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Capital Lease and Finance Obligations - UNS Energy (Details) - UNS Energy CAD in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 CAD | Dec. 31, 2015 CAD | Dec. 31, 2016 USD ($) lease | Dec. 31, 2015 USD ($)
Springerville Common Facilities |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Number of capital lease obligations | lease |  |  | 3 |
Interest expense related to capital lease obligations | CAD | CAD 4 | CAD 5 |  |
Depreciation expense related to assets under capital lease | CAD | CAD 7 | 8 |  |
Springerville Common Facilities | Energy supply costs |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Depreciation expense related to assets under capital lease | CAD |  | CAD 3 |  |
Springerville Common Facilities | Interest rate swaps |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Amortized principal balance |  |  | $ 23 | $ 29
Springerville Common Facilities | Minimum |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Lease renewal term | 2 years |  |  |
Springerville Common Facilities | Capital Leases |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Stated interest rate (percent) |  |  | 5.08% |
Springerville Common Facilities | Capital Leases | LIBOR | Interest rate swaps |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Variable rate (percent) | 1.88% | 1.88% |  |
Springerville Common Facilities, One Lease [Member] |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Fixed-price purchase provision |  |  | $ 38 |
Commitment to purchase capital lease (percent) |  |  | 17.80% |
Fixed purchase price |  |  | $ 38 |
Springerville Common Facilities, Leases Expiring 2021 |  |  |  |
Capital Leased Assets [Line Items] |  |  |  |
Fixed-price purchase provision |  |  | $ 68 |
Lease renewal term | 2 years |  |  |

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