XML 52 R29.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Fair Value Measurements (Tables) | 9 Months Ended
--- | ---
Sep. 30, 2017
Statement [Line Items] |
Fair Value Measurements | Carrying amounts and related estimated fair values of other financial instruments were as follows (in millions):
Alliant Energy | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $29.4 |  |  |  | $— |  |  |  | $2.9 |  |  |  | $26.5 |  |  |  | $29.4 |  |  |  | $41.4 |  |  |  | $— |  |  |  | $4.6 |  |  |  | $36.8 |  |  |  | $41.4 |
Deferred proceeds | 115.3 |  |  | — |  |  | — |  |  | 115.3 |  |  | 115.3 |  |  | 211.1 |  |  | — |  |  | — |  |  | 211.1 |  |  | 211.1 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 45.1 |  |  | — |  |  | 14.9 |  |  | 30.2 |  |  | 45.1 |  |  | 28.6 |  |  | — |  |  | 0.5 |  |  | 28.1 |  |  | 28.6 |
Long-term debt (incl. current maturities) | 4,360.3 |  |  | — |  |  | 4,893.3 |  |  | 2.9 |  |  | 4,896.2 |  |  | 4,320.2 |  |  | — |  |  | 4,795.7 |  |  | 3.3 |  |  | 4,799.0 |
Cumulative preferred stock of IPL | 200.0 |  |  | 202.3 |  |  | — |  |  | — |  |  | 202.3 |  |  | 200.0 |  |  | 194.8 |  |  | — |  |  | — |  |  | 194.8 |
IPL | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $21.1 |  |  |  | $— |  |  |  | $1.6 |  |  |  | $19.5 |  |  |  | $21.1 |  |  |  | $20.8 |  |  |  | $— |  |  |  | $2.8 |  |  |  | $18.0 |  |  |  | $20.8 |
Deferred proceeds | 115.3 |  |  | — |  |  | — |  |  | 115.3 |  |  | 115.3 |  |  | 211.1 |  |  | — |  |  | — |  |  | 211.1 |  |  | 211.1 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 18.7 |  |  | — |  |  | 4.5 |  |  | 14.2 |  |  | 18.7 |  |  | 8.3 |  |  | — |  |  | 0.4 |  |  | 7.9 |  |  | 8.3 |
Long-term debt (incl. current maturities) | 2,195.0 |  |  | — |  |  | 2,430.1 |  |  | — |  |  | 2,430.1 |  |  | 2,153.5 |  |  | — |  |  | 2,352.3 |  |  | — |  |  | 2,352.3 |
Cumulative preferred stock | 200.0 |  |  | 202.3 |  |  | — |  |  | — |  |  | 202.3 |  |  | 200.0 |  |  | 194.8 |  |  | — |  |  | — |  |  | 194.8 |
WPL | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $8.3 |  |  |  | $— |  |  |  | $1.3 |  |  |  | $7.0 |  |  |  | $8.3 |  |  |  | $20.6 |  |  |  | $— |  |  |  | $1.8 |  |  |  | $18.8 |  |  |  | $20.6 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 26.4 |  |  | — |  |  | 10.4 |  |  | 16.0 |  |  | 26.4 |  |  | 20.3 |  |  | — |  |  | 0.1 |  |  | 20.2 |  |  | 20.3 |
Long-term debt | 1,536.2 |  |  | — |  |  | 1,829.3 |  |  | — |  |  | 1,829.3 |  |  | 1,535.2 |  |  | — |  |  | 1,807.4 |  |  | — |  |  | 1,807.4 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions):
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, July 1 |  | $9.2 |  |  |  | $0.6 |  |  |  | $170.0 |  |  |  | $74.4 |
Total net losses included in changes in net assets (realized/unrealized) | (4.3 | ) |  | (5.1 | ) |  | — |  |  | — |
Transfers out of Level 3 | — |  |  | 0.8 |  |  | — |  |  | — |
Sales | (0.1 | ) |  | (0.2 | ) |  | — |  |  | — |
Settlements (a) | (8.5 | ) |  | (4.0 | ) |  | (54.7 | ) |  | 165.3 |
Ending balance, September 30 |  | ($3.7 | ) |  |  | ($7.9 | ) |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at September 30 |  | ($4.2 | ) |  |  | ($5.0 | ) |  |  | $— |  |  |  | $— |
