Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 15.7		$ 13.5		$ 11.4
Interest cost	49.0		51.6		52.0
Expected return on plan assets	(74.0)	[1]	(68.8)	[1]	(63.8)	[1]
Amortization of prior service cost (credit)	0.2	[2]	0.3	[2]	0.7	[2]
Amortization of actuarial loss	36.2	[3]	33.3	[3]	21.1	[3]
Additional benefit costs	9.0	[4],[5]	0.1	[4],[5]	10.2	[4],[5]
Settlement losses	0	[6]	5.4	[6]	1.1	[6]
Total	36.1		35.4		32.7
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6.3		6.9		7.0
Interest cost	8.5		10.2		12.3
Expected return on plan assets	(8.1)	[1]	(7.5)	[1]	(7.9)	[1]
Amortization of prior service cost (credit)	(11.9)	[2]	(12.0)	[2]	(10.0)	[2]
Amortization of actuarial loss	4.9	[3]	6.3	[3]	5.3	[3]
Additional benefit costs	0	[4],[5]	0	[4],[5]	0	[4],[5]
Settlement losses	0	[6]	0	[6]	0	[6]
Total	(0.3)		3.9		6.7
Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	9.0				10.2
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	8.6		7.5		6.1
Interest cost	22.9		24.1		24.0
Expected return on plan assets	(35.2)	[1]	(32.6)	[1]	(29.7)	[1]
Amortization of prior service cost (credit)	0.1	[2]	0.2	[2]	0.3	[2]
Amortization of actuarial loss	15.2	[3]	14.1	[3]	8.7	[3]
Additional benefit costs	2.6	[4],[5]	0	[4],[5]	2.8	[4],[5]
Total	14.2		13.3		12.2
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.9		3.0		2.6
Interest cost	3.6		4.4		5.5
Expected return on plan assets	(5.6)	[1]	(5.1)	[1]	(5.4)	[1]
Amortization of prior service cost (credit)	(6.3)	[2]	(6.3)	[2]	(5.0)	[2]
Amortization of actuarial loss	2.7	[3]	3.5	[3]	2.9	[3]
Additional benefit costs	0	[4],[5]	0	[4],[5]	0	[4],[5]
Total	(2.7)		(0.5)		0.6
IPL [Member] | Directly assigned defined benefit pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	2.6				2.8
IPL [Member] | Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	5.5				6.3
IPL [Member] | Corporate Service Allocation [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	2.9				3.5
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5.9		5.2		4.5
Interest cost	20.7		21.6		21.6
Expected return on plan assets	(31.9)	[1]	(29.6)	[1]	(27.3)	[1]
Amortization of prior service cost (credit)	0.3	[2]	0.4	[2]	0.3	[2]
Amortization of actuarial loss	17.1	[3]	15.7	[3]	10.1	[3]
Additional benefit costs	0.6	[4],[5]	0.1	[4],[5]	0.7	[4],[5]
Total	12.7		13.4		9.9
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.5		2.7		2.9
Interest cost	3.4		4.1		4.9
Expected return on plan assets	(1.3)	[1]	(1.3)	[1]	(1.3)	[1]
Amortization of prior service cost (credit)	(3.9)	[2]	(3.9)	[2]	(3.4)	[2]
Amortization of actuarial loss	1.9	[3]	2.3	[3]	2.1	[3]
Additional benefit costs	0	[4],[5]	0	[4],[5]	0	[4],[5]
Total	2.6		3.9		5.2
WPL [Member] | Directly assigned defined benefit pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	0.6				0.7
WPL [Member] | Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	2.8				3.4
WPL [Member] | Corporate Service Allocation [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs	$ 2.2				$ 2.7

[1] The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.

[2] Unrecognized prior service costs (credits) for the postretirement benefits plans are amortized over the average future service period to full eligibility of the participants of each plan.

[3] Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Cash Balance Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.

[4] 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 by Alliant Energy to certain former participants in the Cash Balance Plan in 2011. Alliant Energy recognized $10.2 million of additional benefits costs in 2011 related to these benefits. IPL recognized $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) and WPL recognized $3.4 million ($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 16(c) for additional information regarding the Cash Balance Plan.

[5] In 2013, Alliant Energy filed a stipulation agreement with the Court related to the class-action lawsuit against the Cash Balance Plan. As a result, Alliant Energy recognized $9.0 million of additional benefits costs in 2013 related to the agreement. IPL recognized $5.5 million ($2.6 million directly assigned and $2.9 million allocated by Corporate Services) and WPL recognized $2.8 million ($0.6 million directly assigned and $2.2 million allocated by Corporate Services) of additional benefits costs in 2013 related to the agreement. Refer to Note 16(c) for additional information regarding the Cash Balance Plan.

[6] Settlement losses related to payments made to retired executives of Alliant Energy.