Derivative Instruments - Net Long Natural Gas Contracts (Details) - Bcf	6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Total volumes	42	30	(9)
Cash Flow Hedges [Member]
Derivative [Line Items]
Total volumes	4	6	5
Not Designated as Hedges [Member]
Derivative [Line Items]
Total volumes	38	24	(14)
Short [Member] | Cash Flow Hedges [Member]
Derivative [Line Items]
Total volumes	(5)	(8)	(6)
Short [Member] | Not Designated as Hedges [Member]
Derivative [Line Items]
Total volumes	(3,092)	(2,930)	(3,089)
Long [Member] | Cash Flow Hedges [Member]
Derivative [Line Items]
Total volumes	9	14	11
Long [Member] | Not Designated as Hedges [Member]
Derivative [Line Items]
Total volumes	3,130	2,954	3,075