Decommission fund to pay for the costs of decontaminating and decommissioning of facilities through collection of revenues derived from utility assessments and government appropriations. Decommission fund investment for the process whereby a power station, at the end of its economic life, is taken permanently out of service and its site made available for other purposes. In the case of a nuclear station this comprises three different states of clearance. Immediately after the final closure, radioactive material such as nuclear fuel and operational waste is removed and the buildings surrounding the reactor shield are dismantled and finally the reactor itself is dismantled.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

Accounting for Asset Retirement Obligations (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)
Accounting For Asset Retirement Obligations [Line Items]
Number of External Trust Funds with Independent Trustee with Legal Restrictions To Settling ARO	6
Decommissioning Fund Investments	$ 286,866	$ 255,708
Asset Retirement Obligation, Revision of Estimate	$ 3,461	0	$ 0
Jointly Owned Nuclear Power Plant
Accounting For Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Revision of Estimate		$ 3,500		$ 1,900