New Accounting Guidance - ASC 606 (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues [Abstract]
Revenue	$ 4,000	$ 3,503
Expense:
Compensation and benefits	2,224	2,005
Other operating expenses	868	749
Operating expenses	3,092	2,754
Operating income	908	749
Other net benefit credits	66	60
Interest income	3	2
Interest expense	(61)	(58)
Income before income taxes	916	753
Income tax expense	220	175
Net Income Before Non-Controlling Interests	690	569
Less: Net income attributable to non-controlling interests	6	9
Net income attributable to the Company	690	569
Assets
Cash and cash equivalents	1,168	930		$ 1,205	$ 1,026
Net receivables	4,562		$ 4,201	4,133
Other current assets	540		542	224
Total current assets	6,270			5,562
Goodwill and intangible assets	9,194			9,089
Fixed assets, net	713			712
Pension related assets	1,857			1,693
Deferred tax assets	554		552	669
Total assets	21,379			20,429
Liabilities
Short-term debt	512			262
Accounts payable and accrued liabilities	2,343		2,205	2,083
Accrued compensation and employee benefits	813			1,718
Accrued income taxes	261			199
Dividends payable	193			0
Total current liabilities	4,122			4,262
Fiduciary liabilities	5,140			4,847
Less – cash and investments held in a fiduciary capacity	(5,140)			(4,847)
Fiduciary Liabilities, Net, Noncurrent	0			0
Long-term debt	5,815			5,225
Pension, post-retirement and post-employment benefits	1,842			1,888
Liabilities for errors and omissions	312			301
Other liabilities	1,267		1,334	1,311
Equity [Abstract]
Retained earnings	13,812		13,504	13,140
Total equity	8,021	6,619		7,442
Total liabilities and stockholders' equity	21,379			$ 20,429
Statement of Cash Flows [Abstract]
Net income before non-controlling interests	696	578
Depreciation and amortization of fixed assets and capitalized software	80	80
Amortization of intangible assets	45	40
Adjustments and payments related to contingent consideration liability	(5)	(20)
Provision for deferred income taxes	11	25
(Gain) loss on disposition of assets	(1)	6
Share-based compensation expense	50	42
Net receivables	(357)	(146)
Other current assets	2	(43)
Other assets	(32)	(25)
Accounts payable and accrued liabilities	135	60
Accrued compensation and employee benefits	(905)	(888)
Accrued income taxes	61	56
Contributions to pension and other benefit plans in excess of current year expense/credit	(96)	(106)
Other liabilities	17	(46)
Effect of exchange rate changes	(65)	(12)
Net cash used for operations	(364)	$ (399)
Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	4,000
Expense:
Compensation and benefits	2,224
Other operating expenses	868
Operating expenses	3,092
Operating income	908
Other net benefit credits	66
Interest income	3
Interest expense	(61)
Income before income taxes	916
Income tax expense	220
Net Income Before Non-Controlling Interests	696
Less: Net income attributable to non-controlling interests	6
Net income attributable to the Company	690
Assets
Cash and cash equivalents	1,168
Net receivables	4,562		68
Other current assets	540		318
Total current assets	6,270
Goodwill and intangible assets	10,450
Fixed assets, net	713
Pension related assets	1,857
Deferred tax assets	554		(103)
Other assets	1,535
Total assets	21,379
Liabilities
Short-term debt	512
Accounts payable and accrued liabilities	2,343		122
Accrued compensation and employee benefits	813
Accrued income taxes	261
Dividends payable	193
Total current liabilities	4,122
Fiduciary liabilities	5,140
Less – cash and investments held in a fiduciary capacity	5,140
Fiduciary Liabilities, Net, Noncurrent	0
Long-term debt	5,815
Pension, post-retirement and post-employment benefits	1,842
Liabilities for errors and omissions	312
Other liabilities	1,267		23
Equity [Abstract]
Retained earnings	364		$ 364
Total equity	8,021
Total liabilities and stockholders' equity	21,379
Statement of Cash Flows [Abstract]
Net income before non-controlling interests	696
Depreciation and amortization of fixed assets and capitalized software	80
Amortization of intangible assets	45
Adjustments and payments related to contingent consideration liability	(5)
Provision for deferred income taxes	11
(Gain) loss on disposition of assets	(1)
