Integration and Restructuring Costs (Restructuring Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restructuring Reserve [Roll Forward]
Beginning of period	$ 102	$ 112
Amounts Accrued	89	112
Cash payments	(100)	(112)
Non-Cash/Other	(10)	(10)
End of period	81	102
Severance
Restructuring Reserve [Roll Forward]
Beginning of period	51	73
Amounts Accrued	39	73
Cash payments	(54)	(91)
Non-Cash/Other		(4)
End of period	36	51
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Beginning of period	51	39
Amounts Accrued	50	39
Cash payments	(46)	(21)
Non-Cash/Other	(10)	(6)
End of period	45	51
JLT Transaction
Restructuring Reserve [Roll Forward]
Beginning of period	47	0
Amounts Accrued	251	335
Cash payments	(226)	(265)
Non-cash charges	(47)	(23)
End of period	25	47
JLT Transaction | Severance
Restructuring Reserve [Roll Forward]
Beginning of period	42	0
Amounts Accrued	43	154
Cash payments	(69)	(112)
Non-cash charges	0	0
End of period	16	42
JLT Transaction | Real Estate Related Costs
Restructuring Reserve [Roll Forward]
Beginning of period	5	0
Amounts Accrued	69	38
Cash payments	(25)	(14)
Non-cash charges	(42)	(19)
End of period	7	5
JLT Transaction | Information Technology
Restructuring Reserve [Roll Forward]
Beginning of period	0	0
Amounts Accrued	62	45
Cash payments	(55)	(45)
Non-cash charges	(5)	0
End of period	2	0
JLT Transaction | Consulting and Other Outside Services
Restructuring Reserve [Roll Forward]
Beginning of period	0	0
Amounts Accrued	77	98
Cash payments	(77)	(94)
Non-cash charges	0	(4)
End of period	$ 0	$ 0