Marsh & McLennan Companies (MMC) 10-K 26 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 20, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	MMC
Entity Registrant Name	MARSH & MCLENNAN COMPANIES, INC.
Entity Central Index Key	62709
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		538,779,514
Entity Public Float			$28,219,746,395

Consolidated_Statements_Of_Inc

Consolidated Statements Of Income (USD $)	3 Months Ended		12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue	$3,246	$3,115	$12,951	$12,261	$11,924
Expense:
Compensation and benefits			7,515	7,226	7,134
Other operating expenses			3,135	2,958	2,961
Operating expenses			10,650	10,184	10,095
Operating income	536	489	2,301	2,077	1,829
Interest income			21	18	24
Interest expense			-165	-167	-181
Cost of extinguishment of debt			-137	-24	0
Investment income			37	69	24
Income before income taxes			2,057	1,973	1,696
Income tax expense			586	594	492
Income from continuing operations	269	307	1,471	1,379	1,204
Discontinued operations, net of tax	30	0	26	6	-3
Net income before non-controlling interests			1,497	1,385	1,201
Less: Net income attributable to non-controlling interests			32	28	25
Net income attributable to the Company	$294	$303	$1,465	$1,357	$1,176
Basic Per Share Data:
Basic net income per share – Continuing operations (in dollars per share)	$0.49	$0.55	$2.64	$2.46	$2.16
Net income attributable to the Company (in dollars per share)	$0.54	$0.55	$2.69	$2.47	$2.16
Diluted Per Share Data:
Continuing operations (in dollars per share)	$0.48	$0.54	$2.61	$2.42	$2.13
Net income attributable to the Company (in dollars per share)	$0.54	$0.54	$2.65	$2.43	$2.13
Average number of shares outstanding
Basic (in shares)			545	549	544
Diluted (in shares)			553	558	552
Shares outstanding at December 31 (in shares)	540	547	540	547	545

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income before non-controlling interests	$1,497	$1,385	$1,201
Other comprehensive income (loss), before tax:
Foreign currency translation adjustments	-527	-86	177
Unrealized investment loss	0	1	-1
(Loss) gain related to pension/post-retirement plans	-1,085	1,213	-447
Other comprehensive (loss) income, before tax	-1,612	1,128	-271
Income tax (credit) expense on other comprehensive loss	-386	442	-152
Other comprehensive (loss) income, net of tax	-1,226	686	-119
Comprehensive income	271	2,071	1,082
Less: Comprehensive income attributable to non-controlling interests	32	28	25
Comprehensive income attributable to the Company	$239	$2,043	$1,057

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Current assets:
Cash and cash equivalents	$1,958	$2,303
Receivables
Commissions and fees	3,142	3,065
Advanced premiums and claims	50	61
Other	280	282
Gross receivables	3,472	3,408
Less-allowance for doubtful accounts and cancellations	-95	-98
Net receivables	3,377	3,310
Current deferred tax assets	521	482
Other current assets	199	205
Total current assets	6,055	6,300
Goodwill and intangible assets	7,933	7,365
Fixed assets, net	809	828
Pension related assets	967	979
Deferred tax assets	876	626
Other assets	1,200	882
Total assets	17,840	16,980
Current liabilities:
Short-term debt	11	334
Accounts payable and accrued liabilities	1,883	1,861
Accrued compensation and employee benefits	1,633	1,466
Accrued income taxes	178	148
Total current liabilities	3,705	3,809
Fiduciary liabilities	4,552	4,234
Less – cash and investments held in a fiduciary capacity	-4,552	-4,234
Fiduciary liabilities, net, noncurrent	0	0
Long-term debt	3,376	2,621
Pension, postretirement and postemployment benefits	2,244	1,150
Liabilities for errors and omissions	341	373
Other liabilities	1,041	1,052
Commitments and contingencies	0	0
Equity:
Preferred stock, $1 par value, authorized 6,000,000 shares, none issued	0	0
1,600,000,000 shares, issued 560,641,640 shares at December 31, 2014 and December 31, 2013	561	561
Additional paid-in capital	930	1,028
Retained earnings	10,335	9,452
Accumulated other comprehensive loss	-3,847	-2,621
Non-controlling interests	79	70
Stockholders Equity Subtotal Before Treasury Stock	8,058	8,490
Less – treasury shares, at cost, 20,499,596 shares at December 31, 2014 and 13,882,204 shares at December 31, 2013	-925	-515
Total equity	7,133	7,975
Total liabilities and stockholders' equity	$17,840	$16,980

