Postretirement Benefits Other Than Pensions - Narrative (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020 USD ($) employees	Dec. 31, 2019	Dec. 31, 2016 USD ($)
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Number of retired employees covered | employees		575
Number of active employees covered | employees		500
Pre-tax reduction to its accumulated postretirement benefit obligation				$ 11.0
Amounts that will be amortized from AOCI in next fiscal Year		$ 2.8
Estimated future employer contributions in next fiscal year		$ 1.3
Medical Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year		6.50%	7.50%
Health care cost trend rate assumed for future years		4.80%	4.80%
Prescription Drug Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next fiscal year		7.50%	10.00%
Health care cost trend rate assumed for future years	4.80%		4.80%