Restructuring Costs (Restructuring Activity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Beginning of period	$ 81	$ 102
Amounts Accrued	70	89
Cash payments	(98)	(100)
Non-Cash/Other	(5)	(10)
End of period	48	81
Severance
Restructuring Reserve [Roll Forward]
Beginning of period	36	51
Amounts Accrued	24	39
Cash payments	(42)	(54)
Non-Cash/Other		0
End of period	18	36
Future rent under non-cancelable leases and other costs
Restructuring Reserve [Roll Forward]
Beginning of period	45	51
Amounts Accrued	46	50
Cash payments	(56)	(46)
Non-Cash/Other	(5)	(10)
End of period	30	45
JLT Transaction
Restructuring Reserve [Roll Forward]
Beginning of period	25	47
Amounts Accrued	93	251
Cash payments	(80)	(226)
Non-cash charges	(17)	(47)
End of period	21	25
JLT Transaction | Severance
Restructuring Reserve [Roll Forward]
Beginning of period	16	42
Amounts Accrued	14	43
Cash payments	(13)	(69)
Non-cash charges	0	0
End of period	17	16
JLT Transaction | Real Estate Related Costs
Restructuring Reserve [Roll Forward]
Beginning of period	7	5
Amounts Accrued	26	69
Cash payments	(12)	(25)
Non-cash charges	(17)	(42)
End of period	4	7
JLT Transaction | Information Technology
Restructuring Reserve [Roll Forward]
Beginning of period	2	0
Amounts Accrued	23	62
Cash payments	(25)	(55)
Non-cash charges	0	(5)
End of period	0	2
JLT Transaction | Consulting and Other Outside Services
Restructuring Reserve [Roll Forward]
Beginning of period	0	0
Amounts Accrued	30	77
Cash payments	(30)	(77)
Non-cash charges	0	0
End of period	$ 0	$ 0