Notes at Three Point Eight Percent Due 2027 [Member]            
Debt Instrument [Line Items]            
Senior notes     $ 300.0 300.0    
Percentage interest on debt instrument     3.80%      
Senior Notes [Member] | Senior Notes at Four Point Seven Five Percent Due 2030 [Member] [Member]            
Debt Instrument [Line Items]            
Senior notes     $ 500.0 $ 0.0   $ 500.0
Percentage interest on debt instrument     4.75%      
Senior notes, aggregate offering price percentage           99.479%
Minimum [Member] | Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Commitment fee on undrawn amounts (percent)     0.125%      
Maximum [Member] | Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Commitment fee on undrawn amounts (percent)     0.20%      
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Line of Credit [Member] | Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Basis spread on variable interest rates (percent)     1.125%      
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Line of Credit [Member] | Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Basis spread on variable interest rates (percent)     1.50%      
Base Rate [Member] | Minimum [Member] | Line of Credit [Member] | Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Basis spread on variable interest rates (percent)     0.125%      
Base Rate [Member] | Maximum [Member] | Line of Credit [Member] | Revolving Credit Facility Due 2025 [Member]            
Debt Instrument [Line Items]            
Basis spread on variable interest rates (percent)     0.50%      
Subsequent Event [Member] | Senior Notes [Member] | Senior Notes at Three Point Three Five Percent Due 2021 [Member]            
Debt Instrument [Line Items]            
Repayment of senior notes $ 300.0