Short-term and Long-term Debt - Schedule of Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Total Long-term Debt	$ 1,373.5	$ 1,110.2
Less: Due Within One Year	100.7	27.2
Net Long-Term Debt	1,272.8	1,083.0
6.94% Series Due 2014 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	0	18.0
7.70% Series Due 2016 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	20.0	20.0
1.83% Series Due 2018 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	50.0	50.0
8.17% Series Due 2019 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	42.0	42.0
5.28% Series Due 2020 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	35.0	35.0
4.85% Series Due 2021 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	15.0	15.0
3.02% Series Due 2021 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	60.0	0
3.40% Series Due 2022 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	75.0	0
4.95% Pollution Control Series F Due 2022 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	0	111.0
6.02% Series Due 2023 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	75.0	75.0
3.69% Series Due 2024 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	60.0	0
4.90% Series Due 2025 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	30.0	30.0
5.10% Series Due 2025 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	30.0	30.0
3.20% Series Due 2026 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	75.0	75.0
5.99% Series Due 2027 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	60.0	60.0
3.30% Series Due 2028 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	40.0	40.0
3.74% Series Due 2029 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	50.0	0
5.69% Series Due 2036 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	50.0	50.0
6.00% Series Due 2040 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	35.0	35.0
5.82% Series Due 2040 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	45.0	45.0
4.08% Series Due 2042 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	85.0	85.0
4.21% Series Due 2043 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	60.0	60.0
4.95% Series Due 2044 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	40.0	0
5.05% Series Due 2044 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	40.0	0
4.39% Series Due 2044 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	50.0	0
Unsecured Term Loan Variable Rate Due 2015 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	75.0	75.0
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	24.6	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
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%-7.50% Due 2015-2037 [Member]
Debt Instrument [Line Items]
Total Long-term Debt	$ 59.1	$ 41.8