Pension and Savings Plans - Changes in the Fair Value of Master Trust's Level 3 Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	$ 4.1	$ 6.2	$ 11.8
Realized gains (losses)	1.6	(1.0)	2.6
Unrealized (losses) gains relating to investments still held at the reporting date	(2.1)	1.4	(2.6)
Purchases, sales and settlements, net	(2.4)	(2.5)	(5.6)
Fair value of plan assets at end of period	1.2	4.1	6.2
Pension Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	855.2	764.8
Fair value of plan assets at end of period	847.5	855.2	$ 764.8
Pension Plan | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	4.1
Fair value of plan assets at end of period	1.2	4.1
Pension Plan | Level 3 | Real estate interests
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Fair value of plan assets at beginning of period	4.1
Fair value of plan assets at end of period	$ 1.2	$ 4.1