Summary of Significant Accounting Policies - ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Dec. 31, 2015
Revenues [Abstract]
Revenue	$ 3,712	$ 3,504	$ 3,734	$ 4,000	$ 3,685	$ 3,341	$ 3,495	$ 3,503	$ 14,950	$ 14,024	$ 13,211
Expense:
Compensation and benefits									8,605	8,085	7,694
Other operating expenses									3,584	3,284	3,086
Operating expenses									12,189	11,369	10,780
Operating income	621	541	691	908	670	535	701	749	2,761	2,655	2,431
Other net benefits credits									215	201	233
Interest income									11	9	5
Interest expense									(290)	(237)	(189)
Acquisition related derivative contracts									(441)	0	0
Income before income taxes									2,244	2,643	2,480
Income tax expense									574	1,133	685
Net income attributable to the Company									1,650	1,490	1,768
Net income before non-controlling interests	159	279	536	696	28	397	507	578	1,670	1,510	1,795
Less: Net income attributable to non-controlling interests									20	20	27
Net income attributable to the Company	153	$ 276	$ 531	$ 690	29	$ 393	$ 501	$ 569	1,650	1,492	1,768
ASSETS
Cash and cash equivalents	1,066				1,205				1,066	1,205	1,026		$ 1,374
Net receivables	4,317				4,133				4,317	4,133		$ 4,201
Other current assets	551				224				551	224		542
Total current assets	5,934				5,562				5,934	5,562
Fixed assets, net	701				712				701	712	725
Pension related assets	1,688				1,693				1,688	1,693
Deferred tax assets					669					669		552
Other assets	1,539				1,430				1,539	1,430		1,656
Total assets	21,578				20,429				21,578	20,429	18,190
LIABILITIES AND EQUITY
Short-term debt	314				262				314	262
Accounts payable and accrued liabilities	2,234				2,083				2,234	2,083		2,205
Accrued compensation and employee benefits	1,778				1,718				1,778	1,718
Acquisition related derivatives	441				0				441	0
Accrued income taxes	157				199				157	199
Total current liabilities	4,924				4,262				4,924	4,262
Fiduciary liabilities	5,001				4,847				5,001	4,847
Less – cash and investments held in a fiduciary capacity	(5,001)				(4,847)				(5,001)	(4,847)
Fiduciary liabilities, net, noncurrent	0				0				0	0
Long-term debt	5,510				5,225				5,510	5,225
Pension, postretirement and postemployment benefits	1,911				1,888				1,911	1,888
Liability for errors and omissions	287				301				287	301
Other liabilities	1,362				1,311				1,362	1,311		1,334
Total equity	7,584				7,442				7,584	7,442	6,272
Total liabilities and stockholders' equity	21,578				$ 20,429				21,578	20,429
Operating cash flows:
Net income before non-controlling interests									1,670	1,512	1,795
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization of fixed assets and capitalized software									311	312	308
Amortization of intangible assets									183	169	130
Adjustments and payments related to contingent consideration liability									(4)	(24)	(33)
Loss (gain) on deconsolidation of entity									11	0	(11)
Benefit for deferred income taxes									(39)	396	68
Loss (Gain) on investments									12	(15)	0
(Gain) Loss on disposition of assets									(48)	10	6
Change in fair value of acquisition-related derivative contracts									441	0	0
Changes in assets and liabilities:
Net receivables									(78)	(454)	(154)
Other current assets									26	(3)	(9)
Other assets									(37)	(199)	34
Accrued compensation and employee benefits									68	63	2
Accrued income taxes									(40)	37	(21)
Contributions to pension and other benefit plans in excess of current year expense/credit									(291)	(457)	(279)
Other liabilities									9	406	(97)
Effect of exchange rate changes									18	(96)	104
Net cash provided by operations									2,428	$ 1,893	$ 2,007
Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue									14,950
Expense:
Compensation and benefits									8,605
Other operating expenses									3,584
Operating expenses									12,189
Operating income									2,761
Other net benefits credits									215
Interest income									11
Interest expense									(290)
Investment (loss) income									(12)
Acquisition related derivative contracts									(441)
Income before income taxes									2,244
Income tax expense									574
Net income before non-controlling interests									1,670
Less: Net income attributable to non-controlling interests									20
Net income attributable to the Company									1,650
ASSETS
Cash and cash equivalents	1,066								1,066
Net receivables	4,317								4,317			68
Other current assets	551								551			318
Total current assets	5,934								5,934
Goodwill and intangible assets	11,036								11,036
Fixed assets, net	701								701
Pension related assets	1,688								1,688
Deferred tax assets	680								680			(103)
Other assets	1,539								1,539			226
Total assets	21,578								21,578
LIABILITIES AND EQUITY
Short-term debt	314								314
Accounts payable and accrued liabilities	2,234								2,234			122
Accrued compensation and employee benefits	1,778								1,778
Acquisition related derivatives	441								441
Accrued income taxes	157								157
Total current liabilities	4,924								4,924
Fiduciary liabilities	5,001								5,001
Less – cash and investments held in a fiduciary capacity	(5,001)								(5,001)
Fiduciary liabilities, net, noncurrent	0								0
Long-term debt	5,510								5,510
Pension, postretirement and postemployment benefits	1,911								1,911
Liability for errors and omissions	287								287
Other liabilities	1,362								1,362			$ 23
Total equity	7,584								7,584
Total liabilities and stockholders' equity	21,578								21,578
Operating cash flows:
Net income before non-controlling interests									1,670
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization of fixed assets and capitalized software									311
Amortization of intangible assets									183
Adjustments and payments related to contingent consideration liability									(4)
Loss (gain) on deconsolidation of entity									11
Benefit for deferred income taxes									(39)
Loss (Gain) on investments									12
(Gain) Loss on disposition of assets									(48)
Change in fair value of acquisition-related derivative contracts									441
