Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 12.6	$ 15.9	$ 13.1
Interest cost		53.0	53.6	54.1
Expected return on plan assets	[1]	(65.5)	(75.0)	(74.9)
Amortization of prior service cost (credit)		(0.3)	(0.2)	0.0
Amortization of actuarial loss	[2]	37.4	35.4	19.5
Additional benefit costs		0.0	0.5	0.0
Total		37.2	30.2	11.8
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		5.3	5.5	5.2
Interest cost		9.4	9.1	9.5
Expected return on plan assets	[1]	(6.1)	(8.4)	(8.3)
Amortization of prior service cost (credit)	[3]	(4.1)	(11.3)	(11.9)
Amortization of actuarial loss	[2]	4.7	4.8	2.4
Additional benefit costs		0.0	0.0	0.0
Total		9.2	(0.3)	(3.1)
IPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		7.5	8.8	7.2
Interest cost		24.5	25.0	25.1
Expected return on plan assets	[1]	(30.9)	(35.8)	(35.7)
Amortization of prior service cost (credit)		(0.2)	(0.1)	0.0
Amortization of actuarial loss	[2]	16.5	15.3	8.0
Total		17.4	13.2	4.6
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.3	2.4	2.4
Interest cost		3.8	3.8	3.9
Expected return on plan assets	[1]	(4.3)	(5.7)	(5.8)
Amortization of prior service cost (credit)	[3]	(2.6)	(6.1)	(6.3)
Amortization of actuarial loss	[2]	2.6	2.3	1.1
Total		1.8	(3.3)	(4.7)
WPL [Member] | Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		4.9	5.8	4.9
Interest cost		22.3	22.6	22.6
Expected return on plan assets	[1]	(28.3)	(32.4)	(32.4)
Amortization of prior service cost (credit)		0.2	0.2	0.3
Amortization of actuarial loss	[2]	17.6	16.8	9.2
Additional benefit costs		0.0	0.5	0.0
Total		16.7	13.5	4.6
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		2.0	2.1	2.0
Interest cost		3.8	3.7	3.8
Expected return on plan assets	[1]	(0.8)	(1.5)	(1.3)
Amortization of prior service cost (credit)	[3]	(0.9)	(3.5)	(3.9)
Amortization of actuarial loss	[2]	1.8	2.2	1.3
Additional benefit costs		0.0	0.0	0.0
Total		$ 5.9	$ 3.0	$ 1.9

[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 Alliant Energy Cash Balance Pension Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.

[3] 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.