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Leases | 12 Months Ended
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Dec. 31, 2015
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 2015, 2014 and 2013, rental expenses associated with operating leases were not material. At December 31, 2015, future minimum operating lease payments, excluding contingent rentals, were as follows (in millions):
| 2016 |  | 2017 |  | 2018 |  | 2019 |  | 2020 |  | Thereafter |  | Total
Alliant Energy |  | $6 |  |  |  | $8 |  |  |  | $3 |  |  |  | $2 |  |  |  | $2 |  |  |  | $19 |  |  |  | $40 |
IPL | 3 |  |  | 3 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | 13 |  |  | 23 |
WPL | 3 |  |  | 5 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | 9 |
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 2015, 2014 and 2013, rental expenses associated with operating leases were not material. At December 31, 2015, future minimum operating lease payments, excluding contingent rentals, were as follows (in millions):
| 2016 |  | 2017 |  | 2018 |  | 2019 |  | 2020 |  | Thereafter |  | Total
Alliant Energy |  | $6 |  |  |  | $8 |  |  |  | $3 |  |  |  | $2 |  |  |  | $2 |  |  |  | $19 |  |  |  | $40 |
IPL | 3 |  |  | 3 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | 13 |  |  | 23 |
WPL | 3 |  |  | 5 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | 9 |
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 2015, 2014 and 2013, rental expenses associated with operating leases were not material. At December 31, 2015, future minimum operating lease payments, excluding contingent rentals, were as follows (in millions):
| 2016 |  | 2017 |  | 2018 |  | 2019 |  | 2020 |  | Thereafter |  | Total
Alliant Energy |  | $6 |  |  |  | $8 |  |  |  | $3 |  |  |  | $2 |  |  |  | $2 |  |  |  | $19 |  |  |  | $40 |
IPL | 3 |  |  | 3 |  |  | 2 |  |  | 1 |  |  | 1 |  |  | 13 |  |  | 23 |
WPL | 3 |  |  | 5 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | 9 |
(b) Capital Leases - WPL - In 2005, WPL entered into a 20-year agreement with Resources’ Non-regulated Generation business to lease Sheboygan Falls, with an option for two lease renewal periods thereafter. The lease became effective in 2005 when Sheboygan Falls began commercial operation. WPL is responsible for the operation of Sheboygan Falls and has exclusive rights to its output. In 2005, the PSCW approved this affiliated lease agreement with initial monthly lease payments of approximately $1.3 million. The lease payments were based on a 50% debt to capital ratio, a return on equity of 10.9%, a cost of debt based on the cost of senior notes issued by Resources’ Non-regulated Generation business in 2005 and certain costs incurred to construct the facility. In accordance with its order approving the lease agreement, the PSCW reserved the right to review the capital structure, return on equity and cost of debt every five years from the date of the order. No revisions to the lease have been made since its inception. The capital lease asset is amortized using the straight-line method over the 20-year lease term. Since the inception of the lease in 2005, WPL’s retail and wholesale rates have included recovery of the monthly Sheboygan Falls lease payments. Sheboygan Falls lease expenses were included in WPL’s income statements as follows (in millions):
| 2015 |  | 2014 |  | 2013
Interest expense |  | $9.9 |  |  |  | $10.4 |  |  |  | $10.9 |
Depreciation and amortization | 6.2 |  |  | 6.2 |  |  | 6.2 |
|  | $16.1 |  |  |  | $16.6 |  |  |  | $17.1 |
At December 31, 2015, WPL’s estimated future minimum capital lease payments for Sheboygan Falls were as follows (in millions):
| 2016 |  | 2017 |  | 2018 |  | 2019 |  | 2020 |  | Thereafter |  | Total |  | Less: amount representing interest |  | Present value of minimum capital lease payments
Sheboygan Falls | $15 |  | $15 |  | $15 |  | $15 |  | $15 |  | $68 |  | $143 |  | $54 |  | $89

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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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