Revision of Prior Period Financial Statements (Prior Period Revisions) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2021	Jun. 30, 2021	Sep. 30, 2021	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Cash Flows [Abstract]
Net cash provided by operations	$ (408)	$ 750	$ 2,074	$ 3,516	$ 3,382	$ 2,361
Financing cash flows:
Change in fiduciary liabilities	190	1,277	1,919	1,183	955	1,025
Other lines	(451)	(1,491)	(2,046)		(1,880)	3,306
Net cash used for financing activities	(261)	(214)	(127)	(1,297)	(925)	4,331
Investing cash flows:
Acquisitions, net of cash and cash held in a fiduciary capacity acquired	0	(350)	(384)	(859)	(647)	(4,229)
Other lines	(67)	(74)	(188)		(146)	(166)
Net cash used for investing activities	(67)	(424)	(572)	(1,164)	(793)	(4,395)
Effect of exchange rate changes on cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	(36)	38	(243)	(355)	511	135
Increase in changes on cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	(772)	150	1,132	700	2,175	2,432
Cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity at beginning of year	10,674	10,674	10,674	10,674	8,499	6,067
Cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity at end of year	9,902	10,824	11,806	11,374	10,674	8,499
Cash and cash equivalents				1,752	2,089	1,155
Cash and cash equivalents held in a fiduciary capacity				9,622	8,585	7,344
Total cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	9,902	10,824	11,806	11,374	10,674	8,499
As Reported
Statement of Cash Flows [Abstract]
Net cash provided by operations	(408)	750	2,074		3,382	2,361
Financing cash flows:
Change in fiduciary liabilities	0	0	0		0	0
Other lines	(451)	(1,491)	(2,046)		(1,880)	3,306
Net cash used for financing activities	(451)	(1,491)	(2,046)		(1,880)	3,306
Investing cash flows:
Acquisitions, net of cash and cash held in a fiduciary capacity acquired	0	(363)	(401)		(668)	(5,505)
Other lines	(67)	(74)	(188)		(146)	(166)
Net cash used for investing activities	(67)	(437)	(589)		(814)	(5,671)
Effect of exchange rate changes on cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	(43)	(23)	(130)		246	93
Increase in changes on cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	(969)	(1,201)	(691)		934	89
Cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity at beginning of year	2,089	2,089	2,089	2,089	1,155	1,066
Cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity at end of year	1,120	888	1,398		2,089	1,155
Total cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	1,120	888	1,398		2,089	1,155
Effect of Change
Statement of Cash Flows [Abstract]
Net cash provided by operations	0	0	0		0	0
Financing cash flows:
Change in fiduciary liabilities	190	1,277	1,919		955	1,025
Other lines	0	0	0		0	0
Net cash used for financing activities	190	1,277	1,919		955	1,025
Investing cash flows:
Acquisitions, net of cash and cash held in a fiduciary capacity acquired	0	13	17		21	1,276
Other lines	0	0	0		0	0
Net cash used for investing activities	0	13	17		21	1,276
Effect of exchange rate changes on cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	7	61	(113)		265	42
Increase in changes on cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	197	1,351	1,823		1,241	2,343
Cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity at beginning of year	8,585	8,585	8,585	$ 8,585	7,344	5,001
Cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity at end of year	8,782	9,936	10,408		8,585	7,344
Total cash, cash equivalents, and cash and cash equivalents held in a fiduciary capacity	$ 8,782	$ 9,936	$ 10,408		$ 8,585	$ 7,344