Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	$ 18.5		$ 22.5
Non-Current Regulatory Assets	336.6		340.3
Total Regulatory Assets	355.1		362.8
Non-Current Regulatory Liabilities	59.2		60.1
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	18.3		19.5
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	18.6		18.1
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	16.0		15.5
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	6.3		7.0
Deferred Fuel [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	18.5	[1]	22.5	[1]
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	256.1		260.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	34.5		36.0
Asset Retirement Obligation [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	12.8		12.1
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	24.1	[2]	18.5	[2]
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	5.0		5.0
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	0.4		4.3
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	$ 3.7		$ 3.7

[1] Current regulatory assets are included in prepayments and other on the Consolidated Balance Sheet.

[2] The cost recovery rider regulatory asset is primarily due to capital expenditures related to our Bison Wind Energy Center.