Benefit Plans (Funded Status of Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	$ 1,081.4		$ 953.0
Service cost	13.5		11.4		11.9
Interest cost	51.6		52.0		52.3
Plan participants' contributions	0		0
Plan amendments	0	[1]	10.2	[1]
Additional benefit costs	0.1		0
Actuarial loss	135.4		126.2
ERRP proceeds	0		0
Gross benefits paid	(74.5)		(71.4)
Federal subsidy on other postretirement benefits paid	0		0
Net projected benefit obligation at December 31	1,207.5		1,081.4		953.0
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	897.4		823.0
Actual return on plan assets	126.9		28.9
Employer contributions	15.8		116.9
Plan participants' contributions	0		0
Gross benefits paid	(74.5)		(71.4)
Fair value of plan assets at December 31	965.6		897.4		823.0
Under funded status at December 31	(241.9)		(184.0)
Pension Plans, Defined Benefit [Member] | IPL [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	344.5		293.8
Service cost	7.5		6.1
Interest cost	17.1		16.7
Plan participants' contributions	0		0
Plan amendments	0	[1]	0	[1]
Actuarial loss	38.2		45.2
ERRP proceeds	0		0
Gross benefits paid	(18.9)		(17.3)
Federal subsidy on other postretirement benefits paid	0		0
Net projected benefit obligation at December 31	388.4		344.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	298.7		256.9
Actual return on plan assets	42.5		9.1
Employer contributions	0		50.0
Plan participants' contributions	0		0
Gross benefits paid	(18.9)		(17.3)
Fair value of plan assets at December 31	322.3		298.7
Under funded status at December 31	(66.1)		(45.8)
Pension Plans, Defined Benefit [Member] | WPL [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	332.5		283.3
Service cost	5.2		4.5
Interest cost	16.4		16.1
Plan participants' contributions	0		0
Plan amendments	0	[1]	0	[1]
Additional benefit costs	0.1		0
Actuarial loss	43.4		43.3
ERRP proceeds	0		0
Gross benefits paid	(17.4)		(14.7)
Federal subsidy on other postretirement benefits paid	0		0
Net projected benefit obligation at December 31	380.2		332.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	289.6		255.2
Actual return on plan assets	41.0		9.1
Employer contributions	0		40.0
Plan participants' contributions	0		0
Gross benefits paid	(17.4)		(14.7)
Fair value of plan assets at December 31	313.2		289.6
Under funded status at December 31	(67.0)		(42.9)
Other Postretirement Benefits Plans [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	224.2		274.9
Service cost	6.9	[2]	7.0	[2]	9.3	[2]
Interest cost	10.2	[2]	12.3	[2]	14.9	[2]
Plan participants' contributions	2.7		6.4
Plan amendments	0	[1]	(56.6)	[1]
Additional benefit costs	0		0
Actuarial loss	(1.6)		(0.8)
ERRP proceeds	0		0.6
Gross benefits paid	(19.2)		(20.8)
Federal subsidy on other postretirement benefits paid	0		1.2
Net projected benefit obligation at December 31	223.2		224.2		274.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	120.4		122.7
Actual return on plan assets	14.3		2.6
Employer contributions	4.9		9.5
Plan participants' contributions	2.7		6.4
Gross benefits paid	(19.2)		(20.8)
Fair value of plan assets at December 31	123.1		120.4		122.7
Under funded status at December 31	(100.1)		(103.8)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	97.5		128.5
Service cost	3.0	[2]	2.6	[2]	3.4	[2]
Interest cost	4.4	[2]	5.5	[2]	6.8	[2]
Plan participants' contributions	0.9		2.0
Plan amendments	0	[1]	(30.1)	[1]
Actuarial loss	(1.4)		(2.7)
ERRP proceeds	0		0.2
Gross benefits paid	(8.4)		(9.1)
Federal subsidy on other postretirement benefits paid	0		0.6
Net projected benefit obligation at December 31	96.0		97.5		128.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	74.7		77.5
Actual return on plan assets	9.4		1.4
Employer contributions	2.2		2.9
Plan participants' contributions	0.9		2.0
Gross benefits paid	(8.4)		(9.1)
Fair value of plan assets at December 31	78.8		74.7		77.5
Under funded status at December 31	(17.2)		(22.8)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in projected benefit obligation:
Net projected benefit obligation at January 1	89.6		103.3
Service cost	2.7	[2]	2.9	[2]	3.6	[2]
Interest cost	4.1	[2]	4.9	[2]	5.5	[2]
Plan participants' contributions	1.2		3.3
Plan amendments	0	[1]	(18.2)	[1]
Additional benefit costs	0		0
Actuarial loss	0.3		2.3
ERRP proceeds	0		0.2
Gross benefits paid	(8.8)		(9.6)
Federal subsidy on other postretirement benefits paid	0		0.5
Net projected benefit obligation at December 31	89.1		89.6		103.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	25.1		25.0
Actual return on plan assets	2.5		0.9
Employer contributions	2.3		5.5
Plan participants' contributions	1.2		3.3
Gross benefits paid	(8.8)		(9.6)
Fair value of plan assets at December 31	22.3		25.1		25.0
Under funded status at December 31	$ (66.8)		$ (64.5)

[1] Refer to “Net Periodic Benefit Costs” above for additional information regarding plan amendments to the defined benefit pension and other postretirement benefits plans in 2011.

[2] In 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 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 December 31, 2010.