Regulatory Matters - Regulatory Assets (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Regulatory Assets [Line Items]
Current Regulatory Assets	$ 15.5		$ 17.5
Non-Current Regulatory Assets	339.3		345.9
Regulatory Assets	354.8		363.4
Deferred Fuel [Member]
Regulatory Assets [Line Items]
Current Regulatory Assets	15.5	[1]	17.5	[1]
Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets [Line Items]
Non-Current Regulatory Assets	287.4		292.8
Income Taxes [Member]
Regulatory Assets [Line Items]
Non-Current Regulatory Assets	28.5		28.6
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Non-Current Regulatory Assets	10.3		9.8
PPACA Income Tax Deferral [Member]
Regulatory Assets [Line Items]
Non-Current Regulatory Assets	5.0		5.0
Conservation Improvement Program [Member]
Regulatory Assets [Line Items]
Non-Current Regulatory Assets	2.4		4.6
Other [Member]
Regulatory Assets [Line Items]
Non-Current Regulatory Assets	$ 5.7		$ 5.1

[1] Current regulatory assets are included in prepayments and other on the consolidated balance sheet.