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Nine Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, January 1 |  | $8.7 |  |  |  | ($32.7 | ) |  |  | $211.1 |  |  |  | $172.0 |
Total net gains (losses) included in changes in net assets (realized/unrealized) | (31.3 | ) |  | 8.0 |  |  | — |  |  | — |
Transfers into Level 3 | — |  |  | 0.9 |  |  | — |  |  | — |
Transfers out of Level 3 | 12.2 |  |  | 1.2 |  |  | — |  |  | — |
Purchases | 28.3 |  |  | 22.0 |  |  | — |  |  | — |
Sales | (0.3 | ) |  | (0.9 | ) |  | — |  |  | — |
Settlements (a) | (21.3 | ) |  | (6.4 | ) |  | (95.8 | ) |  | 67.7 |
Ending balance, September 30 |  | ($3.7 | ) |  |  | ($7.9 | ) |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($29.4 | ) |  |  | $9.7 |  |  |  | $— |  |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, July 1 |  | $17.1 |  |  |  | $18.3 |  |  |  | $170.0 |  |  |  | $74.4 |
Total net losses included in changes in net assets (realized/unrealized) | (4.4 | ) |  | (0.4 | ) |  | — |  |  | — |
Transfers out of Level 3 | — |  |  | 0.3 |  |  | — |  |  | — |
Sales | (0.1 | ) |  | (0.2 | ) |  | — |  |  | — |
Settlements (a) | (7.3 | ) |  | (4.6 | ) |  | (54.7 | ) |  | 165.3 |
Ending balance, September 30 |  | $5.3 |  |  |  | $13.4 |  |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at September 30 |  | ($4.5 | ) |  |  | ($0.4 | ) |  |  | $— |  |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Nine Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, January 1 |  | $10.1 |  |  |  | ($1.9 | ) |  |  | $211.1 |  |  |  | $172.0 |
Total net gains (losses) included in changes in net assets (realized/unrealized) | (13.9 | ) |  | 4.8 |  |  | — |  |  | — |
Transfers into Level 3 | — |  |  | 0.5 |  |  | — |  |  | — |
Transfers out of Level 3 | 3.1 |  |  | 0.2 |  |  | — |  |  | — |
Purchases | 24.6 |  |  | 20.6 |  |  | — |  |  | — |
Sales | (0.2 | ) |  | (0.9 | ) |  | — |  |  | — |
Settlements (a) | (18.4 | ) |  | (9.9 | ) |  | (95.8 | ) |  | 67.7 |
Ending balance, September 30 |  | $5.3 |  |  |  | $13.4 |  |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($12.6 | ) |  |  | $5.7 |  |  |  | $— |  |  |  | $— |
WPL | Commodity Contract Derivative
| Assets and (Liabilities), net
Three Months Ended September 30 | 2017 |  | 2016
Beginning balance, July 1 |  | ($7.9 | ) |  |  | ($17.7 | )
Total net gains (losses) included in changes in net assets (realized/unrealized) | 0.1 |  |  | (4.7 | )
Transfers out of Level 3 | — |  |  | 0.5 |
Settlements | (1.2 | ) |  | 0.6 |
Ending balance, September 30 |  | ($9.0 | ) |  |  | ($21.3 | )
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | $0.3 |  |  |  | ($4.6 | )
WPL | Commodity Contract Derivative
| Assets and (Liabilities), net
Nine Months Ended September 30 | 2017 |  | 2016
Beginning balance, January 1 |  | ($1.4 | ) |  |  | ($30.8 | )
Total net gains (losses) included in changes in net assets (realized/unrealized) | (17.4 | ) |  | 3.2 |
Transfers into Level 3 | — |  |  | 0.4 |
Transfers out of Level 3 | 9.1 |  |  | 1.0 |
Purchases | 3.7 |  |  | 1.4 |
Sales | (0.1 | ) |  | — |
Settlements | (2.9 | ) |  | 3.5 |
