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Leases | 12 Months Ended
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Dec. 31, 2017
Leases | LEASES (a) Operating Leases - Various agreements have been entered into related to property, plant and equipment rights that are accounted for as operating leases. In 2017, 2016 and 2015, rental expenses associated with operating leases were not material. At December 31, 2017, future minimum operating lease payments, excluding contingent rentals, were as follows (in millions):
| 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | Thereafter |  | Total
Alliant Energy |  | $6 |  |  |  | $5 |  |  |  | $2 |  |  |  | $2 |  |  |  | $1 |  |  |  | $13 |  |  |  | $29 |
IPL | 3 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | 1 |  |  | 13 |  |  | 21 |
WPL | 3 |  |  | 3 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | 7 |
IPL [Member] |
Leases | LEASES (a) Operating Leases - Various agreements have been entered into related to property, plant and equipment rights that are accounted for as operating leases. In 2017, 2016 and 2015, rental expenses associated with operating leases were not material. At December 31, 2017, future minimum operating lease payments, excluding contingent rentals, were as follows (in millions):
| 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | Thereafter |  | Total
Alliant Energy |  | $6 |  |  |  | $5 |  |  |  | $2 |  |  |  | $2 |  |  |  | $1 |  |  |  | $13 |  |  |  | $29 |
IPL | 3 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | 1 |  |  | 13 |  |  | 21 |
WPL | 3 |  |  | 3 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | 7 |
WPL [Member] |
Leases | LEASES (a) Operating Leases - Various agreements have been entered into related to property, plant and equipment rights that are accounted for as operating leases. In 2017, 2016 and 2015, rental expenses associated with operating leases were not material. At December 31, 2017, future minimum operating lease payments, excluding contingent rentals, were as follows (in millions):
| 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | Thereafter |  | Total
Alliant Energy |  | $6 |  |  |  | $5 |  |  |  | $2 |  |  |  | $2 |  |  |  | $1 |  |  |  | $13 |  |  |  | $29 |
IPL | 3 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | 1 |  |  | 13 |  |  | 21 |
WPL | 3 |  |  | 3 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | 7 |
(b) Capital Leases - WPL - In 2005, WPL entered into a 20-year agreement with AEF’s Non-utility Generation business to lease the Sheboygan Falls Energy Facility, with an option for two lease renewal periods thereafter. The lease became effective in 2005 when the EGU began commercial operation. WPL is responsible for the operation of the EGU and has exclusive rights to its output, and the PSCW approved this affiliated lease agreement in 2005. The capital lease asset is amortized using the straight-line method over the 20-year lease term. WPL’s retail and wholesale rates include recovery of the Sheboygan Falls Energy Facility lease payments. The Sheboygan Falls Energy Facility lease expenses were included in WPL’s income statements as follows (in millions):
| 2017 |  | 2016 |  | 2015
Interest expense |  | $8.7 |  |  |  | $9.3 |  |  |  | $9.9 |
Depreciation and amortization | 6.2 |  |  | 6.2 |  |  | 6.2 |
|  | $14.9 |  |  |  | $15.5 |  |  |  | $16.1 |
At December 31, 2017, WPL’s estimated future minimum capital lease payments for the Sheboygan Falls Energy Facility were as follows (in millions):
| 2018 |  | 2019 |  | 2020 |  | 2021 |  | 2022 |  | Thereafter |  | Total |  | Less: amount representing interest |  | Present value of minimum capital lease payments
Sheboygan Falls Energy Facility | $15 |  | $15 |  | $15 |  | $15 |  | $15 |  | $35 |  | $110 |  | $33 |  | $77

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- Definition The entire disclosure for lessee entity's leasing arrangements including, but not limited to, all of the following: (a.) The basis on which contingent rental payments are determined, (b.) The existence and terms of renewal or purchase options and escalation clauses, (c.) Restrictions imposed by lease agreements, such as those concerning dividends, additional debt, and further leasing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6452660&loc=d3e36991-112694 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 1,3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 + Details
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