New Accounting Guidance - ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues [Abstract]
Revenue	$ 3,734	$ 3,495	$ 7,734	$ 6,998
Expense:
Compensation and benefits	2,135	1,998	4,359	4,003
Other operating expenses	908	796	1,776	1,545
Operating expenses	3,043	2,794	6,135	5,548
Operating income	691	701	1,599	1,450
Other net benefit credits	65	63	131	123
Interest income	3	2	6	4
Interest expense	(68)	(60)	(129)	(118)
Investment Income	28	5	28	5
Income before income taxes	719	711	1,635	1,464
Income tax expense	183	204	403	379
Net income before non-controlling interests	536	507	1,232	1,085
Less: Net income attributable to non-controlling interests	5	6	11	15
Net income attributable to the Company	531	501	1,221	1,070
Assets
Cash and cash equivalents	1,036	966	1,036	966		$ 1,205	$ 1,026
Net receivables	4,601		4,601		$ 4,201	4,133
Other current assets	538		538		542	224
Total current assets	6,175		6,175			5,562
Fixed assets, net	698		698			712
Pension related assets	1,808		1,808			1,693
Deferred tax assets	532		532		552	669
Total assets	21,159		21,159			20,429
Liabilities
Short-term debt	439		439			262
Accounts payable and accrued liabilities	2,246		2,246		2,205	2,083
Accrued compensation and employee benefits	1,103		1,103			1,718
Accrued income taxes	216		216			199
Dividends payable	212		212			0
Total current liabilities	4,216		4,216			4,262
Fiduciary liabilities	5,118		5,118			4,847
Less – cash and investments held in a fiduciary capacity	(5,118)		(5,118)			(4,847)
Fiduciary Liabilities, Net, Noncurrent	0		0			0
Long-term debt	5,813		5,813			5,225
Pension, post-retirement and post-employment benefits	1,768		1,768			1,888
Liabilities for errors and omissions	303		303			301
Other liabilities	1,262		1,262		1,334	1,311
Equity [Abstract]
Retained earnings	14,131		14,131		13,504	13,140
Total equity	7,797	7,074	7,797	7,074		7,442
Total liabilities and stockholders' equity	21,159		21,159			$ 20,429
Statement of Cash Flows [Abstract]
Net income before non-controlling interests	536	$ 507	1,232	1,085
Depreciation and amortization of fixed assets and capitalized software			159	156
Amortization of intangible assets			88	80
Adjustments and payments related to contingent consideration liability			2	(13)
Provision for deferred income taxes			34	42
(Gain) loss on investments			(28)	(5)
(Gain) loss on disposition of assets			(1)	9
Share-based compensation expense			99	75
Net receivables			(388)	(318)
Other current assets			4	(18)
Other assets			(10)	(34)
Accounts payable and accrued liabilities			30	52
Accrued compensation and employee benefits			(614)	(579)
Accrued income taxes			18	45
Contributions to pension and other benefit plans in excess of current year expense/credit			(178)	(214)
Other liabilities			(10)	9
Effect of exchange rate changes			(24)	(29)
Net cash provided by operations			413	$ 343
Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	3,734		7,734
Expense:
Compensation and benefits	2,135		4,359
Other operating expenses	908		1,776
Operating expenses	3,043		6,135
Operating income	691		1,599
Other net benefit credits	65		131
Interest income	3		6
Interest expense	(68)		(129)
Investment Income	28		28
Income before income taxes	719		1,635
Income tax expense	183		403
Net income before non-controlling interests	536		1,232
Less: Net income attributable to non-controlling interests	5		11
Net income attributable to the Company	531		1,221
Assets
Cash and cash equivalents	1,036		1,036
Net receivables	4,601		4,601		68
Other current assets	538		538		318
Total current assets	6,175		6,175
Goodwill and intangible assets	10,411		10,411
Fixed assets, net	698		698
Pension related assets	1,808		1,808
Deferred tax assets	532		532		(103)
Other assets	1,535		1,535
Total assets	21,159		21,159
Liabilities
Short-term debt	439		439
Accounts payable and accrued liabilities	2,246		2,246		122
Accrued compensation and employee benefits	1,103		1,103
Accrued income taxes	216		216
Dividends payable	212		212
Total current liabilities	4,216		4,216
Fiduciary liabilities	5,118		5,118
Less – cash and investments held in a fiduciary capacity	(5,118)		(5,118)
Fiduciary Liabilities, Net, Noncurrent	0		0
Long-term debt	5,813		5,813
Pension, post-retirement and post-employment benefits	1,768		1,768
Liabilities for errors and omissions	303		303
Other liabilities	1,262		1,262		23
Equity [Abstract]
Retained earnings	364		364		$ 364
Total equity	7,797		7,797
Total liabilities and stockholders' equity	21,159		21,159
Statement of Cash Flows [Abstract]
Net income before non-controlling interests			1,232
Depreciation and amortization of fixed assets and capitalized software			159
Amortization of intangible assets			88
Adjustments and payments related to contingent consideration liability			2
