Restructuring Costs (Restructuring Activity) (Details) - JLT Transaction - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Restructuring Reserve [Roll Forward]
Beginning of period	$ 69	$ 106
Restructuring charges	427	163
Cash payments	(193)	(178)
Non-cash charges	(157)	(22)
End of period	146	69
Severance
Restructuring Reserve [Roll Forward]
Beginning of period	35	52
Restructuring charges	111	38
Cash payments	(58)	(55)
Non-cash charges	0	0
End of period	88	35
Real Estate Related Costs
Restructuring Reserve [Roll Forward]
Beginning of period	34	51
Restructuring charges	195	31
Cash payments	(25)	(26)
Non-cash charges	(148)	(22)
End of period	56	34
Information Technology
Restructuring Reserve [Roll Forward]
Beginning of period	0	2
Restructuring charges	15	23
Cash payments	(6)	(25)
Non-cash charges	(9)	0
End of period	0	0
Consulting and Other Outside Services
Restructuring Reserve [Roll Forward]
Beginning of period	0	1
Restructuring charges	106	71
Cash payments	(104)	(72)
Non-cash charges	0	0
End of period	$ 2	$ 0