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Debt (Prior Credit Facility) (Details) - Prior Credit Facility - USD ($)
12 Months Ended
Mar. 21, 2017
Dec. 31, 2016
Dec. 31, 2017
Mar. 31, 2016
Line Of Credit Facility [Line Items]        
Borrowing base amount       $ 550,000,000
Availability under the facility     $ 0  
Weighted average interest rate   5.25%    
Additional interest rate on outstanding amounts in event of default waived   2.00%    
Repayments of debt $ 444,440,000