Ending balance, September 30 |  | ($9.0 | ) |  |  | ($21.3 | )
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($16.8 | ) |  |  | $4.0 |
(a) | Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets (Liabilities) | The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
September 30, 2017 |  | ($22.2 | ) |  |  | $18.5 |  |  |  | ($10.4 | ) |  |  | $15.7 |  |  |  | ($11.8 | ) |  |  | $2.8 |
December 31, 2016 | (2.3 | ) |  | 11.0 |  |  | 0.1 |  |  | 10.0 |  |  | (2.4 | ) |  | 1.0 |
IPL [Member] |
Statement [Line Items] |
Fair Value Measurements | Carrying amounts and related estimated fair values of other financial instruments were as follows (in millions):
Alliant Energy | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $29.4 |  |  |  | $— |  |  |  | $2.9 |  |  |  | $26.5 |  |  |  | $29.4 |  |  |  | $41.4 |  |  |  | $— |  |  |  | $4.6 |  |  |  | $36.8 |  |  |  | $41.4 |
Deferred proceeds | 115.3 |  |  | — |  |  | — |  |  | 115.3 |  |  | 115.3 |  |  | 211.1 |  |  | — |  |  | — |  |  | 211.1 |  |  | 211.1 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 45.1 |  |  | — |  |  | 14.9 |  |  | 30.2 |  |  | 45.1 |  |  | 28.6 |  |  | — |  |  | 0.5 |  |  | 28.1 |  |  | 28.6 |
Long-term debt (incl. current maturities) | 4,360.3 |  |  | — |  |  | 4,893.3 |  |  | 2.9 |  |  | 4,896.2 |  |  | 4,320.2 |  |  | — |  |  | 4,795.7 |  |  | 3.3 |  |  | 4,799.0 |
Cumulative preferred stock of IPL | 200.0 |  |  | 202.3 |  |  | — |  |  | — |  |  | 202.3 |  |  | 200.0 |  |  | 194.8 |  |  | — |  |  | — |  |  | 194.8 |
IPL | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $21.1 |  |  |  | $— |  |  |  | $1.6 |  |  |  | $19.5 |  |  |  | $21.1 |  |  |  | $20.8 |  |  |  | $— |  |  |  | $2.8 |  |  |  | $18.0 |  |  |  | $20.8 |
Deferred proceeds | 115.3 |  |  | — |  |  | — |  |  | 115.3 |  |  | 115.3 |  |  | 211.1 |  |  | — |  |  | — |  |  | 211.1 |  |  | 211.1 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 18.7 |  |  | — |  |  | 4.5 |  |  | 14.2 |  |  | 18.7 |  |  | 8.3 |  |  | — |  |  | 0.4 |  |  | 7.9 |  |  | 8.3 |
Long-term debt (incl. current maturities) | 2,195.0 |  |  | — |  |  | 2,430.1 |  |  | — |  |  | 2,430.1 |  |  | 2,153.5 |  |  | — |  |  | 2,352.3 |  |  | — |  |  | 2,352.3 |
Cumulative preferred stock | 200.0 |  |  | 202.3 |  |  | — |  |  | — |  |  | 202.3 |  |  | 200.0 |  |  | 194.8 |  |  | — |  |  | — |  |  | 194.8 |
WPL | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $8.3 |  |  |  | $— |  |  |  | $1.3 |  |  |  | $7.0 |  |  |  | $8.3 |  |  |  | $20.6 |  |  |  | $— |  |  |  | $1.8 |  |  |  | $18.8 |  |  |  | $20.6 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 26.4 |  |  | — |  |  | 10.4 |  |  | 16.0 |  |  | 26.4 |  |  | 20.3 |  |  | — |  |  | 0.1 |  |  | 20.2 |  |  | 20.3 |
Long-term debt | 1,536.2 |  |  | — |  |  | 1,829.3 |  |  | — |  |  | 1,829.3 |  |  | 1,535.2 |  |  | — |  |  | 1,807.4 |  |  | — |  |  | 1,807.4 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions):
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, July 1 |  | $9.2 |  |  |  | $0.6 |  |  |  | $170.0 |  |  |  | $74.4 |
Total net losses included in changes in net assets (realized/unrealized) | (4.3 | ) |  | (5.1 | ) |  | — |  |  | — |
