Postretirement Health Care and Life Insurance Benefits Reconciliation of Postretirement Plans' Funded Status to the Balance Sheet (Details) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 01, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) years	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) years
Postretirement Health Care and Life Insurance Benefits
Change in benefit obligation:
Accumulated postretirement benefit obligation at beginning of period			$ 595.4	$ 783.3
Service cost	$ 2.3	$ 6.9	4.8	8.2
Interest cost	8.4	24.2	25.1	28.3
Participant contributions			2.3	0.5
Plan amendments			0.0	(51.7)
Benefits paid			(47.7)	(44.8)
Actuarial loss (gain)			80.0	(128.4)
Accumulated postretirement benefit obligation at end of period		783.3	659.9	595.4	$ 783.3
Change in plan assets:
Fair value of plan assets at beginning of period			15.0	0.0
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss)			2.2	0.0
Employer contributions			62.4	59.3
Participant contributions			2.3	0.5
Benefits paid and administrative fees (net of Medicare Part D reimbursements)			(47.7)	(44.8)
Fair value of plan assets at end of period		0.0	34.2	15.0	$ 0.0
Funded status at end of period			(625.7)	(580.4)
Less: Current portion (included in “Accounts payable and accrued expenses”)			32.3	32.7
Noncurrent obligation (included in “Accrued postretirement benefit costs”)			$ (593.4)	(547.7)
Defined Benefit Plan, Amortization Period | years			5.9		10.3
Annual Benefits | Postretirement Health Care and Life Insurance Benefits
Change in benefit obligation:
Plan amendments				50.2
Limitation of eligibility based on age and service criteria | Postretirement Health Care and Life Insurance Benefits
Change in benefit obligation:
Plan amendments				1.5
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Change in plan assets:
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax	$ 0.0	$ 0.0	$ 0.0	$ 44.6