Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Inventories [Abstract]
Fuel	$ 22.4			$ 28.0
Materials and Supplies	41.9			41.8
Total Inventories	64.3			69.8
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	18.5			22.5
Other	9.6			11.1
Total Prepayments and Other Current Assets	28.1			33.6
Other Current Liabilites [Abstract]
Customer Deposits	28.4	[1]		28.8	[1]
Other	29.4			33.8
Total Other Current Liabilities	57.8			62.6
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	80.6			77.9
Other	46.1			45.4
Total Other Non-Current Liabilities	126.7			123.3
Supplemental Statement of Cash Flows Information [Abstract]
Cash Paid During the Period for Interest - Net of Amounts Capitalized	12.0		12.3
Cash Paid During the Period for Income Taxes	0.5		0.2
Noncash Investing and Financing Activities [Abstract]
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment	(26.2)		(12.3)
Capitalized Asset Retirement Costs	1.9		2.4
AFUDC - Equity	$ 1.1		$ 0.8

[1] Customer deposits are primarily due to security deposits for capital expenditures relating to a transmission project.