Related Parties (Amounts Billed Between Related Parties) (Details) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
ATC Billings to WPL [Member]
Amounts billed between related parties	$ 22.2	$ 22.7	$ 44.4	$ 45.5
WPL Billings to ATC [Member]
Amounts billed between related parties	$ 2.7	$ 2.9	$ 5.9	$ 5.4

Amount of transactions with related party during the financial reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 57
-Paragraph 2
-Subparagraph c

Name:	us-gaap_RelatedPartyTransactionAmountsOfTransaction
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration