Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 3,123.7	$ 2,927.2
Service cost	50.1	47.0	$ 47.1
Interest cost	102.8	120.4	114.3
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial loss (gain)	311.6	269.3
Benefit payments	(241.8)	(240.2)
Transfer	0.0	0.0
Obligation at December 31	3,346.4	3,123.7	2,927.2
Change in fair value of plan assets
Beginning balance at January 1	3,007.0	2,690.8
Actual return on plan assets	348.1	494.1
Employer contributions	111.7	62.3
Participant contributions	0.0	0.0
Benefit payments	(241.8)	(240.2)
Ending balance at December 31	3,225.0	3,007.0	2,690.8
Funded status at December 31	(121.4)	(116.7)
OPEB Benefits
Change in benefit obligation
Obligation at January 1	558.6	608.2
Service cost	15.2	16.3	23.7
Interest cost	18.6	25.7	29.9
Participant contributions	13.3	12.3
Plan amendments	(5.0)	(4.0)
Actuarial loss (gain)	(1.4)	(60.7)
Benefit payments	(46.1)	(42.3)
Federal subsidy on benefits paid	1.3	1.3
Transfer	1.6	1.8
Obligation at December 31	556.1	558.6	608.2
Change in fair value of plan assets
Beginning balance at January 1	879.6	771.7
Actual return on plan assets	103.1	134.3
Employer contributions	1.5	3.6
Participant contributions	13.3	12.3
Benefit payments	(46.1)	(42.3)
Ending balance at December 31	951.4	879.6	$ 771.7
Funded status at December 31	$ 395.3	$ 321.0