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$1	$1
Preferred stock, shares authorized (in shares)	6,000,000	6,000,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$1	$1
Common stock, shares authorized (in shares)	1,600,000,000	1,600,000,000
Common stock, shares issued (in shares)	560,641,640	560,641,640
Treasury shares, shares (in shares)	20,499,596	13,882,204

Consolidated_Statements_Of_Cas

Consolidated Statements Of Cash Flows (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating cash flows:
Net income before non-controlling interests	$1,497	$1,385	$1,201
Adjustments to reconcile net income to cash provided by operations:
Depreciation and amortization of fixed assets and capitalized software	302	286	277
Amortization of intangible assets	86	72	72
Intangible asset impairment	0	5	8
Adjustments to acquisition related contingent consideration liability	19	24	-44
Cost of early extinguishment of debt	137	24	0
Provision for deferred income taxes	127	184	96
Gain on investments	-37	-69	-24
(Gain) Loss on disposition of assets	-38	1	23
Share-based compensation expense	93	129	177
Changes in assets and liabilities:
Net receivables	-58	-245	-144
Other current assets	-32	-70	-37
Other assets	25	-75	44
Accounts payable and accrued liabilities	45	106	-210
Accrued compensation and employee benefits	167	-8	72
Accrued income taxes	43	43	44
Contributions to pension and other benefit plans in excess of current year expense/credit	-152	-432	-117
Other liabilities	-185	13	-81
Effect of exchange rate changes	73	-32	-35
Net cash provided by operations	2,112	1,341	1,322
Financing cash flows:
Purchase of treasury shares	-800	-550	-230
Proceeds from debt	1,393	547	248
Repayments of debt	-331	-260	-259
Payments for early extinguishment of debt	-765	-274	0
Shares withheld for taxes on vested units – treasury shares	-64	-79	-97
Issuance of common stock from treasury shares	263	352	248
Payments of deferred and contingent consideration for acquisitions	-55	-9	-30
Distributions of non-controlling interests	-20	-28	-16
Dividends paid	-582	-533	-497
Net cash used for financing activities	-961	-834	-633
Investing cash flows:
Capital expenditures	-368	-401	-320
Net sales of long-term investments	6	93	20
Purchase of equity investment	-304	0	0
Proceeds from sales of fixed assets	3	5	6
Dispositions	0	5	0
Acquisitions	-554	-142	-292
Other, net	-5	-6	3
Net cash used for investing activities	-1,222	-446	-583
Effect of exchange rate changes on cash and cash equivalents	-274	-59	82
(Decrease) increase in cash and cash equivalents	-345	2	188
Cash and cash equivalents at beginning of period	2,303	2,301	2,113
Cash and cash equivalents at end of period	$1,958	$2,303	$2,301

