Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic benefit (credit) cost:
Defined Benefit Plan, Actuarial Gain (Loss), Immediate Recognition as Component in Net Periodic Benefit (Cost) Credit	$ (21.1)	$ (16.9)	$ (19.7)	$ 4.1	$ (12.8)	$ (18.1)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment			$ 10.2
Pension Plan [Member]
Components of net periodic benefit (credit) cost:
Defined Benefit Plan, Actuarial Gain (Loss), Immediate Recognition as Component in Net Periodic Benefit (Cost) Credit				(13.9)	(38.8)	23.2
Postretirement Benefit Plans [Member]
Components of net periodic benefit (credit) cost:
Service cost				(0.2)	(0.2)	(0.1)
Interest cost				(5.9)	(7.6)	(9.1)
Expected return on plan assets				(3.2)	(3.7)	(5.6)
Amortization of prior service credit				5.4	1.7	1.0
Defined Benefit Plan, Actuarial Gain (Loss), Immediate Recognition as Component in Net Periodic Benefit (Cost) Credit				18.0	16.7	4.0
Net periodic benefit (credit) cost				$ (20.5)	$ (14.3)	$ (1.4)