Subsequent Events (Narrative) (Details) $ in Thousands	1 Months Ended		3 Months Ended		12 Months Ended
	May 31, 2017	Apr. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	May 04, 2017 USD ($)
Subsequent Event [Line Items]
Derivative, Cost of Hedge Net of Cash Received			$ 4,200			$ 4,600
Payments to acquire oil and gas property			$ 7,032	$ 0
Subsequent Event [Member]
Subsequent Event [Line Items]
Derivative, Cost of Hedge Net of Cash Received					$ 2,800
Payments to acquire oil and gas property		$ 9,800
7.50% Senior Notes [Member]
Subsequent Event [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			7.50%
Senior Secured Revolving Credit Facility [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 1,000,000
Line of Credit Facility, Current Borrowing Capacity			$ 600,000
Line of Credit Facility, Commitment Fee Percentage			0.50%
Senior Secured Revolving Credit Facility [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity							$ 2,000,000
Line of Credit Facility, Current Borrowing Capacity							900,000
Line of Credit Facility, Elected Borrowing Capacity							800,000
Line of Credit Facility, Commitment Fee Percentage	0.375%
Utilization of lender commitments	50.00%
Consolidated Cash Balance Threshold							$ 50,000
Credit Facility Availability Threshold							20.00%
Senior Secured Revolving Credit Facility [Member] | Maximum [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Ratio of total debt to EBITDA	4.00
Adjusted ratio of total debt to EBITDA							3.50