Contractual interest rate for funds borrowed, under the debt agreement.

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-Publisher FASB
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-Topic 230
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Debt (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended			9 Months Ended
Debt (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Apr. 30, 2020	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019
Debt [Line Items]
Payments to retire long-term debt				$ 654	$ 253
IPL [Member]
Debt [Line Items]
Payments to retire long-term debt				200	0
WPL [Member]
Debt [Line Items]
Payments to retire long-term debt				$ 150	$ 250
Term Loan Credit Agreement [Member] \| Term Loan Credit Agreement Through March 2022 [Member] \| Alliant Energy Finance, LLC [Member]
Debt [Line Items]
Proceeds from issuance of long-term debt			$ 300
Interest rate, percent				1.00%
Term Loan Credit Agreement [Member] \| Term Loan Credit Agreement Through April 2020 [Member] \| Alliant Energy Finance, LLC [Member]
Debt [Line Items]
Payments to retire long-term debt			$ 300
Debentures [Member] \| 3.65% debenture, due 2050 [Member] \| WPL [Member]
Debt [Line Items]
Proceeds from issuance of long-term debt		$ 350
Interest rate, percent				3.65%
Debentures [Member] \| 4.6% debenture, due 2020 [Member] \| WPL [Member]
Debt [Line Items]
Interest rate, percent	4.60%
Payments to retire long-term debt	$ 150
Senior Debentures [Member] \| 2.3% senior debenture, due 2030 [Member] \| IPL [Member]
Debt [Line Items]
Proceeds from issuance of long-term debt	$ 400
Interest rate, percent				2.30%
Senior Debentures [Member] \| 3.65% senior debenture, due 2020 [Member] \| IPL [Member]
Debt [Line Items]
Interest rate, percent	3.65%
Payments to retire long-term debt	$ 200