Other, Net (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Other, Net [Abstract]
Interest Income	$ 1.1		$ 0.9	$ 2.8		$ 1.8
AFUDC Equity	3.9		3.7	7.6		6.2
Pension and other postretirement non-service cost	6.1		0.5	12.3		1.7
Interest rate swap settlement gain	0.0	[1]	0.0	21.2	[1]	0.0	[1]
Miscellaneous	1.7		(0.9)	0.2		(3.2)
Total Other, net	$ 12.8		$ 4.2	$ 44.1		$ 6.5

[1] See Note 9, "Risk Management Activities," for additional information.