Integration and Restructuring Costs (Restructuring Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Reserve [Roll Forward]
Beginning of period	$ 112	$ 65
Amounts Accrued	112	161	$ 40
Cash payments	(112)	(114)	(68)
Other	(10)	0
End of period	102	112	65
Severance
Restructuring Reserve [Roll Forward]
Beginning of period	73	15
Amounts Accrued	73	137
Cash payments	(91)	(77)
Other	(4)	(2)
End of period	51	73	15
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Beginning of period	39	50
Amounts Accrued	39	24
Cash payments	(21)	(37)
Other	(6)	2
End of period	51	39	$ 50
JLT Transaction
Restructuring Reserve [Roll Forward]
Beginning of period	0
Amounts Accrued	335
Cash payments	(265)
Non-cash charges	(23)
End of period	47	0
JLT Transaction | Severance
Restructuring Reserve [Roll Forward]
Beginning of period	0
Amounts Accrued	154
Cash payments	(112)
Non-cash charges	0
End of period	42	0
JLT Transaction | Real Estate Related Costs
Restructuring Reserve [Roll Forward]
Beginning of period	0
Amounts Accrued	38
Cash payments	(14)
Non-cash charges	(19)
End of period	5	0
JLT Transaction | Information Technology
Restructuring Reserve [Roll Forward]
Beginning of period	0
Amounts Accrued	45
Cash payments	(45)
Non-cash charges	0
End of period	0	0
JLT Transaction | Consulting and Other Outside Services
Restructuring Reserve [Roll Forward]
Beginning of period	0
Amounts Accrued	98
Cash payments	(94)
Non-cash charges	(4)
End of period	$ 0	$ 0