Transfers out of Level 3 | — |  |  | 0.8 |  |  | — |  |  | — |
Sales | (0.1 | ) |  | (0.2 | ) |  | — |  |  | — |
Settlements (a) | (8.5 | ) |  | (4.0 | ) |  | (54.7 | ) |  | 165.3 |
Ending balance, September 30 |  | ($3.7 | ) |  |  | ($7.9 | ) |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at September 30 |  | ($4.2 | ) |  |  | ($5.0 | ) |  |  | $— |  |  |  | $— |
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Nine Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, January 1 |  | $8.7 |  |  |  | ($32.7 | ) |  |  | $211.1 |  |  |  | $172.0 |
Total net gains (losses) included in changes in net assets (realized/unrealized) | (31.3 | ) |  | 8.0 |  |  | — |  |  | — |
Transfers into Level 3 | — |  |  | 0.9 |  |  | — |  |  | — |
Transfers out of Level 3 | 12.2 |  |  | 1.2 |  |  | — |  |  | — |
Purchases | 28.3 |  |  | 22.0 |  |  | — |  |  | — |
Sales | (0.3 | ) |  | (0.9 | ) |  | — |  |  | — |
Settlements (a) | (21.3 | ) |  | (6.4 | ) |  | (95.8 | ) |  | 67.7 |
Ending balance, September 30 |  | ($3.7 | ) |  |  | ($7.9 | ) |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($29.4 | ) |  |  | $9.7 |  |  |  | $— |  |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, July 1 |  | $17.1 |  |  |  | $18.3 |  |  |  | $170.0 |  |  |  | $74.4 |
Total net losses included in changes in net assets (realized/unrealized) | (4.4 | ) |  | (0.4 | ) |  | — |  |  | — |
Transfers out of Level 3 | — |  |  | 0.3 |  |  | — |  |  | — |
Sales | (0.1 | ) |  | (0.2 | ) |  | — |  |  | — |
Settlements (a) | (7.3 | ) |  | (4.6 | ) |  | (54.7 | ) |  | 165.3 |
Ending balance, September 30 |  | $5.3 |  |  |  | $13.4 |  |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at September 30 |  | ($4.5 | ) |  |  | ($0.4 | ) |  |  | $— |  |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Nine Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, January 1 |  | $10.1 |  |  |  | ($1.9 | ) |  |  | $211.1 |  |  |  | $172.0 |
Total net gains (losses) included in changes in net assets (realized/unrealized) | (13.9 | ) |  | 4.8 |  |  | — |  |  | — |
Transfers into Level 3 | — |  |  | 0.5 |  |  | — |  |  | — |
Transfers out of Level 3 | 3.1 |  |  | 0.2 |  |  | — |  |  | — |
Purchases | 24.6 |  |  | 20.6 |  |  | — |  |  | — |
Sales | (0.2 | ) |  | (0.9 | ) |  | — |  |  | — |
Settlements (a) | (18.4 | ) |  | (9.9 | ) |  | (95.8 | ) |  | 67.7 |
Ending balance, September 30 |  | $5.3 |  |  |  | $13.4 |  |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($12.6 | ) |  |  | $5.7 |  |  |  | $— |  |  |  | $— |
WPL | Commodity Contract Derivative
| Assets and (Liabilities), net
Three Months Ended September 30 | 2017 |  | 2016
Beginning balance, July 1 |  | ($7.9 | ) |  |  | ($17.7 | )
Total net gains (losses) included in changes in net assets (realized/unrealized) | 0.1 |  |  | (4.7 | )
Transfers out of Level 3 | — |  |  | 0.5 |
Settlements | (1.2 | ) |  | 0.6 |
Ending balance, September 30 |  | ($9.0 | ) |  |  | ($21.3 | )
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | $0.3 |  |  |  | ($4.6 | )
WPL | Commodity Contract Derivative
| Assets and (Liabilities), net
Nine Months Ended September 30 | 2017 |  | 2016
Beginning balance, January 1 |  | ($1.4 | ) |  |  | ($30.8 | )
