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Debt and Credit Agreements - Narrative (Details) - USD ($)
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
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    
Interest rate   2.30%