Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Costs
Change in benefit obligation
Obligation at January 1	$ 3,163.7	$ 3,058.8
Service cost	47.1	44.6	$ 45.4
Interest cost	114.3	121.8	130.8
Participant contributions	0.0	0.0
Plan amendments	0.0	0.0
Actuarial loss (gain)	(171.8)	162.6
Benefit payments	(226.1)	(224.1)
Transfer	0.0	0.0
Obligation at December 31	2,927.2	3,163.7	3,058.8
Change in fair value of plan assets
Beginning balance at January 1	2,966.8	2,709.2
Actual return on plan assets	(122.2)	368.7
Employer contributions	72.3	113.0
Participant contributions	0.0	0.0
Benefit payments	(226.1)	(224.1)
Ending balance at December 31	2,690.8	2,966.8	2,709.2
Funded status at December 31	(236.4)	(196.9)
OPEB Costs
Change in benefit obligation
Obligation at January 1	818.5	818.4
Service cost	23.7	24.1	26.1
Interest cost	29.9	32.9	37.0
Participant contributions	15.5	13.4
Plan amendments	(3.5)	(36.4)
Actuarial loss (gain)	(222.6)	12.9
Benefit payments	(55.4)	(48.8)
Federal subsidy on benefits paid	1.0	2.0
Transfer	1.1	0.0
Obligation at December 31	608.2	818.5	818.4
Change in fair value of plan assets
Beginning balance at January 1	841.5	773.5
Actual return on plan assets	(35.2)	95.9
Employer contributions	5.3	7.5
Participant contributions	15.5	13.4
Benefit payments	(55.4)	(48.8)
Ending balance at December 31	771.7	841.5	$ 773.5
Funded status at December 31	$ 163.5	$ 23.0

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of contributions received by defined benefit plan from participant which increase benefit obligation.

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Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change, classified as other.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of contributions received by defined benefit plan from participant which increases plan assets.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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