Recently Issued or Adopted Accounting Pronouncements (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Recently Issued or Adopted Accounting Pronouncements
Regulatory assets	$ 594,908,000	$ 585,084,000
ASU 2016-01
Recently Issued or Adopted Accounting Pronouncements
Regulatory assets		$ 618,000

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

Name:	us-gaap_RegulatoryAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant