New Accounting Guidance - ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues [Abstract]
Revenue	$ 3,504	$ 3,341	$ 11,238	$ 10,339
Expense:
Compensation and benefits	2,083	1,968	6,442	5,971
Other operating expenses	880	838	2,656	2,383
Operating expenses	2,963	2,806	9,098	8,354
Operating income	541	535	2,140	1,985
Other net benefit credits	63	62	194	185
Interest income	2	2	8	6
Interest expense	(69)	(60)	(198)	(178)
Investment Income	(52)	(2)	(24)	3
Change in fair value of acquisition related FX contract	(100)	0	(100)	0
Income before income taxes	385	537	2,020	2,001
Income tax expense	106	140	509	519
Net income before non-controlling interests	279	397	1,511	1,482
Less: Net income attributable to non-controlling interests	3	4	14	19
Net income attributable to the Company	276	393	1,497	1,463
Assets
Cash and cash equivalents	951	1,078	951	1,078		$ 1,205	$ 1,026
Net receivables	4,476		4,476		$ 4,201	4,133
Other current assets	539		539		542	224
Total current assets	5,966		5,966			5,562
Fixed assets, net	707		707			712
Pension related assets	1,814		1,814			1,693
Deferred tax assets	497		497		552	669
Other assets	1,381		1,381		1,656	1,430
Total assets	21,129		21,129			20,429
Liabilities
Short-term debt	638		638			262
Accounts payable and accrued liabilities	2,293		2,293		2,205	2,083
Accrued compensation and employee benefits	1,406		1,406			1,718
Accrued income taxes	179		179			199
Dividends payable	211		211			0
Total current liabilities	4,727		4,727			4,262
Fiduciary liabilities	5,185		5,185			4,847
Less – cash and investments held in a fiduciary capacity	(5,185)		(5,185)			(4,847)
Fiduciary Liabilities, Net, Noncurrent	0		0			0
Long-term debt	5,512		5,512			5,225
Pension, post-retirement and post-employment benefits	1,727		1,727			1,888
Liabilities for errors and omissions	303		303			301
Other liabilities	1,322		1,322		1,334	1,311
Equity [Abstract]
Retained earnings	14,196		14,196		13,504	13,140
Total equity	7,538	7,131	7,538	7,131		7,442
Total liabilities and stockholders' equity	21,129		21,129			$ 20,429
Statement of Cash Flows [Abstract]
Net income before non-controlling interests	279	$ 397	1,511	1,482
Depreciation and amortization of fixed assets and capitalized software			236	234
Amortization of intangible assets			135	122
Adjustments and payments related to contingent consideration liability			(10)	(30)
Provision for deferred income taxes			66	52
Loss (gain) on investments			24	(3)
(Gain) loss on disposition of assets			(53)	9
Share-based compensation expense			146	111
Change in fair value of acquisition related FX contract			100	0
Net receivables			(210)	(248)
Other current assets			19	(14)
Other assets			(51)	(18)
Accounts payable and accrued liabilities			(3)	11
Accrued compensation and employee benefits			(312)	(278)
Accrued income taxes			(13)	77
Contributions to pension and other benefit plans in excess of current year expense/credit			(250)	(337)
Other liabilities			11	83
Effect of exchange rate changes			(27)	(116)
Net cash provided by operations			1,319	$ 1,137
Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	3,504		11,238
Expense:
Compensation and benefits	2,083		6,442
Other operating expenses	880		2,656
Operating expenses	2,963		9,098
Operating income	541		2,140
Other net benefit credits	63		194
Interest income	2		8
Interest expense	(69)		(198)
Investment Income	(52)		(24)
Change in fair value of acquisition related FX contract	(100)		(100)
Income before income taxes	385		2,020
Income tax expense	106		509
Net income before non-controlling interests	279		1,511
Less: Net income attributable to non-controlling interests	3		14
Net income attributable to the Company	276		1,497
Assets
Cash and cash equivalents	951		951
Net receivables	4,476		4,476		68
Other current assets	539		539		318
Total current assets	5,966		5,966
Goodwill and intangible assets	10,764		10,764
Fixed assets, net	707		707
Pension related assets	1,814		1,814
Deferred tax assets	497		497		(103)
Other assets	1,381		1,381		226
Total assets	21,129		21,129
Liabilities
Short-term debt	638		638
Accounts payable and accrued liabilities	2,293		2,293		122
Accrued compensation and employee benefits	1,406		1,406
Accrued income taxes	179		179
Dividends payable	211		211
Total current liabilities	4,727		4,727
Fiduciary liabilities	5,185		5,185
Less – cash and investments held in a fiduciary capacity	(5,185)		(5,185)
Fiduciary Liabilities, Net, Noncurrent	0		0
Long-term debt	5,512		5,512
Pension, post-retirement and post-employment benefits	1,727		1,727
Liabilities for errors and omissions	303		303
Other liabilities	1,322		1,322		23
Equity [Abstract]
Retained earnings	364		364		$ 364
Total equity	7,538		7,538
Total liabilities and stockholders' equity	21,129		21,129
Statement of Cash Flows [Abstract]
Net income before non-controlling interests			1,511
Depreciation and amortization of fixed assets and capitalized software			236
Amortization of intangible assets			135
Adjustments and payments related to contingent consideration liability			(10)
Provision for deferred income taxes			66
Loss (gain) on investments			24
(Gain) loss on disposition of assets			(53)
Share-based compensation expense			146
Change in fair value of acquisition related FX contract			100
Net receivables			(210)
