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Restructuring Costs - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Dec. 31, 2019
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | $ 9.0 | $ 112.0
Severance and consulting costs | Marsh Insurance Group |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 2.0 |
Severance and consulting costs | Mercer Consulting Group |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 4.0 |
Severance and future rent under non-cancelable leases and other costs | Corporate |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 3.0 |
Jardine Lloyd Thompson Group plc |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 80.0 | 335.0
Jardine Lloyd Thompson Group plc | Acquisition related |  |
Restructuring Cost and Reserve [Line Items] |  |
Restructuring and related cost, expected cost | 700.0 |
Restructuring and related cost, cost incurred to date | 80.0 | $ 335.0
Amounts accrued | 80.0 |
Jardine Lloyd Thompson Group plc | Acquisition related | Corporate |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 9.0 |
Jardine Lloyd Thompson Group plc | Acquisition related | Risk and Insurance Services Segment |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | 61.0 |
Jardine Lloyd Thompson Group plc | Acquisition related | Consulting Segment |  |
Restructuring Cost and Reserve [Line Items] |  |
Amounts accrued | $ 10.0 |

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- Definition Amount of costs incurred to date for the specified restructuring cost. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 5.P.4(d)) -URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140904-122747 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 5.P.4(b)(1)) -URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140904-122747 + Details
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- Definition Amount expected to be recognized in earnings for the specified restructuring cost. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 5.P.4(b)(1)) -URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140904-122747 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 5.P.4(d)) -URI http://asc.fasb.org/extlink&oid=115931487&loc=d3e140904-122747 + Details
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