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Debt and Credit Agreements - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Apr. 11, 2017
Debt Instrument [Line Items]    
Change in interest rate during period 0.00%  
Revolving credit facility    
Debt Instrument [Line Items]    
Borrowing base   $ 3,200,000,000.0
Remaining borrowing capacity $ 1,700,000,000 $ 1,700,000,000
Line of credit $ 0