Oil and Gas Producing Activities (Unaudited) - Components of proved oil and gas reserves (Details) bbl in Thousands, MMcf in Thousands	Apr. 30, 2017 bbl MMcf	Dec. 31, 2016 bbl MMcf	Dec. 31, 2015 bbl MMcf	Dec. 31, 2014 bbl MMcf
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved developed reserves \| MMcf	0	4,214	4,813
Proved undeveloped reserves \| MMcf	0	0	22
Total proved reserves \| MMcf	0	4,214	4,835	5,611
Crude Oil [Member]
Reserve Quantities [Line Items]
Proved developed reserves \| bbl	0	187	223
Proved undeveloped reserves \| bbl	0	0	3
Total proved reserves \| bbl	0	187	226	318

The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-10.(a)(2))
-URI http://asc.fasb.org/extlink&oid=50486907&loc=d3e511914-122862

+ Details

Name:	us-gaap_ProvedDevelopedAndUndevelopedReservesNet
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61884-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61797-109447

+ Details

Name:	us-gaap_ProvedDevelopedReservesVolume
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

+ References

+ Details

Name:	us-gaap_ProvedUndevelopedReserveVolume
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ReserveQuantitiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ReserveQuantitiesByTypeOfReserveAxis=us-gaap_NaturalGasReservesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReserveQuantitiesByTypeOfReserveAxis=us-gaap_OilReservesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: