Benefit Plans (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 9.3
Intrinsic value	1.3
Minimum rating of amendment in participants interest credit				3.00%
Total plan assets percentage on common stock	less than 1%
Common stock percentage in assets held saving plans	14.60%		12.90%
Percentage of base salary and incentive compensation	100.00%
OIP authorizes common stock issuance	240,000,000		240,000,000
IPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
OIP authorizes common stock issuance	24,000,000		24,000,000
WPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
OIP authorizes common stock issuance	18,000,000		18,000,000
Omnibus Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
OIP authorizes common stock issuance	4,300,000
Defined Benefit Pension Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	10.2
Expected rate of return on plan assets	7.90%		8.00%	8.25%
Discount rate for net periodic cost	5.56%		5.80%	6.15%
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments				0.3
Defined Benefit Pension Plans [Member] | IPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	6.3
Expected rate of return on plan assets	7.90%		8.00%	8.25%
Rate of compensation increase	3.50%		3.50%	3.50%
Discount rate for net periodic cost	5.70%		5.80%	6.15%
Defined Benefit Pension Plans [Member] | WPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	3.4
Expected rate of return on plan assets	7.90%		8.00%	8.25%
Rate of compensation increase	3.50%		3.50%	3.50%
Settlement losses				0.9
Discount rate for net periodic cost	5.70%		5.80%	6.15%
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments				0.7	[1]
Other Postretirement Benefits Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected rate of return on plan assets	7.00%		6.90%	8.25%
Rate of compensation increase	3.50%		3.50%	3.50%
Discount rate for net periodic cost	5.25%		5.55%	6.15%
Other Postretirement Benefits Plans [Member] | IPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected rate of return on plan assets	7.30%		7.10%	8.25%
Rate of compensation increase	3.50%		3.50%	3.50%
Discount rate for net periodic cost	5.25%		5.55%	6.15%
Other Postretirement Benefits Plans [Member] | WPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected rate of return on plan assets	6.30%		6.30%	8.25%
Rate of compensation increase	3.50%		3.50%	3.50%
Discount rate for net periodic cost	5.25%		5.55%	6.15%
Cash Balance Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	10.2
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	2.8	[2]
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Curtailments				(0.1)
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	0.7	[2],[3]
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost recognized over a weighted average period	2
Weighted average remaining contractual term	2
Maximum [Member] | Defined Benefit Pension Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Rate of compensation increase	4.50%		4.50%	4.50%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost recognized over a weighted average period	1
Weighted average remaining contractual term	1
Minimum [Member] | Defined Benefit Pension Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Rate of compensation increase	3.50%		3.50%	3.50%
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares from vested performance shares	1,387			51,189
Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance multiplier range, lower	0.00%
Performance multiplier range, upper	200.00%
U.S. Currency [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Securities lending initial collateral levels	102.00%
Foreign Currency [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Securities lending initial collateral levels	105.00%
Other Securities [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Securities lending initial collateral levels	102.00%
Allocated From Corporate Services [Member] | IPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated amortizations	1.9
Allocated From Corporate Services [Member] | WPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Estimated amortizations	1.2
Allocated From Corporate Services [Member] | Defined Benefit Pension Plans [Member] | IPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	3.5
Allocated From Corporate Services [Member] | Defined Benefit Pension Plans [Member] | WPL [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Additional Payments To Certain Plan Participants	$ 2.7
Affiliated Borrowers [Member] | U.S. Currency [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Securities lending initial collateral levels	102.00%
Non-Affiliated Borrowers [Member] | U.S. Currency [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Securities lending initial collateral levels	100.00%

[1] In 2007, members of the International Brotherhood of Electrical Workers Local 965 ratified a four-year collective bargaining agreement reached with WPL, resulting in changes to WPL's qualified pension plan. One of these changes provided WPL qualified pension plan participants an option to cease participating in the WPL qualified pension plan and begin participating in the Alliant Energy 401(k) Savings Plan with increased levels of contribution by Alliant Energy. The election of this option did not impact a participant's eligibility for benefits previously vested under the WPL qualified pension plan. In 2009, certain of these employees elected to cease participating in the WPL qualified pension plan, resulting in Alliant Energy and WPL recognizing a curtailment loss related to the WPL qualified pension plan of $0.7 million in 2009.

[2] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan in 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in 2011 resulting in aggregate additional benefits of $10.2 million paid to certain former participants in the Cash Balance Plan in 2011. Alliant Energy recognized $10.2 million ($6.3 million at IPL ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) and $3.4 million at WPL ($0.7 million directly assigned and $2.7 million allocated by Corporate Services)) of additional benefits costs in 2011 related to these benefits. Refer to Note 13(b) for additional information regarding the Cash Balance Plan.

[3] In May 2011, Alliant Energy, IPL and WPL amended their defined benefit postretirement health care plans resulting in a revision to the method and level of coverage provided for participants more than 65 years of age. This amendment was determined to be a significant event, which required Alliant Energy, IPL and WPL to remeasure their defined benefit postretirement health care plans in May 2011. The amendment resulted in a decrease in Alliant Energy's, IPL's and WPL's postretirement benefit obligations of $55 million, $30 million and $16 million, respectively, in 2011 with the impact of the remeasurement on net periodic benefit costs being recognized prospectively from the remeasurement date. The impact of the remeasurement decreased Alliant Energy's, IPL's and WPL's net periodic benefit costs by $11.3 million, $7.2 million, and $3.8 million in 2011, respectively. The discount rate used for the remeasurement was 5.20%. All other assumptions used for the remeasurement were consistent with the measurement assumptions used at Dec. 31, 2010.