Postretirement Health Care and Life Insurance Benefits - Reconciliation of Postretirement Plans' Funded Status to the Balance Sheet (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Change in benefit obligation:
Accumulated postretirement benefit obligation at end of period	$ 139.5
Change in plan assets:
Average remaining service period (in years)	2 years 10 months 24 days	3 years 10 months 24 days
Postretirement Health Care and Life Insurance Benefits
Change in benefit obligation:
Accumulated postretirement benefit obligation at beginning of period	$ 476.6	$ 659.9
Service cost	1.0	3.8	$ 4.8
Interest cost	10.5	20.2	25.1
Participant contributions	0.1	2.4
Plan amendments	(139.5)	(185.4)
Benefits paid and administrative fees (net of Medicare Part D reimbursements)	(34.5)	(42.9)
Actuarial (gain) loss	(55.5)	18.6
Accumulated postretirement benefit obligation at end of period	258.7	476.6	659.9
Change in plan assets:
Fair value of plan assets at beginning of period	33.7	34.2
Actual return on plan assets	0.0	3.6
Employer contributions	26.8	36.4
Participant contributions	0.1	2.4
Benefits paid and administrative fees (net of Medicare Part D reimbursements)	(34.5)	(42.9)
Fair value of plan assets at end of period	26.1	33.7	34.2
Funded status at end of period	(232.6)	(442.9)
Less: Current portion (included in “Accounts payable and accrued expenses”)	20.5	29.7
Noncurrent obligation (included in “Accrued postretirement benefit costs”)	$ (212.1)	(413.2)
Average remaining service period (in years)	14 years 2 months 12 days
Postretirement Health Care and Life Insurance Benefits | Retirement Plan Amendment One
Change in benefit obligation:
Plan amendments		$ (174.5)
Change in plan assets:
Average remaining service period (in years)	3 years 10 months 24 days	4 years 10 months 24 days
Postretirement Health Care and Life Insurance Benefits | Retirement Plan Amendment Two
Change in benefit obligation:
Plan amendments		$ (10.9)
Change in plan assets:
Average remaining service period (in years)	9 years 6 months	10 years 6 months
Pension Plan
Change in benefit obligation:
Accumulated postretirement benefit obligation at beginning of period	$ 816.4	$ 853.8
Service cost	0.2	0.3	2.0
Interest cost	20.4	28.0	33.5
Benefits paid and administrative fees (net of Medicare Part D reimbursements)	(55.9)	(57.5)
Actuarial (gain) loss	29.4	(43.4)
Settlement	0.0	(51.6)
Accumulated postretirement benefit obligation at end of period	751.7	816.4	853.8
Change in plan assets:
Fair value of plan assets at beginning of period	847.5	855.2
Actual return on plan assets	(19.2)	101.4
Benefits paid and administrative fees (net of Medicare Part D reimbursements)	(55.9)	(57.5)
Settlement	0.0	(51.6)
Fair value of plan assets at end of period	772.4	847.5	$ 855.2
Funded status at end of period	20.7	31.1
Noncurrent obligation (included in “Accrued postretirement benefit costs”)	$ (7.8)	$ (8.5)