Pension and Other Postretirement Benefits - Benefit Costs (Details 5) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Recognized net actuarial loss (gain):
Pension and OPEB (income) expense			$ (11.6)	$ (64.4)	$ 24.3
Expense for defined contribution plans			31.1	24.9	23.1
Pension Plans, Defined Benefit [Member]
Components of pension and OPEB (income) expense:
Service cost			3.2	2.8	3.0
Interest cost			94.5	108.2	120.4
Expected return on plan assets			(148.8)	(149.9)	(167.1)
Amortization of prior service cost (credit)			3.8	4.7	5.6
Recognized net actuarial loss (gain):
Annual amortization			16.1	10.5	29.2
Pension/OPEB corridor charge/credit			0.0	0.0	78.4
Settlement (gain) loss	$ 14.5	$ 25.0	14.5	0.0	29.9
Pension and OPEB (income) expense			(16.7)	(23.7)	99.4
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of pension and OPEB (income) expense:
Service cost			4.5	4.7	4.9
Interest cost			15.6	17.3	19.9
Expected return on plan assets			0.0	0.0	0.0
Amortization of prior service cost (credit)			(13.6)	(58.5)	(60.4)
Recognized net actuarial loss (gain):
Annual amortization			(1.3)	(4.2)	(4.3)
Pension/OPEB corridor charge/credit			0.0	0.0	(35.3)
Settlement (gain) loss			0.0	0.0	0.0
Pension and OPEB (income) expense			$ 5.2	$ (40.7)	$ (75.2)