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Employee Benefits - Weighted-average assumptions used to determine future benefit obligations (Details) - Pension Plan | Dec. 31, 2017 | Dec. 31, 2016
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U.K. |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Discount rate | 2.63% | 2.77%
Underlying price inflation | 1.87% | 1.83%
U.K. | Minimum |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Rate of compensation increase | 3.70% | 3.70%
U.K. | Maximum |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Rate of compensation increase | 4.20% | 4.20%
U.S. | Minimum |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Discount rate | 3.27% | 3.53%
U.S. | Maximum |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Discount rate | 3.61% | 4.11%
Other |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Underlying price inflation | 2.00% |
Other | Minimum |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Discount rate | 1.78% | 1.85%
Rate of compensation increase | 1.00% | 1.00%
Underlying price inflation |  | 2.00%
Other | Maximum |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Discount rate | 3.39% | 3.81%
Rate of compensation increase | 3.00% | 3.50%
Underlying price inflation |  | 2.50%

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- Definition Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Underlying Price Inflation + References No definition available. + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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