Commitments and Contingencies, Legal Contingencies (Details) - Gas Trading Litigation [Member]	12 Months Ended
Dec. 31, 2011	Dec. 31, 2009
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	7	13
Loss Contingency, Claims Settled, Number		5
Loss Contingency, Claims Dismissed, Number	6	1
NSP-Wisconsin
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number		2

Number of pending claims pertaining to a loss contingency.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

Name:	us-gaap_LossContingencyPendingClaimsNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant