XML 72 R39.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Debt (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2018
Debt Disclosure [Line Items] |
Other Short-Term Borrowings | Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions):
| Alliant Energy |  | IPL |  | WPL
December 31 | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Commercial paper outstanding | $441.2 |  | $320.2 |  | $50.4 |  | $— |  | $105.5 |  | $25.0
Commercial paper weighted average interest rates | 2.8% |  | 2.0% |  | 2.8% |  | N/A |  | 2.5% |  | 1.5%
Available credit facility capacity | $558.8 |  | $679.8 |  | $199.6 |  | $250.0 |  | $244.5 |  | $325.0
| Alliant Energy |  | IPL |  | WPL
For the year ended | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Maximum amount outstanding (based on daily outstanding balances) | $446.5 |  | $424.4 |  | $50.4 |  | $20.0 |  | $126.0 |  | $271.2
Average amount outstanding (based on daily outstanding balances) | $221.4 |  | $294.3 |  | $1.5 |  | $0.5 |  | $36.6 |  | $118.2
Weighted average interest rates | 2.2% |  | 1.2% |  | 2.3% |  | 1.3% |  | 2.1% |  | 1.0%
Schedule of Debt-To-Capital Ratios | The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2018 were as follows:
| Alliant Energy |  | IPL |  | WPL
Requirement, not to exceed | 65% |  | 65% |  | 65%
Actual | 55% |  | 45% |  | 48%
Schedule of Long-term Debt | Long-term debt, net as of December 31 was as follows (dollars in millions):
| 2018 |  | 2017
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Senior Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.65%, due 2020 |  | $200.0 |  |  |  | $200.0 |  |  |  | $— |  |  |  | $200.0 |  |  |  | $200.0 |  |  |  | $— |
3.25%, due 2024 | 500.0 |  |  | 500.0 |  |  | — |  |  | 500.0 |  |  | 500.0 |  |  | — |
3.4%, due 2025 | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
5.5%, due 2025 | 50.0 |  |  | 50.0 |  |  | — |  |  | 50.0 |  |  | 50.0 |  |  | — |
4.1%, due 2028 (b) | 500.0 |  |  | 500.0 |  |  | — |  |  | — |  |  | — |  |  | — |
6.45%, due 2033 | 100.0 |  |  | 100.0 |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |
6.3%, due 2034 | 125.0 |  |  | 125.0 |  |  | — |  |  | 125.0 |  |  | 125.0 |  |  | — |
6.25%, due 2039 | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |
4.7%, due 2043 | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
3.7%, due 2046 | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |
5.875% (Retired in 2018) | — |  |  | — |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |
7.25% (Retired in 2018) | — |  |  | — |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
| 2,575.0 |  |  | 2,575.0 |  |  | — |  |  | 2,425.0 |  |  | 2,425.0 |  |  | — |
Debentures (a): |  |  |  |  |  |  |  |  |  |  |
5%, due 2019 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
4.6%, due 2020 | 150.0 |  |  | — |  |  | 150.0 |  |  | 150.0 |  |  | — |  |  | 150.0 |
2.25%, due 2022 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
3.05%, due 2027 | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |
6.25%, due 2034 | 100.0 |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |  |  | 100.0 |
6.375%, due 2037 | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |
7.6%, due 2038 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
4.1%, due 2044 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
| 1,850.0 |  |  | — |  |  | 1,850.0 |  |  | 1,850.0 |  |  | — |  |  | 1,850.0 |
Other: |  |  |  |  |  |  |  |  |  |  |
AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) | 300.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Corporate Services 3.45% senior notes, due 2022 (a) | 75.0 |  |  | — |  |  | — |  |  | 75.0 |  |  | — |  |  | — |
AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) | 400.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) | 300.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) | 44.3 |  |  | — |  |  | — |  |  | 49.6 |  |  | — |  |  | — |
AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) | — |  |  | — |  |  | — |  |  | 500.0 |  |  | — |  |  | — |
Other, 1% at December 31, 2018, due 2019 to 2025 | 2.4 |  |  | — |  |  | — |  |  | 2.9 |  |  | — |  |  | — |
| 1,121.7 |  |  | — |  |  | — |  |  | 627.5 |  |  | — |  |  | — |
Subtotal | 5,546.7 |  |  | 2,575.0 |  |  | 1,850.0 |  |  | 4,902.5 |  |  | 2,425.0 |  |  | 1,850.0 |
Current maturities | (256.5 | ) |  | — |  |  | (250.0 | ) |  | (855.7 | ) |  | (350.0 | ) |  | — |
Unamortized debt issuance costs | (32.1 | ) |  | (17.2 | ) |  | (9.6 | ) |  | (25.4 | ) |  | (14.3 | ) |  | (10.5 | )
Unamortized debt (discount) and premium, net | (11.8 | ) |  | (5.5 | ) |  | (5.5 | ) |  | (10.8 | ) |  | (4.7 | ) |  | (6.1 | )
Long-term debt, net (e) |  | $5,246.3 |  |  |  | $2,552.3 |  |  |  | $1,584.9 |  |  |  | $4,010.6 |  |  |  | $2,056.0 |  |  |  | $1,833.4 |
(a) | Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.
(b) | In September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects.
(c) | In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.
(d) | In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.
(e) | There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities | At December 31, 2018, long-term debt maturities for 2019 through 2023 were as follows (in millions):
| 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023
IPL |  | $— |  |  |  | $200 |  |  |  | $— |  |  |  | $— |  |  |  | $— |
WPL | 250 |  |  | 150 |  |  | — |  |  | 250 |  |  | — |
Corporate Services | — |  |  | — |  |  | — |  |  | 75 |  |  | — |
AEF | 6 |  |  | 307 |  |  | 8 |  |  | 8 |  |  | 408 |
Alliant Energy |  | $256 |  |  |  | $657 |  |  |  | $8 |  |  |  | $333 |  |  |  | $408 |
IPL [Member] |
Debt Disclosure [Line Items] |
Other Short-Term Borrowings | Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions):
| Alliant Energy |  | IPL |  | WPL
December 31 | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Commercial paper outstanding | $441.2 |  | $320.2 |  | $50.4 |  | $— |  | $105.5 |  | $25.0
Commercial paper weighted average interest rates | 2.8% |  | 2.0% |  | 2.8% |  | N/A |  | 2.5% |  | 1.5%
Available credit facility capacity | $558.8 |  | $679.8 |  | $199.6 |  | $250.0 |  | $244.5 |  | $325.0
| Alliant Energy |  | IPL |  | WPL
For the year ended | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Maximum amount outstanding (based on daily outstanding balances) | $446.5 |  | $424.4 |  | $50.4 |  | $20.0 |  | $126.0 |  | $271.2
Average amount outstanding (based on daily outstanding balances) | $221.4 |  | $294.3 |  | $1.5 |  | $0.5 |  | $36.6 |  | $118.2
Weighted average interest rates | 2.2% |  | 1.2% |  | 2.3% |  | 1.3% |  | 2.1% |  | 1.0%
Schedule of Debt-To-Capital Ratios | The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2018 were as follows:
| Alliant Energy |  | IPL |  | WPL
Requirement, not to exceed | 65% |  | 65% |  | 65%
Actual | 55% |  | 45% |  | 48%
Schedule of Long-term Debt | Long-term debt, net as of December 31 was as follows (dollars in millions):
| 2018 |  | 2017
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Senior Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.65%, due 2020 |  | $200.0 |  |  |  | $200.0 |  |  |  | $— |  |  |  | $200.0 |  |  |  | $200.0 |  |  |  | $— |
3.25%, due 2024 | 500.0 |  |  | 500.0 |  |  | — |  |  | 500.0 |  |  | 500.0 |  |  | — |
3.4%, due 2025 | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
5.5%, due 2025 | 50.0 |  |  | 50.0 |  |  | — |  |  | 50.0 |  |  | 50.0 |  |  | — |
4.1%, due 2028 (b) | 500.0 |  |  | 500.0 |  |  | — |  |  | — |  |  | — |  |  | — |
6.45%, due 2033 | 100.0 |  |  | 100.0 |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |
6.3%, due 2034 | 125.0 |  |  | 125.0 |  |  | — |  |  | 125.0 |  |  | 125.0 |  |  | — |
6.25%, due 2039 | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |
4.7%, due 2043 | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
3.7%, due 2046 | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |
5.875% (Retired in 2018) | — |  |  | — |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |
7.25% (Retired in 2018) | — |  |  | — |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
| 2,575.0 |  |  | 2,575.0 |  |  | — |  |  | 2,425.0 |  |  | 2,425.0 |  |  | — |
Debentures (a): |  |  |  |  |  |  |  |  |  |  |
5%, due 2019 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
4.6%, due 2020 | 150.0 |  |  | — |  |  | 150.0 |  |  | 150.0 |  |  | — |  |  | 150.0 |
2.25%, due 2022 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
3.05%, due 2027 | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |
6.25%, due 2034 | 100.0 |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |  |  | 100.0 |
6.375%, due 2037 | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |
