Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 3,346.4	$ 3,123.7
Service cost	54.3	50.1	$ 47.0
Interest cost	87.5	102.8	120.4
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial loss (gain)	(101.3)	311.6
Benefit payments	(250.3)	(241.8)
Transfer	0.0	0.0
Obligation at December 31	3,136.6	3,346.4	3,123.7
Change in fair value of plan assets
Beginning balance at January 1	3,225.0	3,007.0
Actual return on plan assets	291.8	348.1
Employer contributions	62.4	111.7
Participant contributions	0.0	0.0
Benefit payments	(250.3)	(241.8)
Ending balance at December 31	3,328.9	3,225.0	3,007.0
Funded status at December 31	192.3	(121.4)
OPEB Benefits
Change in benefit obligation
Obligation at January 1	556.1	558.6
Service cost	15.7	15.2	16.3
Interest cost	14.5	18.6	25.7
Participant contributions	12.5	13.3
Plan amendments	(3.9)	(5.0)
Actuarial loss (gain)	(20.3)	(1.4)
Benefit payments	(47.5)	(46.1)
Federal subsidy on benefits paid	1.2	1.3
Transfer	1.9	1.6
Obligation at December 31	530.2	556.1	558.6
Change in fair value of plan assets
Beginning balance at January 1	951.4	879.6
Actual return on plan assets	79.9	103.1
Employer contributions	3.9	1.5
Participant contributions	12.5	13.3
Benefit payments	(47.5)	(46.1)
Ending balance at December 31	1,000.2	951.4	$ 879.6
Funded status at December 31	$ 470.0	$ 395.3