Short-term and Long-term Debt - Schedule of Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term Debt	$ 1,018.1	$ 863.3
Less: Due Within One Year	84.5	5.4
Net Long-Term Debt	933.6	857.9
4.86% Series Due 2013 [Member]
Debt Instrument [Line Items]
Long-term Debt	60.0	60.0
Less: Due Within One Year	60.0
6.94% Series Due 2014 [Member]
Debt Instrument [Line Items]
Long-term Debt	18.0	18.0
7.70% Series Due 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt	20.0	20.0
8.17% Series Due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt	42.0	42.0
5.28% Series Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	35.0	35.0
4.85% Series Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt	15.0	15.0
4.95% Pollution Control Series F Due 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt	111.0	111.0
6.02% Series Due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt	75.0	75.0
4.90% Series Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	30.0	30.0
5.10% Series Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	30.0	30.0
3.20% Series Due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt	75.0	0
5.99% Series Due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt	60.0	60.0
5.69% Series Due 2036 [Member]
Debt Instrument [Line Items]
Long-term Debt	50.0	50.0
6.00% Series Due 2040 [Member]
Debt Instrument [Line Items]
Long-term Debt	35.0	35.0
5.82% Series Due 2040 [Member]
Debt Instrument [Line Items]
Long-term Debt	45.0	45.0
4.08% Series Due 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt	85.0	0
SWL&P First Mortgage Bonds 7.25% Series Due 2013 [Member]
Debt Instrument [Line Items]
Long-term Debt	10.0	10.0
Senior Unsecured Notes 5.99% Due 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt	50.0	50.0
Variable Demand Revenue Refunding Bonds Series 1997 A, B, and C Due 2013-2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	27.5	28.2
Industrial Development Revenue Bonds 6.5% Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	0	6.0
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006 Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	27.8	27.8
Unsecured Term Loan Variable Rate Due 2014 [Member]
Debt Instrument [Line Items]
Long-term Debt	75.0	75.0
Other Long-Term Debt, 1.0%-8.0% Due 2013-2037 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 41.8	$ 40.3