Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Accumulated Other Comprehensive Loss	$ (20.6)		$ (16.8)		$ (21.1)		$ (17.1)
Other Comprehensive Income Before Reclassifications	0.0		0.3		0.3		0.3
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)	0.4		0.2		0.6		0.5
Net Other Comprehensive Income	0.4		0.5		0.9		0.8
Ending Accumulated Other Comprehensive Income (Loss)	(20.2)		(16.3)		(20.2)		(16.3)
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Before Tax	(0.6)		(0.5)		(1.1)		(1.0)
Other Income (Expense) [Member]
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract]
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Before Tax	0.1	[1]	0.2	[1]	0.1	[2]	0.2	[2]
Income Tax Expense [Member]
Reclassifications Out of Accumulated Other Comprehensive Income [Abstract]
AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Tax	(0.1)	[3]	(0.1)	[3]	0.0	[4]	(0.1)	[4]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Tax	0.2	[3]	0.2	[3]	0.4	[4]	0.4	[4]
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, Prior Service Costs, Before Tax	0.1	[5]	0.1	[5]	0.2	[6]	0.2	[6]
AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Actuarial Gains and Losses, Before Tax	(0.7)	[5]	(0.6)	[5]	(1.3)	[6]	(1.2)	[6]
Unrealized Gains and Losses on Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Accumulated Other Comprehensive Loss	(0.2)		(0.1)		(0.3)		(0.1)
Other Comprehensive Income Before Reclassifications	0.0		0.3		0.2		0.3
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)	0.0		(0.1)		(0.1)		(0.1)
Net Other Comprehensive Income	0.0		0.2		0.1		0.2
Ending Accumulated Other Comprehensive Income (Loss)	(0.2)		0.1		(0.2)		0.1
Defined Benefit Pension and Other Postretirement Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Accumulated Other Comprehensive Loss	(20.4)		(16.4)		(20.7)		(16.7)
Other Comprehensive Income Before Reclassifications	0.0		0.0		0.0		0.0
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)	0.4		0.3		0.7		0.6
Net Other Comprehensive Income	0.4		0.3		0.7		0.6
Ending Accumulated Other Comprehensive Income (Loss)	(20.0)		(16.1)		(20.0)		(16.1)
Gains and Losses on Cash Flow Hedge [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Accumulated Other Comprehensive Loss	0.0		(0.3)		(0.1)		(0.3)
Other Comprehensive Income Before Reclassifications	0.0		0.0		0.1		0.0
Amounts Reclassified From Accumulated Other Comprehensive Income (Loss)	0.0		0.0		0.0		0.0
Net Other Comprehensive Income	0.0		0.0		0.1		0.0
Ending Accumulated Other Comprehensive Income (Loss)	$ 0.0		$ (0.3)		$ 0.0		$ (0.3)

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

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

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

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

[5] 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 15. Pension and Other Postretirement Benefit Plans.)

[6] 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 15. Pension and Other Postretirement Benefit Plans.)