Pension and Other Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	$ 7.2	$ 20.2
Amortization of Prior Service (Cost) Credit	0.0	(0.2)
Amortization of Loss	(9.5)	(17.9)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities	(2.3)	2.1
Postretirement Health and Life [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	13.5	0.0
Amortization of Prior Service (Cost) Credit	2.9	3.0
Amortization of Loss	(0.2)	(0.4)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities	$ 16.2	$ 2.6