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Employee Benefits - Weighted-average assumptions used to determine the net periodic benefit cost (Details) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
U.K. | Minimum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Rate of compensation increase | 3.70% | 3.70% | 3.63%
U.K. | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Rate of compensation increase | 4.20% | 4.20% | 4.13%
U.K. | Pension Plan |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 2.63% | 2.77% | 3.96%
Expected return on plan assets, net of administration expenses | 3.34% | 3.36% | 4.55%
U.S. | Pension Plan |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Expected return on plan assets, net of administration expenses | 7.71% | 7.88% | 7.81%
U.S. | Pension Plan | Minimum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.27% | 3.53% | 3.69%
U.S. | Pension Plan | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.61% | 4.11% | 4.43%
Other | Minimum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Rate of compensation increase | 1.00% | 1.00% | 2.00%
Other | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Rate of compensation increase | 3.00% | 3.50% | 3.50%
Other | Pension Plan | Minimum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 1.78% | 1.85% | 2.43%
Expected return on plan assets, net of administration expenses | 1.70% | 2.68% | 3.47%
Other | Pension Plan | Maximum |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Discount rate | 3.39% | 3.81% | 3.96%
Expected return on plan assets, net of administration expenses | 4.85% | 5.15% | 4.95%

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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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