Employee Benefits - Accumulated Benefit Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 3,194.3	$ 2,992.9
Information for pension or OPEB plans with an accumulated benefit obligation in excess of plan assets
Accumulated benefit obligation	1,555.5	1,754.2
Fair value of plan assets	1,298.3	1,504.6
Information for pension plans with a projected benefit obligation in excess of plan assets
Projected benefit obligation	2,034.1	1,810.1
Fair value of plan assets	1,729.8	1,504.6
OPEB Benefits
Information for pension or OPEB plans with an accumulated benefit obligation in excess of plan assets
Accumulated benefit obligation	25.7	31.1
Fair value of plan assets	$ 3.0	$ 10.4