Supplemental Information on Oil and Gas Producing Activities (Unaudited) - Proved reserves (Details) MMcfe in Thousands	12 Months Ended
	Dec. 31, 2016 MMcfe Bcf MMBbls	Dec. 31, 2015 MMcfe Bcf MMBbls	Dec. 31, 2014 MMcfe Bcf MMBbls
Proved developed and undeveloped reserves:
Balance at the beginning of the period | MMcfe	13,215	12,683	7,632
Revisions | MMcfe	(404)	(1,801)	(1,054)
Extensions, discoveries and other additions | MMcfe	2,637	2,878	6,444
Production | MMcfe	(676)	(545)	(368)
Purchase of reserves | MMcfe	624		29
Sales of reserves in place | MMcfe	(10)
Proved Developed and Undeveloped Reserve, Net (Energy), Ending Balance | MMcfe	15,386	13,215	12,683
Oil and Gas Reserves
Proved developed reserves | MMcfe	6,914	5,838	3,803
Proved undeveloped reserves | MMcfe	8,472	7,377	8,880
Natural gas
Proved developed and undeveloped reserves:
Balance at the beginning of the period | Bcf	9,533	10,535	6,753
Revisions | Bcf	(2,069)	(2,816)	(1,025)
Extensions, discoveries and other additions | Bcf	1,990	2,253	5,095
Production | Bcf	(505)	(439)	(317)
Purchase of reserves | Bcf	475		29
Sales of reserves in place | Bcf	(10)
Balance at the end of the period | Bcf	9,414	9,533	10,535
Oil and Gas Reserves
Proved developed reserves | Bcf	4,426	3,627	3,285
Proved undeveloped reserves | Bcf	4,988	5,906	7,250
NGLS
Proved developed and undeveloped reserves:
Balance at the beginning of the period	587	330	137
Revisions	275	176	(6)
Extensions, discoveries and other additions	99	97	206
Production	(27)	(16)	(7)
Purchase of reserves	23
Balance at the end of the period	957	587	330
Oil and Gas Reserves
Proved developed reserves	401	360	80
Proved undeveloped reserves	556	227	250
Oil and condensate
Proved developed and undeveloped reserves:
Balance at the beginning of the period	26	28	10
Revisions	3	(8)
Extensions, discoveries and other additions	9	8	19
Production	(2)	(2)	(1)
Purchase of reserves	2
Balance at the end of the period	38	26	28
Oil and Gas Reserves
Proved developed reserves	13	8	6
Proved undeveloped reserves	25	18	22