Restructuring Costs (Restructuring Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Restructuring Reserve [Roll Forward]
Restructuring charges	$ 301	$ 427
JLT Transaction
Restructuring Reserve [Roll Forward]
Beginning of period	146	69
Restructuring charges	301	427
Cash payments	(271)	(193)
Non-cash charges	(46)	(157)
End of period	130	146
JLT Transaction | Severance
Restructuring Reserve [Roll Forward]
Beginning of period	88	35
Restructuring charges	148	111
Cash payments	(147)	(58)
Non-cash charges	0	0
End of period	89	88
JLT Transaction | Real Estate Related Costs
Restructuring Reserve [Roll Forward]
Beginning of period	56	34
Restructuring charges	96	195
Cash payments	(69)	(25)
Non-cash charges	(44)	(148)
End of period	39	56
JLT Transaction | Information Technology
Restructuring Reserve [Roll Forward]
Beginning of period	0	0
Restructuring charges	15	15
Cash payments	(13)	(6)
Non-cash charges	(2)	(9)
End of period	0	0
JLT Transaction | Consulting and Other Outside Services
Restructuring Reserve [Roll Forward]
Beginning of period	2	0
Restructuring charges	42	106
Cash payments	(42)	(104)
Non-cash charges	0	0
End of period	$ 2	$ 2