Significant Accounting Policies - Dependence on Major Customers (Details) - Oil and gas sales - Customer concentration	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valero Energy Corp.
Dependence on Major Customers
Concentration risk (as a percent)	18.00%	22.00%	13.00%
ETC Field Services LLC
Dependence on Major Customers
Concentration risk (as a percent)	17.00%
NGL Energy Partners LP
Dependence on Major Customers
Concentration risk (as a percent)	15.00%	12.00%
PVR Midstream
Dependence on Major Customers
Concentration risk (as a percent)		12.00%	15.00%
Plains Marketing
Dependence on Major Customers
Concentration risk (as a percent)	16.00%	10.00%	6.00%
Monarch Natural Gas, LLC
Dependence on Major Customers
Concentration risk (as a percent)		10.00%
Unimark LLC
Dependence on Major Customers
Concentration risk (as a percent)	7.00%		13.00%
Mercuria
Dependence on Major Customers
Concentration risk (as a percent)			13.00%