Note 4 - Fair Value Measurements (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Mar. 31, 2011
Weather Derivative Premium	$ 3	$ 1
Medium-term Notes, Current	15
First Mortgage Bonds Noncurrent	$ 500

Carrying value at the balance sheet date of the current portion of a financial instrument used to receive debt funding on a regular basis with original maturities ranging from 5-10 years.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20

-Article 5

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 210.5-02.20)

-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 03

-Paragraph 16

-Article 9

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