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