XML 131 R97.htm IDEA: XBRL DOCUMENT v3.22.4
Benefit Plans (Fair Value Of Plan Assets By Asset Category And Fair Value Hierarchy Level) (Details) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Pension Plans, Defined Benefit [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 706.0 $ 1,011.0 $ 984.0
Assets measured at net asset value 276.0 477.0  
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 79.0 35.0  
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 79.0 35.0  
Pension Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 185.0 256.0  
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 185.0 256.0  
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Global asset securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 35.0 52.0  
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 35.0 52.0  
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 130.0 191.0  
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 30.0 47.0  
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 100.0 144.0  
Pension Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Subtotal [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total assets in fair value hierarchy 429.0 534.0  
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 250.0 355.0  
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 179.0 179.0  
Pension Plans, Defined Benefit [Member] | Subtotal [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | Accrued investment income [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Pension Plans, Defined Benefit [Member] | IPL [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 344.0 462.0 453.0
Assets measured at net asset value 124.0 218.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 62.0 16.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 62.0 16.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 83.0 117.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 83.0 117.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 16.0 24.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 16.0 24.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Global asset securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 58.0 87.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.0 21.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 45.0 66.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total assets in fair value hierarchy 219.0 244.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 112.0 162.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 107.0 82.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Subtotal [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | IPL [Member] | Accrued investment income [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 291.0 450.0 436.0
Assets measured at net asset value 122.0 212.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.0 16.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.0 16.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 82.0 114.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 82.0 114.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 16.0 23.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 16.0 23.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Global asset securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 57.0 85.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.0 21.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 44.0 64.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total assets in fair value hierarchy 168.0 238.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 111.0 158.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 57.0 80.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Subtotal [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Pension Plans, Defined Benefit [Member] | WPL [Member] | Accrued investment income [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 83.0 106.0 108.0
Assets measured at net asset value 23.0 29.0  
Total assets in fair value hierarchy 60.0 77.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 56.0 70.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 4.0 7.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 3.0 5.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 3.0 5.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and equivalents [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 9.0 11.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 9.0 11.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Equity securites - U.S. [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Global asset securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 47.0 60.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 46.0 58.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 2.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fixed income securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 58.0 74.0 74.0
Assets measured at net asset value 18.0 23.0  
Total assets in fair value hierarchy 40.0 51.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 39.0 50.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Cash and equivalents [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 5.0 8.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 5.0 8.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 34.0 42.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 34.0 42.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member] | Fixed income securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 14.0 17.0 $ 18.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 14.0 16.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Cash and equivalents [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 1.0 1.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Equity securites - U.S. [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.0 15.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 13.0 15.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member] | Fixed income securities [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets 0.0 0.0  
Defined Benefit Plan Due To Brokers Net [Member] | Pension Plans, Defined Benefit [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of plan assets $ 0.0 $ (1.0)