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Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Debt Instrument [Line Items] |  |
Total Long-Term Debt | $ 1,605.0 | $ 1,373.5
Less: Due Within One Year | 36.3 | 100.7
Net Long-Term Debt | 1,568.7 | 1,272.8
First Mortgage Bonds - 7.70% Series Due 2016 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 20.0 | 20.0
First Mortgage Bonds - 1.83% Series Due 2018 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 50.0 | 50.0
First Mortgage Bonds - 8.17% Series Due 2019 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 42.0 | 42.0
First Mortgage Bonds - 5.28% Series Due 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 35.0 | 35.0
First Mortgage Bonds - 2.80% Series Due 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 40.0 | 0.0
First Mortgage Bonds - 4.85% Series Due 2021 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 15.0 | 15.0
First Mortgage Bonds - 3.02% Series Due 2021 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 60.0 | 60.0
First Mortgage Bonds - 3.40% Series Due 2022 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 75.0 | 75.0
First Mortgage Bonds - 6.02% Series Due 2023 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 75.0 | 75.0
First Mortgage Bonds - 3.69% Series Due 2024 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 60.0 | 60.0
First Mortgage Bonds - 4.90% Series Due 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 30.0 | 30.0
First Mortgage Bonds - 5.10% Series Due 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 30.0 | 30.0
First Mortgage Bonds - 3.20% Series Due 2026 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 75.0 | 75.0
First Mortgage Bonds - 5.99% Series Due 2027 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 60.0 | 60.0
First Mortgage Bonds - 3.30% Series Due 2028 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 40.0 | 40.0
First Mortgage Bonds - 3.74% Series Due 2029 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 50.0 | 50.0
First Mortgage Bonds - 3.86% Series Due 2030 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 60.0 | 0.0
First Mortgage Bonds - 5.69% Series Due 2036 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 50.0 | 50.0
First Mortgage Bonds - 6.00% Series Due 2040 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 35.0 | 35.0
First Mortgage Bonds - 5.82% Series Due 2040 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 45.0 | 45.0
First Mortgage Bonds - 4.08% Series Due 2042 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 85.0 | 85.0
First Mortgage Bonds - 4.21% Series Due 2043 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 60.0 | 60.0
First Mortgage Bonds - 4.95% Series Due 2044 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 40.0 | 40.0
First Mortgage Bonds - 5.05% Series Due 2044 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 40.0 | 40.0
First Mortgage Bonds - 4.39% Series Due 2044 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 50.0 | 50.0
Unsecured Term Loan Variable Rate Due 2015 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 0.0 | 75.0
Unsecured Term Loan Variable Rate Due 2017 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 125.0 | 0.0
Senior Unsecured Notes 5.99% Due 2017 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 50.0 | 50.0
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2015 – 2020 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 13.5 | 24.6
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 27.8 | 27.8
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 83.3 | 0.0
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | 15.0 | 15.0
Other Long-Term Debt, 0.08% – 7.45% Due 2016 – 2037 [Member] |  |
Debt Instrument [Line Items] |  |
Total Long-Term Debt | $ 68.4 | $ 59.1

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