Property, Plant and Equipment	3 Months Ended
Property, Plant and Equipment	Mar. 31, 2018
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	PROPERTY, PLANT AND EQUIPMENT Utility - Natural Gas-Fired Generation Project - WPL’s West Riverside Energy Center - WPL is currently constructing West Riverside, an approximate 730 MW natural gas-fired combined-cycle EGU. Construction began in 2016 and is currently expected to be completed by early 2020. As of March 31, 2018, Alliant Energy and WPL recorded capitalized expenditures for construction work in progress of $347 million and AFUDC of $20 million for West Riverside in “Property, plant and equipment, net” on their balance sheets. These capital expenditures reflect WPL’s portion of West Riverside. Certain WPL electric cooperatives acquired approximately 60 MW of West Riverside in January 2018, and will fund their share of capital expenditures during construction. As part of the electric cooperatives’ acquisitions, the current wholesale power supply agreements with the various electric cooperatives were extended by at least four years until 2026 with automatic continuation of such agreements unless terminated by either party, with a five-year notice requirement. Wind Generation - IPL’s Expansion of Wind Generation - IPL currently plans to add up to 1,000 MW of new wind generation to its existing generation portfolio. These wind projects are expected to be placed into service in 2019 and 2020. As of March 31, 2018, Alliant Energy and IPL recorded capitalized expenditures for construction work in progress of $385 million and AFUDC of $16 million for this expansion of wind generation in “Property, plant and equipment, net” on their balance sheets. WPL’s Acquisition of Forward Wind Energy Center (FWEC) - In January 2018 and March 2018, WPL received approval from FERC and the PSCW, respectively, to acquire the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. In April 2018, WPL acquired 55 MW of FWEC for approximately $74 million.
IPL [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	PROPERTY, PLANT AND EQUIPMENT Utility - Natural Gas-Fired Generation Project - WPL’s West Riverside Energy Center - WPL is currently constructing West Riverside, an approximate 730 MW natural gas-fired combined-cycle EGU. Construction began in 2016 and is currently expected to be completed by early 2020. As of March 31, 2018, Alliant Energy and WPL recorded capitalized expenditures for construction work in progress of $347 million and AFUDC of $20 million for West Riverside in “Property, plant and equipment, net” on their balance sheets. These capital expenditures reflect WPL’s portion of West Riverside. Certain WPL electric cooperatives acquired approximately 60 MW of West Riverside in January 2018, and will fund their share of capital expenditures during construction. As part of the electric cooperatives’ acquisitions, the current wholesale power supply agreements with the various electric cooperatives were extended by at least four years until 2026 with automatic continuation of such agreements unless terminated by either party, with a five-year notice requirement. Wind Generation - IPL’s Expansion of Wind Generation - IPL currently plans to add up to 1,000 MW of new wind generation to its existing generation portfolio. These wind projects are expected to be placed into service in 2019 and 2020. As of March 31, 2018, Alliant Energy and IPL recorded capitalized expenditures for construction work in progress of $385 million and AFUDC of $16 million for this expansion of wind generation in “Property, plant and equipment, net” on their balance sheets.
WPL [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	PROPERTY, PLANT AND EQUIPMENT Utility - Natural Gas-Fired Generation Project - WPL’s West Riverside Energy Center - WPL is currently constructing West Riverside, an approximate 730 MW natural gas-fired combined-cycle EGU. Construction began in 2016 and is currently expected to be completed by early 2020. As of March 31, 2018, Alliant Energy and WPL recorded capitalized expenditures for construction work in progress of $347 million and AFUDC of $20 million for West Riverside in “Property, plant and equipment, net” on their balance sheets. These capital expenditures reflect WPL’s portion of West Riverside. Certain WPL electric cooperatives acquired approximately 60 MW of West Riverside in January 2018, and will fund their share of capital expenditures during construction. As part of the electric cooperatives’ acquisitions, the current wholesale power supply agreements with the various electric cooperatives were extended by at least four years until 2026 with automatic continuation of such agreements unless terminated by either party, with a five-year notice requirement. Wind Generation - IPL’s Expansion of Wind Generation - IPL currently plans to add up to 1,000 MW of new wind generation to its existing generation portfolio. These wind projects are expected to be placed into service in 2019 and 2020. As of March 31, 2018, Alliant Energy and IPL recorded capitalized expenditures for construction work in progress of $385 million and AFUDC of $16 million for this expansion of wind generation in “Property, plant and equipment, net” on their balance sheets. WPL’s Acquisition of Forward Wind Energy Center (FWEC) - In January 2018 and March 2018, WPL received approval from FERC and the PSCW, respectively, to acquire the assets of FWEC, which is a 129 MW wind farm located in Wisconsin. In April 2018, WPL acquired 55 MW of FWEC for approximately $74 million.