Debt (Credit Facility) (Details)	6 Months Ended
	Jun. 30, 2019 USD ($) qtr	Mar. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Borrowing base amount	$ 750,000,000
Current borrowing capacity	$ 325,000,000	$ 325,000,000
Credit Facility
Line of Credit Facility [Line Items]
Number of consecutive quarters | qtr	4
Low | Credit Facility
Line of Credit Facility [Line Items]
Current Ratio covenant	1.00
High | Credit Facility
Line of Credit Facility [Line Items]
Ratio of debt to EBITDAX	4.0
Adjusted LIBO Rate | Credit Facility
Line of Credit Facility [Line Items]
Weighted average interest rate	4.66%