Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 15.9	$ 13.1	$ 15.7
Interest cost		53.6	54.1	49.0
Expected return on plan assets	[1]	(75.0)	(74.9)	(74.0)
Amortization of prior service cost (credit)		(0.2)	0.0	0.2
Amortization of actuarial loss	[2]	35.4	19.5	36.2
Additional benefit costs		0.5	0.0	9.0	[3]
Total		30.2	11.8	36.1
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		5.5	5.2	6.3
Interest cost		9.1	9.5	8.5
Expected return on plan assets	[1]	(8.4)	(8.3)	(8.1)
Amortization of prior service cost (credit)	[4]	(11.3)	(11.9)	(11.9)
Amortization of actuarial loss	[2]	4.8	2.4	4.9
Additional benefit costs		0.0	0.0	0.0
Total		(0.3)	(3.1)	(0.3)
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		8.8	7.2	8.6
Interest cost		25.0	25.1	22.9
Expected return on plan assets	[1]	(35.8)	(35.7)	(35.2)
Amortization of prior service cost (credit)		(0.1)	0.0	0.1
Amortization of actuarial loss	[2]	15.3	8.0	15.2
Additional benefit costs		0.0	0.0	2.6	[3]
Total		13.2	4.6	14.2
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.4	2.4	2.9
Interest cost		3.8	3.9	3.6
Expected return on plan assets	[1]	(5.7)	(5.8)	(5.6)
Amortization of prior service cost (credit)	[4]	(6.1)	(6.3)	(6.3)
Amortization of actuarial loss	[2]	2.3	1.1	2.7
Additional benefit costs		0.0	0.0	0.0
Total		(3.3)	(4.7)	(2.7)
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		5.8	4.9	5.9
Interest cost		22.6	22.6	20.7
Expected return on plan assets	[1]	(32.4)	(32.4)	(31.9)
Amortization of prior service cost (credit)		0.2	0.3	0.3
Amortization of actuarial loss	[2]	16.8	9.2	17.1
Additional benefit costs		0.5	0.0	0.6	[3]
Total		13.5	4.6	12.7
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.1	2.0	2.5
Interest cost		3.7	3.8	3.4
Expected return on plan assets	[1]	(1.5)	(1.3)	(1.3)
Amortization of prior service cost (credit)	[4]	(3.5)	(3.9)	(3.9)
Amortization of actuarial loss	[2]	2.2	1.3	1.9
Additional benefit costs		0.0	0.0	0.0
Total		$ 3.0	$ 1.9	2.6
Corporate Service Allocation [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				2.9
Corporate Service Allocation [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				2.2
Cash Balance Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				9.0
Cash Balance Plan [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				5.5
Cash Balance Plan [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				2.8
Directly assigned defined benefit pension plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				2.6
Directly assigned defined benefit pension plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional benefit costs				$ 0.6

[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 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.

[3] In 2013, Alliant Energy filed a stipulation agreement with the Western Wisconsin Court related to a 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.

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