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, 2015	Dec. 31, 2014
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	$ 20.2	$ 69.8
Amortization of Prior Service (Cost) Credit	(0.2)	(0.3)
Amortization of Loss	(17.9)	(14.2)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities	2.1	55.3
Postretirement Health and Life [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	0.0	16.4
Prior Service Credit Arising During the Period	0.0	(3.0)
Amortization of Prior Service (Cost) Credit	3.0	2.5
Amortization of Loss	(0.4)	(0.5)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities	$ 2.6	$ 15.4