Short-Term Borrowings (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		9 Months Ended
Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2012 Avista Corp [Member]	Sep. 30, 2012 Line Of Credit [Member] Ecova [Member]	Jul. 31, 2012 Line Of Credit [Member] Ecova [Member] Subsequent Event [Member]
Short-term Debt [Line Items]
Total amount of committed line of credit agreement	$ 400.0	$ 400.0		$ 60.0	$ 125.0
Expiration date			2017-02	2017-07
Maximum percentage ratio of consolidated total debt to consolidated total capitalization needed to comply with covenants	65.00%

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant