Integration and Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	$ 89	$ 112
Severance
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	39	73
Future Rent
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	32
Mercer Plan | Severance
Restructuring Cost and Reserve [Line Items]
Incurred severance and consulting costs	54
JLT Transaction
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	251	335
JLT Transaction | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Program to date	251	335
JLT Transaction | Severance
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	43	$ 154
JLT Transaction | RIS | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	171
JLT Transaction | Consulting | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	51
JLT Transaction | Corporate | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	$ 29