Other current assets			19
Other assets			(51)
Accounts payable and accrued liabilities			(3)
Accrued compensation and employee benefits			(312)
Accrued income taxes			(13)
Contributions to pension and other benefit plans in excess of current year expense/credit			(250)
Other liabilities			11
Effect of exchange rate changes			(27)
Net cash provided by operations			1,319
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	3,562		11,111
Expense:
Compensation and benefits	2,095		6,384
Other operating expenses	880		2,656
Operating expenses	2,975		9,040
Operating income	587		2,071
Other net benefit credits	63		194
Interest income	2		8
Interest expense	(69)		(198)
Investment Income	(52)		(24)
Change in fair value of acquisition related FX contract	(100)		(100)
Income before income taxes	431		1,951
Income tax expense	118		491
Net income before non-controlling interests	313		1,460
Less: Net income attributable to non-controlling interests	3		14
Net income attributable to the Company	310		1,446
Assets
Cash and cash equivalents	951		951
Net receivables	4,301		4,301
Other current assets	249		249
Total current assets	5,501		5,501
Goodwill and intangible assets	10,764		10,764
Fixed assets, net	707		707
Pension related assets	1,814		1,814
Deferred tax assets	618		618
Other assets	1,143		1,143
Total assets	20,547		20,547
Liabilities
Short-term debt	638		638
Accounts payable and accrued liabilities	2,150		2,150
Accrued compensation and employee benefits	1,406		1,406
Accrued income taxes	179		179
Dividends payable	211		211
Total current liabilities	4,584		4,584
Fiduciary liabilities	5,185		5,185
Less – cash and investments held in a fiduciary capacity	(5,185)		(5,185)
Fiduciary Liabilities, Net, Noncurrent	0		0
Long-term debt	5,512		5,512
Pension, post-retirement and post-employment benefits	1,727		1,727
Liabilities for errors and omissions	303		303
Other liabilities	1,298		1,298
Equity [Abstract]
Total equity	7,123		7,123
Total liabilities and stockholders' equity	20,547		20,547
Statement of Cash Flows [Abstract]
Net income before non-controlling interests			1,460
Depreciation and amortization of fixed assets and capitalized software			236
Amortization of intangible assets			135
Adjustments and payments related to contingent consideration liability			(10)
Provision for deferred income taxes			66
Loss (gain) on investments			24
(Gain) loss on disposition of assets			(53)
Share-based compensation expense			146
Change in fair value of acquisition related FX contract			100
Net receivables			(103)
Other current assets			(9)
Other assets			(57)
Accounts payable and accrued liabilities			(24)
Accrued compensation and employee benefits			(312)
Accrued income taxes			(13)
Contributions to pension and other benefit plans in excess of current year expense/credit			(250)
Other liabilities			10
Effect of exchange rate changes			(27)
Net cash provided by operations			1,319
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	58		(127)
Expense:
Compensation and benefits	12		(58)
Other operating expenses	0		0
Operating expenses	12		(58)
Operating income	46		(69)
Other net benefit credits	0		0
Interest income	0		0
Interest expense	0		0
Investment Income	0		0
Change in fair value of acquisition related FX contract	0		0
Income before income taxes	46		(69)
Income tax expense	12		(18)
Net income before non-controlling interests	34		(51)
Less: Net income attributable to non-controlling interests	0		0
Net income attributable to the Company	34		(51)
Assets
Cash and cash equivalents	0		0
Net receivables	(175)		(175)
Other current assets	(290)		(290)
Total current assets	(465)		(465)
Goodwill and intangible assets	0		0
Fixed assets, net	0		0
Pension related assets	0		0
Deferred tax assets	121		121
Other assets	(238)		(238)
Total assets	(582)		(582)
Liabilities
Short-term debt	0		0
Accounts payable and accrued liabilities	(143)		(143)
Accrued compensation and employee benefits	0		0
Accrued income taxes	0		0
Dividends payable	0		0
Total current liabilities	(143)		(143)
Fiduciary liabilities	0		0
Less – cash and investments held in a fiduciary capacity	0		0
Fiduciary Liabilities, Net, Noncurrent	0		0
Long-term debt	0		0
Pension, post-retirement and post-employment benefits	0		0
Liabilities for errors and omissions	0		0
Other liabilities	(24)		(24)
Equity [Abstract]
Total equity	(415)		(415)
Total liabilities and stockholders' equity	$ (582)		(582)
Statement of Cash Flows [Abstract]
Net income before non-controlling interests			(51)
Depreciation and amortization of fixed assets and capitalized software			0
Amortization of intangible assets			0
Adjustments and payments related to contingent consideration liability			0
Provision for deferred income taxes			0
Loss (gain) on investments			0
(Gain) loss on disposition of assets			0
Share-based compensation expense			0
Change in fair value of acquisition related FX contract			0
Net receivables			107
Other current assets			(28)
Other assets			(6)
Accounts payable and accrued liabilities			(21)
Accrued compensation and employee benefits			0
Accrued income taxes			0
Contributions to pension and other benefit plans in excess of current year expense/credit			0
Other liabilities			(1)
Effect of exchange rate changes			0
Net cash provided by operations			$ 0