7.6%, due 2038 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
4.1%, due 2044 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
| 1,850.0 |  |  | — |  |  | 1,850.0 |  |  | 1,850.0 |  |  | — |  |  | 1,850.0 |
Other: |  |  |  |  |  |  |  |  |  |  |
AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) | 300.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Corporate Services 3.45% senior notes, due 2022 (a) | 75.0 |  |  | — |  |  | — |  |  | 75.0 |  |  | — |  |  | — |
AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) | 400.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) | 300.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) | 44.3 |  |  | — |  |  | — |  |  | 49.6 |  |  | — |  |  | — |
AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) | — |  |  | — |  |  | — |  |  | 500.0 |  |  | — |  |  | — |
Other, 1% at December 31, 2018, due 2019 to 2025 | 2.4 |  |  | — |  |  | — |  |  | 2.9 |  |  | — |  |  | — |
| 1,121.7 |  |  | — |  |  | — |  |  | 627.5 |  |  | — |  |  | — |
Subtotal | 5,546.7 |  |  | 2,575.0 |  |  | 1,850.0 |  |  | 4,902.5 |  |  | 2,425.0 |  |  | 1,850.0 |
Current maturities | (256.5 | ) |  | — |  |  | (250.0 | ) |  | (855.7 | ) |  | (350.0 | ) |  | — |
Unamortized debt issuance costs | (32.1 | ) |  | (17.2 | ) |  | (9.6 | ) |  | (25.4 | ) |  | (14.3 | ) |  | (10.5 | )
Unamortized debt (discount) and premium, net | (11.8 | ) |  | (5.5 | ) |  | (5.5 | ) |  | (10.8 | ) |  | (4.7 | ) |  | (6.1 | )
Long-term debt, net (e) |  | $5,246.3 |  |  |  | $2,552.3 |  |  |  | $1,584.9 |  |  |  | $4,010.6 |  |  |  | $2,056.0 |  |  |  | $1,833.4 |
(a) | Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.
(b) | In September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects.
(c) | In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.
(d) | In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.
(e) | There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities | At December 31, 2018, long-term debt maturities for 2019 through 2023 were as follows (in millions):
| 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023
IPL |  | $— |  |  |  | $200 |  |  |  | $— |  |  |  | $— |  |  |  | $— |
WPL | 250 |  |  | 150 |  |  | — |  |  | 250 |  |  | — |
Corporate Services | — |  |  | — |  |  | — |  |  | 75 |  |  | — |
AEF | 6 |  |  | 307 |  |  | 8 |  |  | 8 |  |  | 408 |
Alliant Energy |  | $256 |  |  |  | $657 |  |  |  | $8 |  |  |  | $333 |  |  |  | $408 |
WPL [Member] |
Debt Disclosure [Line Items] |
Other Short-Term Borrowings | Information regarding commercial paper classified as short-term debt and back-stopped by the credit facility was as follows (dollars in millions):
| Alliant Energy |  | IPL |  | WPL
December 31 | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Commercial paper outstanding | $441.2 |  | $320.2 |  | $50.4 |  | $— |  | $105.5 |  | $25.0
Commercial paper weighted average interest rates | 2.8% |  | 2.0% |  | 2.8% |  | N/A |  | 2.5% |  | 1.5%
Available credit facility capacity | $558.8 |  | $679.8 |  | $199.6 |  | $250.0 |  | $244.5 |  | $325.0
| Alliant Energy |  | IPL |  | WPL
For the year ended | 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Maximum amount outstanding (based on daily outstanding balances) | $446.5 |  | $424.4 |  | $50.4 |  | $20.0 |  | $126.0 |  | $271.2
Average amount outstanding (based on daily outstanding balances) | $221.4 |  | $294.3 |  | $1.5 |  | $0.5 |  | $36.6 |  | $118.2
Weighted average interest rates | 2.2% |  | 1.2% |  | 2.3% |  | 1.3% |  | 2.1% |  | 1.0%
Schedule of Debt-To-Capital Ratios | The required debt-to-capital ratios compared to the actual debt-to-capital ratios at December 31, 2018 were as follows:
| Alliant Energy |  | IPL |  | WPL
Requirement, not to exceed | 65% |  | 65% |  | 65%
Actual | 55% |  | 45% |  | 48%
Schedule of Long-term Debt | Long-term debt, net as of December 31 was as follows (dollars in millions):
| 2018 |  | 2017
| Alliant Energy |  | IPL |  | WPL |  | Alliant Energy |  | IPL |  | WPL
Senior Debentures (a): |  |  |  |  |  |  |  |  |  |  |
3.65%, due 2020 |  | $200.0 |  |  |  | $200.0 |  |  |  | $— |  |  |  | $200.0 |  |  |  | $200.0 |  |  |  | $— |
3.25%, due 2024 | 500.0 |  |  | 500.0 |  |  | — |  |  | 500.0 |  |  | 500.0 |  |  | — |
3.4%, due 2025 | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
5.5%, due 2025 | 50.0 |  |  | 50.0 |  |  | — |  |  | 50.0 |  |  | 50.0 |  |  | — |
4.1%, due 2028 (b) | 500.0 |  |  | 500.0 |  |  | — |  |  | — |  |  | — |  |  | — |
6.45%, due 2033 | 100.0 |  |  | 100.0 |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |
6.3%, due 2034 | 125.0 |  |  | 125.0 |  |  | — |  |  | 125.0 |  |  | 125.0 |  |  | — |
6.25%, due 2039 | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |
4.7%, due 2043 | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
3.7%, due 2046 | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |
5.875% (Retired in 2018) | — |  |  | — |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |
7.25% (Retired in 2018) | — |  |  | — |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |
| 2,575.0 |  |  | 2,575.0 |  |  | — |  |  | 2,425.0 |  |  | 2,425.0 |  |  | — |
Debentures (a): |  |  |  |  |  |  |  |  |  |  |
5%, due 2019 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
4.6%, due 2020 | 150.0 |  |  | — |  |  | 150.0 |  |  | 150.0 |  |  | — |  |  | 150.0 |
2.25%, due 2022 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
3.05%, due 2027 | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |
6.25%, due 2034 | 100.0 |  |  | — |  |  | 100.0 |  |  | 100.0 |  |  | — |  |  | 100.0 |
6.375%, due 2037 | 300.0 |  |  | — |  |  | 300.0 |  |  | 300.0 |  |  | — |  |  | 300.0 |
7.6%, due 2038 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
4.1%, due 2044 | 250.0 |  |  | — |  |  | 250.0 |  |  | 250.0 |  |  | — |  |  | 250.0 |
| 1,850.0 |  |  | — |  |  | 1,850.0 |  |  | 1,850.0 |  |  | — |  |  | 1,850.0 |
Other: |  |  |  |  |  |  |  |  |  |  |
AEF term loan credit agreement through April 2020, 3% at December 31, 2018 (with Alliant Energy as guarantor) (c) | 300.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Corporate Services 3.45% senior notes, due 2022 (a) | 75.0 |  |  | — |  |  | — |  |  | 75.0 |  |  | — |  |  | — |
AEF 3.75% senior notes, due 2023 (with Alliant Energy as guarantor) (a)(d) | 400.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
AEF 4.25% senior notes, due 2028 (with Alliant Energy as guarantor) (a)(d) | 300.0 |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Sheboygan Power, LLC 5.06% senior secured notes, due 2019 to 2024 (secured by the Sheboygan Falls Energy Facility and related assets) (a) | 44.3 |  |  | — |  |  | — |  |  | 49.6 |  |  | — |  |  | — |
AEF term loan credit agreement, 2% at December 31, 2017 (with Alliant Energy as guarantor) (Retired in 2018) (d) | — |  |  | — |  |  | — |  |  | 500.0 |  |  | — |  |  | — |
Other, 1% at December 31, 2018, due 2019 to 2025 | 2.4 |  |  | — |  |  | — |  |  | 2.9 |  |  | — |  |  | — |
| 1,121.7 |  |  | — |  |  | — |  |  | 627.5 |  |  | — |  |  | — |
Subtotal | 5,546.7 |  |  | 2,575.0 |  |  | 1,850.0 |  |  | 4,902.5 |  |  | 2,425.0 |  |  | 1,850.0 |
Current maturities | (256.5 | ) |  | — |  |  | (250.0 | ) |  | (855.7 | ) |  | (350.0 | ) |  | — |
Unamortized debt issuance costs | (32.1 | ) |  | (17.2 | ) |  | (9.6 | ) |  | (25.4 | ) |  | (14.3 | ) |  | (10.5 | )
Unamortized debt (discount) and premium, net | (11.8 | ) |  | (5.5 | ) |  | (5.5 | ) |  | (10.8 | ) |  | (4.7 | ) |  | (6.1 | )
Long-term debt, net (e) |  | $5,246.3 |  |  |  | $2,552.3 |  |  |  | $1,584.9 |  |  |  | $4,010.6 |  |  |  | $2,056.0 |  |  |  | $1,833.4 |