Consolidated_Statements_Of_Equ

Consolidated Statements Of Equity (USD $)	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Gain (Loss) [Member]	Treasury Shares [Member]	Non-Controlling Interests [Member]
In Millions, unless otherwise specified
Beginning balance at Dec. 31, 2011		$561	$1,156	$7,949	($3,188)	($595)	$57
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in accrued stock compensation costs			-16
Issuance of shares under stock compensation plans and employee stock purchase plans and related tax impact			-34			378
Purchase of subsidiary shares from non-controlling interests			1
Net income attributable to the Company	1,201			1,176			25
Dividend equivalents declared - (per share amounts: $1.06 in 2014, $0.96 in 2013, and $0.90 in 2012)				-8
Dividends declared – (per share amounts: $1.06 in 2014, $0.96 in 2013, and $0.90 in 2012)				-489
Other comprehensive (loss) income, net of tax	-119				-119
Issuance of shares for acquisitions						0
Purchase of treasury shares						-230
Distributions							-16
Other changes							-2
Ending balance at Dec. 31, 2012	6,606	561	1,107	8,628	-3,307	-447	64
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in accrued stock compensation costs			-22
Issuance of shares under stock compensation plans and employee stock purchase plans and related tax impact			-57			481
Purchase of subsidiary shares from non-controlling interests			0
Net income attributable to the Company	1,385			1,357			28
Dividend equivalents declared - (per share amounts: $1.06 in 2014, $0.96 in 2013, and $0.90 in 2012)				-6
Dividends declared – (per share amounts: $1.06 in 2014, $0.96 in 2013, and $0.90 in 2012)				-527
Other comprehensive (loss) income, net of tax	686				686
Issuance of shares for acquisitions						1
Purchase of treasury shares						-550
Distributions							-28
Other changes							6
Ending balance at Dec. 31, 2013	7,975	561	1,028	9,452	-2,621	-515	70
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Change in accrued stock compensation costs			-15
Issuance of shares under stock compensation plans and employee stock purchase plans and related tax impact			-83			387
Purchase of subsidiary shares from non-controlling interests			0
Net income attributable to the Company	1,497			1,465			32
Dividend equivalents declared - (per share amounts: $1.06 in 2014, $0.96 in 2013, and $0.90 in 2012)				-3
Dividends declared – (per share amounts: $1.06 in 2014, $0.96 in 2013, and $0.90 in 2012)				-579
Other comprehensive (loss) income, net of tax	-1,226				-1,226
Issuance of shares for acquisitions						3
Purchase of treasury shares						-800
Distributions							-20
Other changes							-3
Ending balance at Dec. 31, 2014	$7,133	$561	$930	$10,335	($3,847)	($925)	$79

Consolidated_Statements_Of_Equ1

Consolidated Statements Of Equity (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Stockholders' Equity [Abstract]
Dividends equivalents declared per share (in dollars per share)	$1.06	$0.96	$0.90
Dividends declared per share (in dollars per share)	$1.06	$0.96	$0.90

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies
	Nature of Operations:  Marsh & McLennan Companies, Inc. (the "Company”), a global professional services firm, is organized based on the different services that it offers. Under this organizational structure, the Company’s two business segments are Risk and Insurance Services and Consulting.
	The Risk and Insurance Services segment provides risk management activities and insurance broking, reinsurance broking and insurance program management services for businesses, public entities, insurance companies, associations, professional services organizations, and private clients. The Company conducts business in this segment through Marsh and Guy Carpenter.
	The Company conducts business in its Consulting segment through two main business groups. Mercer provides consulting expertise, advice, services and solutions in the areas of health, retirement, talent and investments. Oliver Wyman Group provides specialized management and economic and brand consulting services.
	Acquisitions impacting the Risk and Insurance Services and Consulting segments are discussed in Note 4 below.
	Principles of Consolidation:  The accompanying consolidated financial statements include all wholly-owned and majority-owned subsidiaries. All significant inter-company transactions and balances have been eliminated.
	Fiduciary Assets and Liabilities:  In its capacity as an insurance broker or agent, the Company generally collects premiums from insureds and, after deducting its commissions, remits the premiums to the respective insurance underwriters. The Company also collects claims or refunds from underwriters on behalf of insureds. Unremitted insurance premiums and claims proceeds are held by the Company in a fiduciary capacity. Risk and Insurance Services revenue includes interest on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively. The Consulting segment recorded fiduciary interest income of $6 million, $5 million and $4 million in 2014, 2013 and 2012, respectively. Since fiduciary assets are not available for corporate use, they are shown in the consolidated balance sheets as an offset to fiduciary liabilities.
	Net uncollected premiums and claims and the related payables amounted to $7.3 billion and $8.2 billion at December 31, 2014 and 2013, respectively. The Company is not a principal to the contracts under which the right to receive premiums or the right to receive reimbursement of insured losses arises. Net uncollected premiums and claims and the related payables are, therefore, not assets and liabilities of the Company and are not included in the accompanying consolidated balance sheets.
	In certain instances, the Company advances premiums, refunds or claims to insurance underwriters or insureds prior to collection. These advances are made from corporate funds and are reflected in the accompanying consolidated balance sheets as receivables.
	Mercer manages approximately $21 billion of assets in trusts or funds for which Mercer’s management or trustee fee is considered a variable interest. Mercer is not the primary beneficiary of these trusts or funds. Mercer’s only variable interest in any of these trusts or funds is its unpaid fees, if any. Mercer’s maximum exposure to loss of its interests is, therefore, limited to collection of its fees.
	Revenue:  Risk and Insurance Services revenue includes insurance commissions, fees for services rendered and interest income on certain fiduciary funds. Insurance commissions and fees for risk transfer services generally are recorded as of the effective date of the applicable policies or, in certain cases (primarily in the Company's reinsurance broking operations), as of the effective date or billing date, whichever is later. A reserve for policy cancellation is provided based on historic and current data on cancellations. Consideration for fee arrangements covering multiple insurance placements, the provision of risk management and/or other services are allocated to all deliverables on the basis of their relative selling prices. Fees for non-risk transfer services provided to clients are recognized over the period in which the services are provided, using a proportional performance model. Fees resulting from achievement of certain performance thresholds are recorded when such levels are attained and such fees are not subject to forfeiture.
	Consulting revenue includes fees paid by clients for advice and services and commissions from insurance companies for the placement of individual and group contracts. Fee revenue for engagements where remuneration is based on time plus out-of-pocket expenses is recognized based on the amount of time consulting professionals expend on the engagement. For fixed fee engagements, revenue is recognized using a proportional performance model. Revenue from insurance commissions not subject to a fee arrangement is recorded over the effective period of the applicable policies. Revenue for asset based fees is recognized on an accrual basis by applying the daily/monthly rate as contractually agreed with the client to the applicable net asset value. On a limited number of engagements, performance fees may also be earned for achieving certain prescribed performance criteria. Such fees are recognized when the performance criteria have been achieved and, when required, agreed to by the client. Reimbursable expenses incurred by professional staff in the generation of revenue and sub-advisory fees related to the majority of funds in the investment management business are included in revenue and the related expenses are included in other operating expenses.
	Cash and Cash Equivalents:  Cash and cash equivalents primarily consist of certificates of deposit and time deposits, with original maturities of three months or less, and money market funds. The estimated fair value of the Company's cash and cash equivalents approximates their carrying value. The Company is required to maintain operating funds related to regulatory requirements outside the U.S. or as collateral under captive insurance arrangements. At December 31, 2014, the Company maintained $179 million related to these regulatory requirements.
	Fixed Assets:  Fixed assets are stated at cost less accumulated depreciation and amortization. Expenditures for improvements are capitalized. Upon sale or retirement, the cost and related accumulated depreciation and amortization are removed from the accounts and any gain or loss is reflected in income. Expenditures for maintenance and repairs are charged to operations as incurred.
