Integration and Restructuring Costs (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2017
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	$ 112	$ 161	$ 40
Liability balance	102	112	65	$ 93
Cash payments and other adjustments	112	114	68
Severance
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	73	137
Liability balance	51	73	$ 15
Cash payments and other adjustments	91	77
Future Rent
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	22
JLT Transaction
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	335
Liability balance	47	0
Cash payments and other adjustments	265
JLT Transaction | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Estimated total program	700
Program to date	335
JLT Transaction | Severance
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	154
Liability balance	42	0
Cash payments and other adjustments	112
JLT Transaction | Information Technology
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	45
Liability balance	0	$ 0
Cash payments and other adjustments	45
JLT Transaction | RIS | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	229
JLT Transaction | Consulting | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	18
JLT Transaction | Corporate | Acquisition Related
Restructuring Cost and Reserve [Line Items]
Costs incurred in other restructuring	88
Marsh Plan | Severance
Restructuring Cost and Reserve [Line Items]
Incurred severance and consulting costs	34
Mercer Plan | Severance
Restructuring Cost and Reserve [Line Items]
Incurred severance and consulting costs	$ 56