Derivatives (Basis Swap Derivative Contracts) (Details) - Natural Gas Basis Swap Derivative Contracts	12 Months Ended
	Dec. 31, 2018 MMBTU $ / MMBTU
Jul 19 - Dec 19
Derivative [Line Items]
Volume in MMbtu | MMBTU	460,000
Weighted average spread | $ / MMBTU	(0.93)
Jan 19 - Dec 19
Derivative [Line Items]
Volume in MMbtu | MMBTU	17,950,000
Weighted average spread | $ / MMBTU	(0.68)
Jan 20 - Mar 20
Derivative [Line Items]
Volume in MMbtu | MMBTU	910,000
Weighted average spread | $ / MMBTU	(0.49)
Jan 19 - Oct 19
Derivative [Line Items]
Volume in MMbtu | MMBTU	2,365,000
Weighted average spread | $ / MMBTU	(0.78)