Benefit Plans Regulatory Asset (Details) - Postretirement pensions and other medical benefits [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Funded status of obligation	$ 74,046	$ 71,297
Accrued benefit cost	(5,490)	(6,239)
Amount to be recovered in future rates	68,556	65,058
Tax gross-up	0	44,737
Regulatory asset	$ 68,556	$ 109,795

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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