	Depreciation of buildings, building improvements, furniture, and equipment is provided on a straight-line basis over the estimated useful lives of these assets. Furniture and equipment is depreciated over periods ranging from three to ten years. Leasehold improvements are amortized on a straight-line basis over the periods covered by the applicable leases or the estimated useful life of the improvement, whichever is less. Buildings are depreciated over periods ranging from thirty to forty years. The Company periodically reviews long-lived assets for impairment whenever events or changes indicate that the carrying value of assets may not be recoverable.
	The components of fixed assets are as follows:
	December 31,
	(In millions of dollars)		2014				2013
	Furniture and equipment		$	1,193			$	1,201
	Land and buildings		401				408
	Leasehold and building improvements		854				816
			2,448				2,425
	Less-accumulated depreciation and amortization		(1,639		)		(1,597		)
			$	809			$	828
	Investments:  The Company holds investments in private companies and private equity funds.
	Investments in private equity funds are accounted for under the equity method of accounting using a consistently applied three-month lag period adjusted for any known significant changes from the lag period to the reporting date of the Company. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. The Company records in earnings, investment gains/losses for its proportionate share of the change in fair value of the funds. Investments using the equity method of accounting are included in other assets in the consolidated balance sheets.
	As part of the sale of MMC Capital in 2005, the Company retained the rights to receive certain performance fees related to the Trident II and Trident III private equity partnerships. The Company recognizes performance fee income when such fees are no longer subject to forfeiture, which may take a number of years to resolve. This income is based on the investment performance over the life of each investment in the private equity fund, and future declines in the fund performance from current levels may result in forfeiture of such revenue. Since Trident II fully harvested all its portfolio investments and made final distributions to its partners in 2013, the Company no longer holds any rights to Trident II performance fees. In 2014, the Company recorded investment income of $37 million compared to $69 million in 2013 and $24 million in 2012. The Company recorded Investment income related to its general partner carried interest from Trident III no longer subject to clawback of $31 million and $41 million in 2014 and 2013, respectively. In 2013, the Company recorded $15 million of general partner carried interest from Trident II. There was no carried interest recorded in 2012. The Company also recorded net investment income gains of $6 million, $13 million and $33 million in 2014, 2013 and 2012, respectively, primarily related to its equity method investments. The Company had deferred performance fees of approximately $16 million and $38 million, at December 31, 2014 and 2013, respectively, related to Trident III. Recognition of these deferred performance fees will only occur as the Trident III investments are harvested and the performance fees are no longer subject to clawback. The timing of recognition of the remaining deferred performance fees is unknown and is not controlled by the Company.
	Subsequent Event
	On February 24, 2015, Mercer announced that it had made an investment to purchase approximately 10% of Benefitfocus (NASDAQ: BNFT), a benefits technology provider, with the option to increase its ownership over time.
	Goodwill and Other Intangible Assets:  Goodwill represents acquisition costs in excess of the fair value of net assets acquired. Goodwill is reviewed at least annually for impairment. The Company performs an annual impairment test for each of its reporting units during the third quarter of each year. When a step 1 test is performed, fair values of the reporting units are estimated using either a market approach or a discounted cash flow model. Carrying values for the reporting units are based on balances at the prior quarter end and include directly identified assets and liabilities as well as an allocation of those assets and liabilities not recorded at the reporting unit level. As discussed in Note 6, the Company may elect to assess qualitative factors to determine if a step 1 test is necessary. Other intangible assets, which primarily consist of customer lists, that are not deemed to have an indefinite life are amortized over their estimated lives and reviewed for impairment upon the occurrence of certain triggering events in accordance with applicable accounting literature. The Company had no indefinite lived identified intangible assets at December 31, 2014 and 2013.
	Capitalized Software Costs:  The Company capitalizes certain costs to develop, purchase or modify software for the internal use of the Company. These costs are amortized on a straight-line basis over periods ranging from 3 to 10 years. Costs incurred during the preliminary project stage and post implementation stage, are expensed as incurred. Costs incurred during the application development stage are capitalized. Costs related to updates and enhancements are only capitalized if they will result in additional functionality. Capitalized computer software costs of $501 million and $399 million, net of accumulated amortization of $837 million and $748 million at December 31, 2014 and 2013, respectively, are included in other assets in the consolidated balance sheets.
	Legal and Other Loss Contingencies:  The Company and its subsidiaries are subject to a significant number of claims, lawsuits and proceedings including claims for errors and omissions ("E&O"). GAAP requires that a liability be recorded when a loss is both probable and reasonably estimable. Significant management judgment is required to apply this guidance. The Company utilizes case level reviews by inside and outside counsel, an internal actuarial analysis and other analysis to estimate potential losses. The liability is reviewed quarterly and adjusted as developments warrant. In many cases, the Company has not recorded a liability, other than for legal fees to defend the claim, because we are unable, at the present time, to make a determination that a loss is both probable and reasonably estimable. Given the unpredictability of E&O claims and of litigation that could flow from them, it is possible that an adverse outcome in a particular matter could have a material adverse effect on the Company’s businesses, results of operations, financial condition or cash flow in a given quarterly or annual period.
