Revenue (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 990.2	$ 928.6	$ 2,767.6	$ 2,661.0
Electric [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	915.9	861.2	2,350.5	2,296.2
Electric [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	331.6	312.9	839.6	820.6
Electric [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	230.9	215.4	583.3	561.7
Electric [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	263.4	247.8	686.3	675.1
Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	49.8	50.0	137.3	146.9
Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	40.2	35.1	104.0	91.9
Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41.5	44.8	322.5	299.0
Gas [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	20.4	21.8	187.4	170.1
Gas [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10.0	11.0	89.7	87.3
Gas [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.3	3.0	9.7	11.4
Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.8	9.0	35.7	30.2
Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11.2	12.3	33.2	36.2
Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9.1	8.7	26.7	26.5
Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2.1	3.6	6.5	9.7
Non-utility and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21.6	10.3	61.4	29.6
Non-utility and Other [Member] | Transportation and other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	21.6	10.3	61.4	29.6
IPL [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	596.5	547.6	1,592.8	1,548.2
IPL [Member] | Electric [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	560.9	509.2	1,373.0	1,337.0
IPL [Member] | Electric [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	196.0	180.1	469.4	461.6
IPL [Member] | Electric [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	158.4	146.0	386.4	371.8
IPL [Member] | Electric [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	157.5	141.4	392.0	385.0
IPL [Member] | Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17.7	18.2	49.2	56.4
IPL [Member] | Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	31.3	23.5	76.0	62.2
IPL [Member] | Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	24.8	26.7	187.8	177.0
IPL [Member] | Gas [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11.7	12.6	109.6	100.9
IPL [Member] | Gas [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5.9	6.4	49.5	49.8
IPL [Member] | Gas [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.9	2.0	6.7	6.1
IPL [Member] | Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5.3	5.7	22.0	20.2
IPL [Member] | Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10.8	11.7	32.0	34.2
IPL [Member] | Other Utility [Member] | Steam [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9.1	8.7	26.7	26.5
IPL [Member] | Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.7	3.0	5.3	7.7
WPL [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	372.1	370.7	1,113.4	1,083.2
WPL [Member] | Electric [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	355.0	352.0	977.5	959.2
WPL [Member] | Electric [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	135.6	132.8	370.2	359.0
WPL [Member] | Electric [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	72.5	69.4	196.9	189.9
WPL [Member] | Electric [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	105.9	106.4	294.3	290.1
WPL [Member] | Electric [Member] | Wholesale [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	32.1	31.8	88.1	90.5
WPL [Member] | Electric [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.9	11.6	28.0	29.7
WPL [Member] | Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	16.7	18.1	134.7	122.0
WPL [Member] | Gas [Member] | Retail - residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	8.7	9.2	77.8	69.2
WPL [Member] | Gas [Member] | Retail - commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	4.1	4.6	40.2	37.5
WPL [Member] | Gas [Member] | Retail - industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.4	1.0	3.0	5.3
WPL [Member] | Gas [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3.5	3.3	13.7	10.0
WPL [Member] | Other Utility [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.4	0.6	1.2	2.0
WPL [Member] | Other Utility [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 0.4	$ 0.6	$ 1.2	$ 2.0