Change in fair value of acquisition related derivative contracts									193
Changes in assets and liabilities:
Net receivables									(78)
Other current assets									26
Other assets									(37)
Accounts payable and accrued liabilities									23
Accrued compensation and employee benefits									68
Accrued income taxes									(40)
Contributions to pension and other benefit plans in excess of current year expense/credit									(291)
Other liabilities									9
Effect of exchange rate changes									18
Net cash provided by operations									2,428
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue									2
Expense:
Compensation and benefits									17
Other operating expenses									0
Operating expenses									17
Operating income									(15)
Other net benefits credits									0
Interest income									0
Interest expense									0
Investment (loss) income									0
Acquisition related derivative contracts									0
Income before income taxes									(15)
Income tax expense									(4)
Net income before non-controlling interests									(11)
Less: Net income attributable to non-controlling interests									0
Net income attributable to the Company									(11)
ASSETS
Cash and cash equivalents	0								0
Net receivables	(68)								(68)
Other current assets	(326)								(326)
Total current assets	(394)								(394)
Goodwill and intangible assets	0								0
Fixed assets, net	0								0
Pension related assets	0								0
Deferred tax assets	107								107
Other assets	(242)								(242)
Total assets	(529)								(529)
LIABILITIES AND EQUITY
Short-term debt	0								0
Accounts payable and accrued liabilities	(129)								(129)
Accrued compensation and employee benefits	0								0
Acquisition related derivatives	0								0
Accrued income taxes	0								0
Total current liabilities	(129)								(129)
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, postretirement and postemployment benefits	0								0
Liability for errors and omissions	0								0
Other liabilities	(25)								(25)
Total equity	(375)								(375)
Total liabilities and stockholders' equity	(529)								(529)
Operating cash flows:
Net income before non-controlling interests									(11)
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization of fixed assets and capitalized software									0
Amortization of intangible assets									0
Adjustments and payments related to contingent consideration liability									0
Loss (gain) on deconsolidation of entity									0
Benefit for deferred income taxes									0
Loss (Gain) on investments									0
(Gain) Loss on disposition of assets									0
Change in fair value of acquisition-related derivative contracts									0
Change in fair value of acquisition related derivative contracts									0
Changes in assets and liabilities:
Net receivables									0
Other current assets									8
Other assets									12
Accounts payable and accrued liabilities									(7)
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									(2)
Effect of exchange rate changes									0
Net cash provided by operations									0
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue									14,952
Expense:
Compensation and benefits									8,622
Other operating expenses									3,584
Operating expenses									12,206
Operating income									2,746
Other net benefits credits									215
Interest income									11
Interest expense									(290)
Investment (loss) income									(12)
Acquisition related derivative contracts									(441)
Income before income taxes									2,229
Income tax expense									570
Net income before non-controlling interests									1,659
Less: Net income attributable to non-controlling interests									20
Net income attributable to the Company									1,639
ASSETS
Cash and cash equivalents	1,066								1,066
Net receivables	4,249								4,249
Other current assets	225								225
Total current assets	5,540								5,540
Goodwill and intangible assets	11,036								11,036
Fixed assets, net	701								701
Pension related assets	1,688								1,688
Deferred tax assets	787								787
Other assets	1,297								1,297
Total assets	21,049								21,049
LIABILITIES AND EQUITY
Short-term debt	314								314
Accounts payable and accrued liabilities	2,105								2,105
Accrued compensation and employee benefits	1,778								1,778
Acquisition related derivatives	441								441
Accrued income taxes	157								157
Total current liabilities	4,795								4,795
Fiduciary liabilities	5,001								5,001
Less – cash and investments held in a fiduciary capacity	(5,001)								(5,001)
Fiduciary liabilities, net, noncurrent	0								0
Long-term debt	5,510								5,510
Pension, postretirement and postemployment benefits	1,911								1,911
Liability for errors and omissions	287								287
Other liabilities	1,337								1,337
Total equity	7,209								7,209
Total liabilities and stockholders' equity	$ 21,049								21,049
Operating cash flows:
Net income before non-controlling interests									1,659
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization of fixed assets and capitalized software									311
Amortization of intangible assets									183
Adjustments and payments related to contingent consideration liability									(4)
Loss (gain) on deconsolidation of entity									11
Benefit for deferred income taxes									(39)
Loss (Gain) on investments									12
(Gain) Loss on disposition of assets									(48)
Change in fair value of acquisition-related derivative contracts									441
Change in fair value of acquisition related derivative contracts									193
Changes in assets and liabilities:
Net receivables									(78)
Other current assets									34
Other assets									(25)
Accounts payable and accrued liabilities									16
Accrued compensation and employee benefits									68
Accrued income taxes									(40)
Contributions to pension and other benefit plans in excess of current year expense/credit									(291)
Other liabilities									7
Effect of exchange rate changes									18
Net cash provided by operations									$ 2,428