Pension and Other Postretirement Benefit Plans - Reconciliation of Net Pension Amounts Recognized in Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Long-term Investments	$ 49.1	$ 53.1
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	20.4	16.5
Prior Service Credit	(4.6)	(4.6)
Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset)	(1.3)	(2.6)
Total Net Pension Amounts Recognized in Consolidated Balance Sheet	(21.7)	(19.1)
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	(230.5)	(236.2)
Prior Service Credit	1.4	0.0
Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset)	80.1	71.2
Total Net Pension Amounts Recognized in Consolidated Balance Sheet	(149.0)	(165.0)
Trust for Benefit of Employees [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Long-term Investments	$ 18.3	$ 19.2