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