Acquisitions and Dispositions (Allocation Of Acquisition Costs) (Details) $ in Millions, £ in Billions		12 Months Ended
	Sep. 20, 2018 GBP (£)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Apr. 01, 2019 USD ($)
Business Acquisition [Line Items]
Cash			$ 910
Estimated fair value of deferred/contingent consideration		$ 66	133	$ 109
Total consideration		5,927	1,040
Allocation of purchase price:
Goodwill		14,671	$ 9,599	$ 9,089
JLT
Business Acquisition [Line Items]
Cash		5,568
Estimated fair value of deferred/contingent consideration		0
Total consideration	£ 5.2	5,568
Allocation of purchase price:
Cash and cash equivalents		349
Accounts receivable, net		695
Other current assets		132
Property, plant, and equipment		89
Other intangible assets		1,545
Goodwill		4,912
Right of use assets		382
Deferred tax assets		66
Other assets		473
Total assets acquired		8,643
Current liabilities		708
Fiduciary liabilities		1,275
Less - fiduciary assets		(1,275)
Long-term debt		1,044			$ 1,000
Long-term lease liability		389
Pension, post-retirement and post-employment liabilities		248
Liabilities for errors and omissions		31
Other liabilities		375
Total liabilities assumed		2,795
Non-controlling interests		280
Net assets acquired		5,568
Other
Business Acquisition [Line Items]
Cash		293
Estimated fair value of deferred/contingent consideration		66
Total consideration		359
Allocation of purchase price:
Cash and cash equivalents		7
Accounts receivable, net		6
Other current assets		0
Property, plant, and equipment		1
Other intangible assets		134
Goodwill		212
Right of use assets		0
Deferred tax assets		0
Other assets		8
Total assets acquired		368
Current liabilities		9
Fiduciary liabilities		0
Less - fiduciary assets		0
Long-term debt		0
Long-term lease liability		0
Pension, post-retirement and post-employment liabilities		0
Liabilities for errors and omissions		0
Other liabilities		0
Total liabilities assumed		9
Non-controlling interests		0
Net assets acquired		359
Total Acquisitions
Business Acquisition [Line Items]
Cash		5,861
Estimated fair value of deferred/contingent consideration		66
Total consideration		5,927
Allocation of purchase price:
Cash and cash equivalents		356
Accounts receivable, net		701
Other current assets		132
Property, plant, and equipment		90
Other intangible assets		1,679
Goodwill		5,124
Right of use assets		382
Deferred tax assets		66
Other assets		481
Total assets acquired		9,011
Current liabilities		717
Fiduciary liabilities		1,275
Less - fiduciary assets		(1,275)
Long-term debt		1,044
Long-term lease liability		389
Pension, post-retirement and post-employment liabilities		248
Liabilities for errors and omissions		31
Other liabilities		375
Total liabilities assumed		2,804
Non-controlling interests		280
Net assets acquired		$ 5,927