Regulatory Matters (Regulatory Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,493.2	$ 1,497.2
Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	890.6	899.4
Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	401.2	410.0
Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	104.0	90.4
WPL earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.4	8.0
Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	16.8	21.0
IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	25.0
Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	48.8	43.4
IPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	754.9	755.4
IPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	390.1	399.5
IPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	273.3	274.5
IPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	47.7	26.4
IPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	11.9	14.6
IPL [Member] | IPL's tax benefit riders [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6.4	25.0
IPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.5	15.4
WPL [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	738.3	741.8
WPL [Member] | Tax-related [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	500.5	499.9
WPL [Member] | Cost of removal obligations [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	127.9	135.5
WPL [Member] | Electric transmission cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	56.3	64.0
WPL [Member] | WPL earnings sharing mechanism [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.4	8.0
WPL [Member] | Commodity cost recovery [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	4.9	6.4
WPL [Member] | Other [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 23.3	$ 28.0