Regulatory Matters (Electric Generating Units Retired Early (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 1,737.3	$ 1,666.7
EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	111.6	63.8
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,279.0	1,231.6
IPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	55.4	31.6
IPL [Member] | EGUs retired early [Member] | Sutherland Units 1 And 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 27.6
Recovery period, authorized	10 years
IPL [Member] | EGUs retired early [Member] | ML Kapp [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 27.8
Recovery period, authorized	10 years
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 458.3	435.1
WPL [Member] | EGUs retired early [Member]
Regulatory Assets [Line Items]
Regulatory assets	56.2	$ 32.2
WPL [Member] | EGUs retired early [Member] | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 27.4
Recovery period, authorized	10 years
WPL [Member] | EGUs retired early [Member] | Edgewater Unit 4 [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 28.8
Recovery period, authorized	10 years