Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined benefit pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 3.1	$ 3.2	$ 9.3	$ 9.5
Interest cost		12.7	13.2	38.3	39.7
Expected return on plan assets		(16.3)	(16.3)	(49.1)	(49.1)
Amortization of prior service cost (credit)		(0.1)	(0.1)	(0.3)	(0.2)
Amortization of actuarial loss		9.4	9.3	28.2	28.0
Settlement losses	[1]	0.9	0.0	0.9	0.0
Total		9.7	9.3	27.3	27.9
OPEB Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.2	1.4	3.7	4.0
Interest cost		2.2	2.3	6.5	7.0
Expected return on plan assets		(1.5)	(1.6)	(4.6)	(4.6)
Amortization of prior service cost (credit)		(0.1)	(1.0)	(0.2)	(3.1)
Amortization of actuarial loss		1.0	1.2	2.9	3.6
Settlement losses	[1]	0.0	0.0	0.0	0.0
Total		2.8	2.3	8.3	6.9
IPL [Member] | Defined benefit pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.8	1.8	5.5	5.6
Interest cost		5.9	6.1	17.6	18.4
Expected return on plan assets		(7.7)	(7.7)	(23.1)	(23.2)
Amortization of prior service cost (credit)		0.0	0.0	(0.1)	(0.1)
Amortization of actuarial loss		4.0	4.2	12.1	12.4
Total		4.0	4.4	12.0	13.1
IPL [Member] | OPEB Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.5	0.5	1.6	1.7
Interest cost		0.8	1.0	2.6	2.9
Expected return on plan assets		(1.0)	(1.0)	(3.2)	(3.2)
Amortization of prior service cost (credit)		0.0	(0.7)	0.0	(2.0)
Amortization of actuarial loss		0.5	0.7	1.5	2.0
Total		0.8	0.5	2.5	1.4
WPL [Member] | Defined benefit pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1.2	1.3	3.6	3.7
Interest cost		5.5	5.5	16.4	16.7
Expected return on plan assets		(7.2)	(7.0)	(21.4)	(21.2)
Amortization of prior service cost (credit)		0.1	0.0	0.1	0.1
Amortization of actuarial loss		4.6	4.4	13.9	13.2
Total		4.2	4.2	12.6	12.5
WPL [Member] | OPEB Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.5	0.5	1.4	1.5
Interest cost		0.9	0.9	2.6	2.8
Expected return on plan assets		(0.2)	(0.2)	(0.6)	(0.6)
Amortization of prior service cost (credit)		(0.1)	(0.3)	(0.2)	(0.7)
Amortization of actuarial loss		0.4	0.5	1.2	1.4
Total		$ 1.5	$ 1.4	$ 4.4	$ 4.4

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