Pension and Other Postretirement Benefit Plans - Costs Recorded in Regulatory Long-Term Assets or Liabilities and AOCI Expected to be Recognized as a Component of Net Pension and Postretirement Benefit Costs in the Next Fiscal Year (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	$ (8.4)
Prior Service Credit	0.0
Total Pension and Postretirement Health and Life Credit	(8.4)
Postretirement Health and Life [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	0.2
Prior Service Credit	(2.9)
Total Pension and Postretirement Health and Life Credit	$ (2.7)