Regulatory Matters - Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	$ 22.1	[1]	$ 23.0	[1]
Non-Current Regulatory Assets	267.0		263.8
Total Regulatory Assets	289.1		286.8
Non-Current Regulatory Liabilities	89.9		81.0
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	17.1		17.0
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	20.4		19.7
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities	23.7		19.7
Deferred Fuel [Member]
Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	22.1	[1]	23.0	[1]
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	164.7	[2]	164.1	[2]
Non-Current Regulatory Liabilities	15.9	[2]	16.3	[2]
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	36.2		35.3
Asset Retirement Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	16.7		16.0
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	40.3	[3]	39.6	[3]
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	5.0		5.0
Other
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets	4.1		3.8
Non-Current Regulatory Liabilities	$ 12.8		$ 8.3

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

[2] Defined benefit pension and other postretirement items included in our Regulated Operations, which are otherwise required to be recognized in accumulated other comprehensive income, are recognized as regulatory assets or regulatory liabilities on the Consolidated Balance Sheet (See Note 14. Pension and Other Postretirement Benefit Plans).

[3] The cost recovery rider regulatory asset is primarily due to capital expenditures related to our Bison Wind Energy Center and is recognized in accordance with the accounting standards for alternative revenue programs.