Share-based compensation expense	50
Net receivables	(357)
Other current assets	2
Other assets	(32)
Accounts payable and accrued liabilities	135
Accrued compensation and employee benefits	(905)
Accrued income taxes	61
Contributions to pension and other benefit plans in excess of current year expense/credit	(96)
Other liabilities	17
Effect of exchange rate changes	(65)
Net cash used for operations	(364)
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	3,839
Expense:
Compensation and benefits	2,164
Other operating expenses	868
Operating expenses	3,032
Operating income	807
Other net benefit credits	66
Interest income	3
Interest expense	(61)
Income before income taxes	815
Income tax expense	194
Net Income Before Non-Controlling Interests	621
Less: Net income attributable to non-controlling interests	6
Net income attributable to the Company	615
Assets
Cash and cash equivalents	1,168
Net receivables	4,320
Other current assets	246
Total current assets	5,734
Goodwill and intangible assets	10,450
Fixed assets, net	713
Pension related assets	1,857
Deferred tax assets	673
Other assets	1,304
Total assets	20,731
Liabilities
Short-term debt	512
Accounts payable and accrued liabilities	2,167
Accrued compensation and employee benefits	813
Accrued income taxes	261
Dividends payable	193
Total current liabilities	3,946
Fiduciary liabilities	5,140
Less – cash and investments held in a fiduciary capacity	5,140
Fiduciary Liabilities, Net, Noncurrent	0
Long-term debt	5,815
Pension, post-retirement and post-employment benefits	1,842
Liabilities for errors and omissions	312
Other liabilities	1,234
Equity [Abstract]
Total equity	7,582
Total liabilities and stockholders' equity	20,731
Statement of Cash Flows [Abstract]
Net income before non-controlling interests	621
Depreciation and amortization of fixed assets and capitalized software	80
Amortization of intangible assets	45
Adjustments and payments related to contingent consideration liability	(5)
Provision for deferred income taxes	11
(Gain) loss on disposition of assets	(1)
Share-based compensation expense	50
Net receivables	(183)
Other current assets	(22)
Other assets	(43)
Accounts payable and accrued liabilities	81
Accrued compensation and employee benefits	(905)
Accrued income taxes	61
Contributions to pension and other benefit plans in excess of current year expense/credit	(96)
Other liabilities	7
Effect of exchange rate changes	(65)
Net cash used for operations	(364)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenues [Abstract]
Revenue	(161)
Expense:
Compensation and benefits	(60)
Other operating expenses	0
Operating expenses	(60)
Operating income	(101)
Other net benefit credits	0
Interest income	0
Interest expense	0
Income before income taxes	(101)
Income tax expense	(26)
Net Income Before Non-Controlling Interests	(75)
Less: Net income attributable to non-controlling interests	0
Net income attributable to the Company	(75)
Assets
Cash and cash equivalents	0
Net receivables	(242)
Other current assets	(294)
Total current assets	(536)
Goodwill and intangible assets	0
Fixed assets, net	0
Pension related assets	0
Deferred tax assets	119
Other assets	(231)
Total assets	(648)
Liabilities
Short-term debt	0
Accounts payable and accrued liabilities	(176)
Accrued compensation and employee benefits	0
Accrued income taxes	0
Dividends payable	0
Total current liabilities	(176)
Fiduciary liabilities	0
Less – cash and investments held in a fiduciary capacity	0
Fiduciary Liabilities, Net, Noncurrent	0
Long-term debt	0
Pension, post-retirement and post-employment benefits	0
Liabilities for errors and omissions	0
Other liabilities	(33)
Equity [Abstract]
Total equity	(439)
Total liabilities and stockholders' equity	(648)
Statement of Cash Flows [Abstract]
Net income before non-controlling interests	(75)
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 disposition of assets	0
Share-based compensation expense	0
Net receivables	174
Other current assets	(24)
Other assets	(11)
Accounts payable and accrued liabilities	(54)
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	(10)
Effect of exchange rate changes	0
Net cash used for operations	$ 0