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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

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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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