Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Change in benefit obligation
Obligation at January 1	$ 2,927.2	$ 3,163.7
Service cost	47.0	47.1	$ 44.6
Interest cost	120.4	114.3	121.8
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial loss (gain)	269.3	(171.8)
Benefit payments	(240.2)	(226.1)
Transfer	0.0	0.0
Obligation at December 31	3,123.7	2,927.2	3,163.7
Change in fair value of plan assets
Beginning balance at January 1	2,690.8	2,966.8
Actual return on plan assets	494.1	(122.2)
Employer contributions	62.3	72.3
Participant contributions	0.0	0.0
Benefit payments	(240.2)	(226.1)
Ending balance at December 31	3,007.0	2,690.8	2,966.8
Funded status at December 31	(116.7)	(236.4)
OPEB Benefits
Change in benefit obligation
Obligation at January 1	608.2	818.5
Service cost	16.3	23.7	24.1
Interest cost	25.7	29.9	32.9
Participant contributions	12.3	15.5
Plan amendments	(4.0)	(3.5)
Actuarial loss (gain)	(60.7)	(222.6)
Benefit payments	(42.3)	(55.4)
Federal subsidy on benefits paid	1.3	1.0
Transfer	1.8	1.1
Obligation at December 31	558.6	608.2	818.5
Change in fair value of plan assets
Beginning balance at January 1	771.7	841.5
Actual return on plan assets	134.3	(35.2)
Employer contributions	3.6	5.3
Participant contributions	12.3	15.5
Benefit payments	(42.3)	(55.4)
Ending balance at December 31	879.6	771.7	$ 841.5
Funded status at December 31	$ 321.0	$ 163.5