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Fair-Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Jul. 24, 2015
Jul. 23, 2015
Fair Value Disclosures [Abstract]        
Senior unsecured notes $ 1,813,169 $ 1,807,054    
Fair value of debt $ 1,900,000 $ 1,500,000    
Enterprise Offshore Acquisition [Member] | SEKCO [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity interest step up acquired percentage 100.00% 50.00% 100.00% 50.00%
Enterprise Offshore Acquisition [Member] | CHOPS [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Equity interest step up acquired percentage   50.00% 100.00% 50.00%