Selected Balance Sheet Data, Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Public Utilities, Inventory [Line Items]
Inventories	$ 308,578	$ 343,916
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	208,090	200,888
Fuel
Public Utilities, Inventory [Line Items]
Inventories	88,287	104,499
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 12,201	$ 38,529

The period end amount for a type of inventory held by the utility in a schedule of inventories.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Inventory
-URI http://asc.fasb.org/extlink&oid=6516093

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-05.6(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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