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Long-Term Debt - Revolving Credit Facility (Details) - Revolving Credit Agreement - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]        
Letters of credit $ 5,000,000      
Revolving credit facility maximum borrowing capacity       $ 500,000,000
Funds available for other borrowings 995,000,000      
Outstanding borrowings 0      
A&R Credit Agreement        
Line of Credit Facility [Line Items]        
Revolving credit facility maximum borrowing capacity $ 1,000,000,000 $ 1,000,000,000 $ 800,000,000