Amount of liability for consideration received or receivable from customer which is not included in transaction price, when consideration is expected to be refunded to customer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 55
-Paragraph 25
-URI http://asc.fasb.org/extlink&oid=96862333&loc=SL49130609-203046-203046

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 55
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=96862333&loc=SL49130607-203046-203046

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 32
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82859839&loc=SL51751984-203043

Name:	us-gaap_ContractWithCustomerRefundLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Revenue Recognition - Miscellaneous items (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2018	Dec. 31, 2017
Revenue Recognition
Number of electric distribution cooperative members \| item	38
Number of services provided \| item	2
Margins for interest ratio	1.10
Achieved margins for interest ratio				1.14
Targeted margins for interest ratio			1.14
Refund liability \| $	$ 0	$ 0
Impairment losses on receivables \| $	$ 0	$ 0