Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt	$ 1,569.1	$ 1,605.0
Unamortized Debt Issuance Costs	(11.0)	(12.6)
Total Long-Term Debt	1,558.1	1,592.4
Less: Due Within One Year	187.7	35.7
Net Long-Term Debt	1,370.4	1,556.7
First Mortgage Bonds - 7.70% Series Due 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 0.0	20.0
Interest Rate	7.70%
First Mortgage Bonds - 1.83% Series Due 2018 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50.0	50.0
Interest Rate	1.83%
First Mortgage Bonds - 8.17% Series Due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 42.0	42.0
Interest Rate	8.17%
First Mortgage Bonds - 5.28% Series Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 35.0	35.0
Interest Rate	5.28%
First Mortgage Bonds - 2.80% Series Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 40.0	40.0
Interest Rate	2.80%
First Mortgage Bonds - 4.85% Series Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 15.0	15.0
Interest Rate	4.85%
First Mortgage Bonds - 3.02% Series Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 60.0	60.0
Interest Rate	3.02%
First Mortgage Bonds - 3.40% Series Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 75.0	75.0
Interest Rate	3.40%
First Mortgage Bonds - 6.02% Series Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 75.0	75.0
Interest Rate	6.02%
First Mortgage Bonds - 3.69% Series Due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 60.0	60.0
Interest Rate	3.69%
First Mortgage Bonds - 4.90% Series Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 30.0	30.0
Interest Rate	4.90%
First Mortgage Bonds - 5.10% Series Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 30.0	30.0
Interest Rate	5.10%
First Mortgage Bonds - 3.20% Series Due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 75.0	75.0
Interest Rate	3.20%
First Mortgage Bonds - 5.99% Series Due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 60.0	60.0
Interest Rate	5.99%
First Mortgage Bonds - 3.30% Series Due 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 40.0	40.0
Interest Rate	3.30%
First Mortgage Bonds - 3.74% Series Due 2029 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50.0	50.0
Interest Rate	3.74%
First Mortgage Bonds - 3.86% Series Due 2030 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 60.0	60.0
Interest Rate	3.86%
First Mortgage Bonds - 5.69% Series Due 2036 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50.0	50.0
Interest Rate	5.69%
First Mortgage Bonds - 6.00% Series Due 2040 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 35.0	35.0
Interest Rate	6.00%
First Mortgage Bonds - 5.82% Series Due 2040 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 45.0	45.0
Interest Rate	5.82%
First Mortgage Bonds - 4.08% Series Due 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 85.0	85.0
Interest Rate	4.08%
First Mortgage Bonds - 4.21% Series Due 2043 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 60.0	60.0
Interest Rate	4.21%
First Mortgage Bonds - 4.95% Series Due 2044 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 40.0	40.0
Interest Rate	4.95%
First Mortgage Bonds - 5.05% Series Due 2044 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 40.0	40.0
Interest Rate	5.05%
First Mortgage Bonds - 4.39% Series Due 2044 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50.0	50.0
Interest Rate	4.39%
Unsecured Term Loan Variable Rate Due 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 125.0	125.0
Senior Unsecured Notes 5.99% Due 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50.0	50.0
Interest Rate	5.99%
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 13.5	13.5
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	27.8	27.8
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 74.6	83.3
Interest Rate	3.26%
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 15.0	15.0
Interest Rate	4.15%
Other Long-Term Debt, 3.11% – 6.20% Due 2017 – 2037 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 61.2	$ 68.4
Minimum [Member] | Other Long-Term Debt, 3.11% – 6.20% Due 2017 – 2037 [Member]
Debt Instrument [Line Items]
Interest Rate	3.11%
Maximum [Member] | Other Long-Term Debt, 3.11% – 6.20% Due 2017 – 2037 [Member]
Debt Instrument [Line Items]
Interest Rate	6.20%