(a) | Contains optional redemption provisions which, if elected by the issuer at its sole discretion, could require material redemption premium payments by the issuer. The redemption premium payments under these optional redemption provisions are variable and dependent on applicable U.S. Treasury rates at the time of redemption.
(b) | In September 2018, IPL issued $500 million of 4.1% senior debentures due 2028. The senior debentures were issued as green bonds, and all of the net proceeds were allocated for the construction and development of wind and solar projects.
(c) | In April 2018, AEF entered into a $300 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement for general corporate purposes. Refer to Note 9(a) for discussion of a financial covenant contained in AEF’s term loan credit agreement.
(d) | In June 2018, AEF issued $400 million of 3.75% senior notes due 2023 and $300 million of 4.25% senior notes due 2028. The proceeds from the issuances were used by AEF to retire its $500 million and $95 million variable-rate term loan credit agreements expiring in 2018, to reduce Alliant Energy’s outstanding commercial paper and for general corporate purposes.
(e) | There were no significant sinking fund requirements related to the outstanding long-term debt.
Schedule of Debt Maturities | At December 31, 2018, long-term debt maturities for 2019 through 2023 were as follows (in millions):
| 2019 |  | 2020 |  | 2021 |  | 2022 |  | 2023
IPL |  | $— |  |  |  | $200 |  |  |  | $— |  |  |  | $— |  |  |  | $— |
WPL | 250 |  |  | 150 |  |  | — |  |  | 250 |  |  | — |
Corporate Services | — |  |  | — |  |  | — |  |  | 75 |  |  | — |
AEF | 6 |  |  | 307 |  |  | 8 |  |  | 8 |  |  | 408 |
Alliant Energy |  | $256 |  |  |  | $657 |  |  |  | $8 |  |  |  | $333 |  |  |  | $408 |

X
- Definition [Line Items] for Debt Disclosure [Table] [Table] + References No definition available. + Details
Name: | lnt_DebtDisclosureLineItems
Namespace Prefix: | lnt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the capital amounts and ratios as of the balance sheet date, indicating whether the entity or entities are in compliance with regulatory capital requirements, by entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1G -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958577-112826 + Details
Name: | us-gaap_ScheduleOfComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e),(f)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399 Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644 + Details
Name: | us-gaap_ScheduleOfDebtInstrumentsTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of the combined aggregate amount of maturities and sinking fund requirements for all long-term borrowings for each of the five years following the date of the latest balance sheet date presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601 + Details
Name: | us-gaap_ScheduleOfMaturitiesOfLongTermDebtTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of short-term debt arrangements (having initial terms of repayment within one year or the normal operating cycle, if longer) including: (1) description of the short-term debt arrangement; (2) identification of the lender or type of lender; (3) repayment terms; (4) weighted average interest rate; (5) carrying amount of funds borrowed under the specified short-term debt arrangement as of the balance sheet date; (6) description of the refinancing of a short-term obligation when that obligation is excluded from current liabilities in the balance sheet; and (7) amount of a short-term obligation that has been excluded from current liabilities in the balance sheet because of a refinancing of the obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_ScheduleOfShortTermDebtTextBlock
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: |