Total net gains (losses) included in changes in net assets (realized/unrealized) | (17.4 | ) |  | 3.2 |
Transfers into Level 3 | — |  |  | 0.4 |
Transfers out of Level 3 | 9.1 |  |  | 1.0 |
Purchases | 3.7 |  |  | 1.4 |
Sales | (0.1 | ) |  | — |
Settlements | (2.9 | ) |  | 3.5 |
Ending balance, September 30 |  | ($9.0 | ) |  |  | ($21.3 | )
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($16.8 | ) |  |  | $4.0 |
(a) | Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets (Liabilities) | The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
September 30, 2017 |  | ($22.2 | ) |  |  | $18.5 |  |  |  | ($10.4 | ) |  |  | $15.7 |  |  |  | ($11.8 | ) |  |  | $2.8 |
December 31, 2016 | (2.3 | ) |  | 11.0 |  |  | 0.1 |  |  | 10.0 |  |  | (2.4 | ) |  | 1.0 |
WPL [Member] |
Statement [Line Items] |
Fair Value Measurements | Carrying amounts and related estimated fair values of other financial instruments were as follows (in millions):
Alliant Energy | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $29.4 |  |  |  | $— |  |  |  | $2.9 |  |  |  | $26.5 |  |  |  | $29.4 |  |  |  | $41.4 |  |  |  | $— |  |  |  | $4.6 |  |  |  | $36.8 |  |  |  | $41.4 |
Deferred proceeds | 115.3 |  |  | — |  |  | — |  |  | 115.3 |  |  | 115.3 |  |  | 211.1 |  |  | — |  |  | — |  |  | 211.1 |  |  | 211.1 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 45.1 |  |  | — |  |  | 14.9 |  |  | 30.2 |  |  | 45.1 |  |  | 28.6 |  |  | — |  |  | 0.5 |  |  | 28.1 |  |  | 28.6 |
Long-term debt (incl. current maturities) | 4,360.3 |  |  | — |  |  | 4,893.3 |  |  | 2.9 |  |  | 4,896.2 |  |  | 4,320.2 |  |  | — |  |  | 4,795.7 |  |  | 3.3 |  |  | 4,799.0 |
Cumulative preferred stock of IPL | 200.0 |  |  | 202.3 |  |  | — |  |  | — |  |  | 202.3 |  |  | 200.0 |  |  | 194.8 |  |  | — |  |  | — |  |  | 194.8 |
IPL | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $21.1 |  |  |  | $— |  |  |  | $1.6 |  |  |  | $19.5 |  |  |  | $21.1 |  |  |  | $20.8 |  |  |  | $— |  |  |  | $2.8 |  |  |  | $18.0 |  |  |  | $20.8 |
Deferred proceeds | 115.3 |  |  | — |  |  | — |  |  | 115.3 |  |  | 115.3 |  |  | 211.1 |  |  | — |  |  | — |  |  | 211.1 |  |  | 211.1 |
Liabilities and equity: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 18.7 |  |  | — |  |  | 4.5 |  |  | 14.2 |  |  | 18.7 |  |  | 8.3 |  |  | — |  |  | 0.4 |  |  | 7.9 |  |  | 8.3 |
Long-term debt (incl. current maturities) | 2,195.0 |  |  | — |  |  | 2,430.1 |  |  | — |  |  | 2,430.1 |  |  | 2,153.5 |  |  | — |  |  | 2,352.3 |  |  | — |  |  | 2,352.3 |
Cumulative preferred stock | 200.0 |  |  | 202.3 |  |  | — |  |  | — |  |  | 202.3 |  |  | 200.0 |  |  | 194.8 |  |  | — |  |  | — |  |  | 194.8 |
WPL | September 30, 2017 |  | December 31, 2016
|  |  | Fair Value |  |  |  | Fair Value
| Carrying |  | Level |  | Level |  | Level |  |  |  | Carrying |  | Level |  | Level |  | Level |  |
| Amount |  | 1 |  | 2 |  | 3 |  | Total |  | Amount |  | 1 |  | 2 |  | 3 |  | Total
Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives |  | $8.3 |  |  |  | $— |  |  |  | $1.3 |  |  |  | $7.0 |  |  |  | $8.3 |  |  |  | $20.6 |  |  |  | $— |  |  |  | $1.8 |  |  |  | $18.8 |  |  |  | $20.6 |
Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives | 26.4 |  |  | — |  |  | 10.4 |  |  | 16.0 |  |  | 26.4 |  |  | 20.3 |  |  | — |  |  | 0.1 |  |  | 20.2 |  |  | 20.3 |
Long-term debt | 1,536.2 |  |  | — |  |  | 1,829.3 |  |  | — |  |  | 1,829.3 |  |  | 1,535.2 |  |  | — |  |  | 1,807.4 |  |  | — |  |  | 1,807.4 |
Fair Value Measurements Using Significant Unobservable Inputs | Information for fair value measurements using significant unobservable inputs (Level 3 inputs) was as follows (in millions):
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, July 1 |  | $9.2 |  |  |  | $0.6 |  |  |  | $170.0 |  |  |  | $74.4 |
Total net losses included in changes in net assets (realized/unrealized) | (4.3 | ) |  | (5.1 | ) |  | — |  |  | — |
Transfers out of Level 3 | — |  |  | 0.8 |  |  | — |  |  | — |
Sales | (0.1 | ) |  | (0.2 | ) |  | — |  |  | — |
Settlements (a) | (8.5 | ) |  | (4.0 | ) |  | (54.7 | ) |  | 165.3 |
Ending balance, September 30 |  | ($3.7 | ) |  |  | ($7.9 | ) |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at September 30 |  | ($4.2 | ) |  |  | ($5.0 | ) |  |  | $— |  |  |  | $— |
Alliant Energy | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Nine Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, January 1 |  | $8.7 |  |  |  | ($32.7 | ) |  |  | $211.1 |  |  |  | $172.0 |
Total net gains (losses) included in changes in net assets (realized/unrealized) | (31.3 | ) |  | 8.0 |  |  | — |  |  | — |
Transfers into Level 3 | — |  |  | 0.9 |  |  | — |  |  | — |
Transfers out of Level 3 | 12.2 |  |  | 1.2 |  |  | — |  |  | — |
Purchases | 28.3 |  |  | 22.0 |  |  | — |  |  | — |
Sales | (0.3 | ) |  | (0.9 | ) |  | — |  |  | — |
Settlements (a) | (21.3 | ) |  | (6.4 | ) |  | (95.8 | ) |  | 67.7 |
Ending balance, September 30 |  | ($3.7 | ) |  |  | ($7.9 | ) |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($29.4 | ) |  |  | $9.7 |  |  |  | $— |  |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Three Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, July 1 |  | $17.1 |  |  |  | $18.3 |  |  |  | $170.0 |  |  |  | $74.4 |
Total net losses included in changes in net assets (realized/unrealized) | (4.4 | ) |  | (0.4 | ) |  | — |  |  | — |
Transfers out of Level 3 | — |  |  | 0.3 |  |  | — |  |  | — |
Sales | (0.1 | ) |  | (0.2 | ) |  | — |  |  | — |
Settlements (a) | (7.3 | ) |  | (4.6 | ) |  | (54.7 | ) |  | 165.3 |
Ending balance, September 30 |  | $5.3 |  |  |  | $13.4 |  |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net losses for the period included in changes in net assets attributable to the change in unrealized losses relating to assets and liabilities held at September 30 |  | ($4.5 | ) |  |  | ($0.4 | ) |  |  | $— |  |  |  | $— |
IPL | Commodity Contract Derivative |  |
| Assets and (Liabilities), net |  | Deferred Proceeds
Nine Months Ended September 30 | 2017 |  | 2016 |  | 2017 |  | 2016
Beginning balance, January 1 |  | $10.1 |  |  |  | ($1.9 | ) |  |  | $211.1 |  |  |  | $172.0 |
Total net gains (losses) included in changes in net assets (realized/unrealized) | (13.9 | ) |  | 4.8 |  |  | — |  |  | — |
Transfers into Level 3 | — |  |  | 0.5 |  |  | — |  |  | — |
