Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Accumulated Other Comprehensive Loss	$ (17.1)		$ (22.0)
Other Comprehensive Income (Loss) Before Reclassifications	(5.3)		4.6
Amounts Reclassified from Accumulated Other Comprehensive Loss	1.3		0.3
Net Other Comprehensive Income (Loss)	(4.0)		4.9		6.9
Ending Accumulated Other Comprehensive Loss	(21.1)		(17.1)		(22.0)
Other Income (Expense) - Other [Member]
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract]
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Before Tax	(0.2)	[1]	2.2	[1]
Income Tax Expense [Member]
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract]
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Tax	0.1	[2]	(0.9)	[2]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Tax	0.8	[2]	1.1	[2]
Operating Expenses - Operating and Maintenance [Member]
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Before Tax	(2.0)		(2.7)
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement items, Prior Service Costs, Before Tax	0.3	[3]	0.8	[3]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Actuarial Gains and Losses, Before Tax	(2.3)	[3]	(3.5)	[3]
Unrealized Gains and Losses on Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Accumulated Other Comprehensive Loss	(0.1)		(0.1)
Other Comprehensive Income (Loss) Before Reclassifications	(0.3)		1.3
Amounts Reclassified from Accumulated Other Comprehensive Loss	0.1		(1.3)
Net Other Comprehensive Income (Loss)	(0.2)		0
Ending Accumulated Other Comprehensive Loss	(0.3)		(0.1)
Defined Benefit Pension, Other Postretirement Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Accumulated Other Comprehensive Loss	(16.7)		(21.5)
Other Comprehensive Income (Loss) Before Reclassifications	(5.2)		3.2
Amounts Reclassified from Accumulated Other Comprehensive Loss	1.2		1.6
Net Other Comprehensive Income (Loss)	(4.0)		4.8
Ending Accumulated Other Comprehensive Loss	(20.7)		(16.7)
Gains and Losses on Cash Flow Hedge [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Accumulated Other Comprehensive Loss	(0.3)		(0.4)
Other Comprehensive Income (Loss) Before Reclassifications	0.2		0.1
Amounts Reclassified from Accumulated Other Comprehensive Loss	0		0
Net Other Comprehensive Income (Loss)	0.2		0.1
Ending Accumulated Other Comprehensive Loss	$ (0.1)		$ (0.3)

[1] Included in Other Income (Expense) – Other on our Consolidated Statement of Income.

[2] Included in Income Tax Expense on our Consolidated Statement of Income.

[3] Defined benefit pension and other postretirement items excluded from our Regulated Operations are recognized in accumulated other comprehensive loss and are subsequently reclassified out of accumulated other comprehensive loss as components of net periodic pension and other postretirement benefit expense. (See Note 17. Pension and Other Postretirement Benefit Plans.)