Provision for deferred income taxes			34
(Gain) loss on investments			(28)
(Gain) loss on disposition of assets			(1)
Share-based compensation expense			99
Net receivables			(388)
Other current assets			4
Other assets			(10)
Accounts payable and accrued liabilities			30
Accrued compensation and employee benefits			(614)
Accrued income taxes			18
Contributions to pension and other benefit plans in excess of current year expense/credit			(178)
Other liabilities			(10)
Effect of exchange rate changes			(24)
Net cash provided by operations			413
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	3,710		7,549
Expense:
Compensation and benefits	2,125		4,289
Other operating expenses	908		1,776
Operating expenses	3,033		6,065
Operating income	677		1,484
Other net benefit credits	65		131
Interest income	3		6
Interest expense	(68)		(129)
Investment Income	28		28
Income before income taxes	705		1,520
Income tax expense	179		373
Net income before non-controlling interests	526		1,147
Less: Net income attributable to non-controlling interests	5		11
Net income attributable to the Company	521		1,136
Assets
Cash and cash equivalents	1,036		1,036
Net receivables	4,347		4,347
Other current assets	240		240
Total current assets	5,623		5,623
Goodwill and intangible assets	10,411		10,411
Fixed assets, net	698		698
Pension related assets	1,808		1,808
Deferred tax assets	665		665
Other assets	1,305		1,305
Total assets	20,510		20,510
Liabilities
Short-term debt	439		439
Accounts payable and accrued liabilities	2,069		2,069
Accrued compensation and employee benefits	1,103		1,103
Accrued income taxes	216		216
Dividends payable	212		212
Total current liabilities	4,039		4,039
Fiduciary liabilities	5,118		5,118
Less – cash and investments held in a fiduciary capacity	(5,118)		(5,118)
Fiduciary Liabilities, Net, Noncurrent	0		0
Long-term debt	5,813		5,813
Pension, post-retirement and post-employment benefits	1,768		1,768
Liabilities for errors and omissions	303		303
Other liabilities	1,239		1,239
Equity [Abstract]
Total equity	7,348		7,348
Total liabilities and stockholders' equity	20,510		20,510
Statement of Cash Flows [Abstract]
Net income before non-controlling interests			1,147
Depreciation and amortization of fixed assets and capitalized software			159
Amortization of intangible assets			88
Adjustments and payments related to contingent consideration liability			2
Provision for deferred income taxes			34
(Gain) loss on investments			(28)
(Gain) loss on disposition of assets			(1)
Share-based compensation expense			99
Net receivables			(202)
Other current assets			(16)
Other assets			(36)
Accounts payable and accrued liabilities			(25)
Accrued compensation and employee benefits			(614)
Accrued income taxes			18
Contributions to pension and other benefit plans in excess of current year expense/credit			(178)
Other liabilities			(10)
Effect of exchange rate changes			(24)
Net cash provided by operations			413
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	(24)		(185)
Expense:
Compensation and benefits	(10)		(70)
Other operating expenses	0		0
Operating expenses	(10)		(70)
Operating income	(14)		(115)
Other net benefit credits	0		0
Interest income	0		0
Interest expense	0		0
Investment Income	0		0
Income before income taxes	(14)		(115)
Income tax expense	(4)		(30)
Net income before non-controlling interests	(10)		(85)
Less: Net income attributable to non-controlling interests	0		0
Net income attributable to the Company	(10)		(85)
Assets
Cash and cash equivalents	0		0
Net receivables	(254)		(254)
Other current assets	(298)		(298)
Total current assets	(552)		(552)
Goodwill and intangible assets	0		0
Fixed assets, net	0		0
Pension related assets	0		0
Deferred tax assets	133		133
Other assets	(230)		(230)
Total assets	(649)		(649)
Liabilities
Short-term debt	0		0
Accounts payable and accrued liabilities	(177)		(177)
Accrued compensation and employee benefits	0		0
Accrued income taxes	0		0
Dividends payable	0		0
Total current liabilities	(177)		(177)
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	(23)		(23)
Equity [Abstract]
Total equity	(449)		(449)
Total liabilities and stockholders' equity	$ (649)		(649)
Statement of Cash Flows [Abstract]
Net income before non-controlling interests			(85)
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
(Gain) loss on investments			0
(Gain) loss on disposition of assets			0
Share-based compensation expense			0
Net receivables			186
Other current assets			(20)
Other assets			(26)
Accounts payable and accrued liabilities			(55)
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			0
Effect of exchange rate changes			0
Net cash provided by operations			$ 0