Transfers out of Level 3 | 3.1 |  |  | 0.2 |  |  | — |  |  | — |
Purchases | 24.6 |  |  | 20.6 |  |  | — |  |  | — |
Sales | (0.2 | ) |  | (0.9 | ) |  | — |  |  | — |
Settlements (a) | (18.4 | ) |  | (9.9 | ) |  | (95.8 | ) |  | 67.7 |
Ending balance, September 30 |  | $5.3 |  |  |  | $13.4 |  |  |  | $115.3 |  |  |  | $239.7 |
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($12.6 | ) |  |  | $5.7 |  |  |  | $— |  |  |  | $— |
WPL | Commodity Contract Derivative
| Assets and (Liabilities), net
Three Months Ended September 30 | 2017 |  | 2016
Beginning balance, July 1 |  | ($7.9 | ) |  |  | ($17.7 | )
Total net gains (losses) included in changes in net assets (realized/unrealized) | 0.1 |  |  | (4.7 | )
Transfers out of Level 3 | — |  |  | 0.5 |
Settlements | (1.2 | ) |  | 0.6 |
Ending balance, September 30 |  | ($9.0 | ) |  |  | ($21.3 | )
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | $0.3 |  |  |  | ($4.6 | )
WPL | Commodity Contract Derivative
| Assets and (Liabilities), net
Nine Months Ended September 30 | 2017 |  | 2016
Beginning balance, January 1 |  | ($1.4 | ) |  |  | ($30.8 | )
Total net gains (losses) included in changes in net assets (realized/unrealized) | (17.4 | ) |  | 3.2 |
Transfers into Level 3 | — |  |  | 0.4 |
Transfers out of Level 3 | 9.1 |  |  | 1.0 |
Purchases | 3.7 |  |  | 1.4 |
Sales | (0.1 | ) |  | — |
Settlements | (2.9 | ) |  | 3.5 |
Ending balance, September 30 |  | ($9.0 | ) |  |  | ($21.3 | )
The amount of total net gains (losses) for the period included in changes in net assets attributable to the change in unrealized gains (losses) relating to assets and liabilities held at September 30 |  | ($16.8 | ) |  |  | $4.0 |
(a) | Settlements related to deferred proceeds are due to the change in the carrying amount of receivables sold less the allowance for doubtful accounts associated with the receivables sold and cash amounts received from the receivables sold.
Fair Value Of Net Derivative Assets (Liabilities) | The fair value of electric, natural gas, coal and diesel fuel commodity contracts categorized as Level 3 was recognized as net derivative assets (liabilities) as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs |  | Excluding FTRs |  | FTRs
September 30, 2017 |  | ($22.2 | ) |  |  | $18.5 |  |  |  | ($10.4 | ) |  |  | $15.7 |  |  |  | ($11.8 | ) |  |  | $2.8 |
December 31, 2016 | (2.3 | ) |  | 11.0 |  |  | 0.1 |  |  | 10.0 |  |  | (2.4 | ) |  | 1.0 |

X
- Definition Net Derivative Assets And Liabilities [Table Text Block] + References No definition available. + Details
Name: | lnt_NetDerivativeAssetsAndLiabilitiesTableTextBlock
Namespace Prefix: | lnt_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the fair value measurement of assets using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets) and gains or losses recognized in other comprehensive income (loss), and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs), by class of asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 + Details
Name: | us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
Name: | us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |