Benefit Plans Regulatory Asset (Details) - Postretirement pensions and other medical benefits [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Funded status of obligation	$ 71,297	$ 71,273
Accrued benefit cost	(6,239)	(6,587)
Amount to be recovered in future rates	65,058	64,686
Tax gross-up	44,737	44,482
Regulatory asset	$ 109,795	$ 109,168

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