Employee Benefits - Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net periodic cost
Weighted-average healthcare cost rate (as a percent)	6.50%	7.00%	7.00%
Effects created by one percentage point change in assumed healthcare cost trend
Effect of 1% point increase on postretirement obligation	$ 0
Effect of 1% point decrease on postretirement obligation	$ 0
Retirement Plans
Benefit obligation
Discount rate (as a percent)	4.40%	3.68%	4.14%
Net periodic cost
Return on assets (as a percent)	6.00%	6.25%	6.25%
Retirement Plans \| United States Retirement Plans
Net periodic cost
Discount rate (as a percent)	3.68%	4.13%	4.47%
Retirement Plans \| United Kingdom Retirement Plan
Net periodic cost
Discount rate (as a percent)	2.41%	2.58%	3.84%
Postretirement Plans
Benefit obligation
Discount rate (as a percent)	4.15%	3.40%	3.69%
Net periodic cost
Weighted-average healthcare cost rate (as a percent)	5.00%
Discount rate (as a percent)	3.40%	3.69%	3.94%

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

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Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

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Ultimate trend rate for health care cost for defined benefit postretirement plan.

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