Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 3,136.6	$ 3,346.4
Service cost	50.8	54.3	$ 50.1
Interest cost	91.8	87.5	102.8
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial gain	(682.3)	(101.3)
Benefit payments	(281.0)	(250.3)
Transfer	0.0	0.0
Obligation at December 31	2,315.9	3,136.6	3,346.4
Change in fair value of plan assets
Beginning balance at January 1	3,328.9	3,225.0
Actual return on plan assets	(431.3)	291.8
Employer contributions	11.4	62.4
Participant contributions	0.0	0.0
Benefit payments	(281.0)	(250.3)
Ending balance at December 31	2,628.0	3,328.9	3,225.0
Funded status at December 31	312.1	192.3
OPEB Benefits
Change in benefit obligation
Obligation at January 1	530.2	556.1
Service cost	14.3	15.7	15.2
Interest cost	15.4	14.5	18.6
Participant contributions	12.5	12.5
Plan amendments	0.2	(3.9)
Actuarial gain	(127.9)	(20.3)
Benefit payments	(45.7)	(47.5)
Federal subsidy on benefits paid	1.4	1.2
Transfer	1.9	1.9
Obligation at December 31	402.3	530.2	556.1
Change in fair value of plan assets
Beginning balance at January 1	1,000.2	951.4
Actual return on plan assets	(135.4)	79.9
Employer contributions	3.7	3.9
Participant contributions	12.5	12.5
Benefit payments	(45.7)	(47.5)
Ending balance at December 31	835.3	1,000.2	$ 951.4
Funded status at December 31	$ 433.0	$ 470.0