Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	$ 22.8	[1]	$ 22.5	[1]
Non-Current Regulatory Assets	340.3		340.3
Total Regulatory Assets	363.1		362.8
Non-Current Regulatory Liabilities	68.7		60.1
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	17.7		19.5
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	20.4		18.1
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	17.7		15.5
Conservation Improvement Program [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	5.8		0
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	7.1		7.0
Deferred Fuel [Member]
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	22.8	[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	247.0		260.7
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	34.4		36.0
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	14.9		12.1
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	35.5	[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		4.3
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	$ 3.5		$ 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.