Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance		$ (24.5)	$ (21.1)	$ (17.1)
Other Comprehensive Income (Loss) Before Reclassifications		(4.1)	(4.6)	(5.3)
Amounts Reclassified from Accumulated Other Comprehensive Loss		0.4	1.2	1.3
Net Other Comprehensive Income (Loss)		(3.7)	(3.4)	(4.0)
Ending Balance		(28.2)	(24.5)	(21.1)
Unrealized Gain (Loss) on Available-for-sale Securities [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance		(0.8)	(0.3)	(0.1)
Other Comprehensive Income (Loss) Before Reclassifications		0.0	(0.4)	(0.3)
Amounts Reclassified from Accumulated Other Comprehensive Loss		(0.2)	(0.1)	0.1
Net Other Comprehensive Income (Loss)		(0.2)	(0.5)	(0.2)
Ending Balance		(1.0)	(0.8)	(0.3)
Defined Benefit Pension, Other Postretirement Items [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance	[1]	(23.7)	(20.7)	(16.7)
Other Comprehensive Income (Loss) Before Reclassifications	[1]	(4.1)	(4.3)	(5.2)
Amounts Reclassified from Accumulated Other Comprehensive Loss	[1]	0.6	1.3	1.2
Net Other Comprehensive Income (Loss)	[1]	(3.5)	(3.0)	(4.0)
Ending Balance	[1]	(27.2)	(23.7)	(20.7)
Gains (Loss) on Cash Flow Hedge [Member]
Accumulated Other Comprehensive Loss [Roll Forward]
Beginning Balance		0.0	(0.1)	(0.3)
Other Comprehensive Income (Loss) Before Reclassifications		0.0	0.1	0.2
Amounts Reclassified from Accumulated Other Comprehensive Loss		0.0	0.0	0.0
Net Other Comprehensive Income (Loss)		0.0	0.1	0.2
Ending Balance		$ 0.0	$ 0.0	$ (0.1)

[1] 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.)