	In addition, to the extent that insurance coverage is available, significant management judgment is required to determine the amount of recoveries that are probable of collection under the Company’s various insurance programs.
	The legal and other contingent liabilities described above are not discounted.
	Income Taxes:  The Company's effective tax rate reflects its income, statutory tax rates and tax planning in the various jurisdictions in which it operates. Significant judgment is required in determining the annual effective tax rate and in evaluating uncertain tax positions and the ability to realize deferred tax assets.
	The Company reports a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in a tax return. The evaluation of a tax position is a two-step process. The first step involves recognition. The Company determines whether it is more likely than not that a tax position will be sustained upon tax examination, including resolution of any related appeals or litigation, based on only the technical merits of the position. The technical merits of a tax position derive from both statutory and judicial authority (legislation and statutes, legislative intent, regulations, rulings, and case law) and their applicability to the facts and circumstances of the tax position. If a tax position does not meet the more likely than not recognition threshold, the benefit of that position is not recognized in the financial statements. The second step is measurement. A tax position that meets the more likely than not recognition threshold is measured to determine the amount of benefit to recognize in the financial statements. The tax position is measured as the largest amount of benefit that is greater than 50 percent likely of being realized upon ultimate resolution with a taxing authority. Uncertain tax positions are evaluated based upon the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based upon new information may lead to changes in recognition, de-recognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. The Company recognizes in income tax expense, interest and penalties, if any, related to unrecognized tax benefits.
	Tax law requires items be included in the Company's tax returns at different times than the items are reflected in the financial statements. As a result, the annual tax expense reflected in the consolidated statements of income is different than that reported in the income tax returns. Some of these differences are permanent, such as expenses that are not deductible in the returns, and some differences are temporary and reverse over time, such as depreciation expense. Temporary differences create deferred tax assets and liabilities. Deferred tax assets generally represent items that can be used as a tax deduction or credit in tax returns in future years for which benefit has already been recorded in the financial statements. Valuation allowances are established for deferred tax assets when it is estimated that future taxable income will be insufficient to use a deduction or credit in that jurisdiction. Deferred tax liabilities generally represent tax expense recognized in the financial statements for which payment has been deferred, or expense for which a deduction has been taken already in the tax return but the expense has not yet been recognized in the financial statements.
	Derivative Instruments:  All derivatives, whether designated in hedging relationships or not, are recorded on the balance sheet at fair value. If the derivative is designated as a fair value hedge, the changes in the fair value of the derivative and of the hedged item attributable to the hedged risk are recognized in earnings. If the derivative is designated as a cash flow hedge, the effective portions of changes in the fair value of the derivative are recorded in other comprehensive income and are recognized in the income statement when the hedged item affects earnings. Changes in the fair value attributable to the ineffective portion of cash flow hedges are recognized in earnings.
	Concentrations of Credit Risk:  Financial instruments which potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents, commissions and fees receivable and insurance recoverable. The Company maintains a policy providing for the diversification of cash and cash equivalent investments and places its investments in a large number of high quality financial institutions to limit the amount of credit risk exposure. Concentrations of credit risk with respect to receivables are generally limited due to the large number of clients and markets in which the Company does business, as well as the dispersion across many geographic areas.
	Per Share Data: Basic net income per share attributable to the Company and income from continuing operations per share are calculated by dividing the respective after-tax income attributable to common shares by the weighted average number of outstanding shares of the Company’s common stock.
	Diluted net income per share attributable to the Company and income from continuing operations per share are calculated by dividing the respective after-tax income attributable to common shares by the weighted average number of outstanding shares of the Company’s common stock, which have been adjusted for the dilutive effect of potentially issuable common shares. Reconciliations of the applicable income components used for diluted EPS - Continuing Operations and basic weighted average common shares outstanding to diluted weighted average common shares outstanding are presented below. The reconciling items related to the calculation of diluted weighted average common shares outstanding are the same for net income attributable to the Company.
	Basic and Diluted EPS Calculation - Continuing Operations
	(In millions, except per share figures)	2014				2013				2012
	Net income from continuing operations	$	1,471			$	1,379			$	1,204
	Less: Net income attributable to non-controlling interests	32				28				25
		$	1,439			$	1,351			$	1,179
	Basic weighted average common shares outstanding	545				549				544
	Dilutive effect of potentially issuable common shares	8				9				8
	Diluted weighted average common shares outstanding	553				558				552
	Average stock price used to calculate common stock equivalents	$	51.15			$	40.97			$	33.1
	There were 18.0 million, 22.6 million and 32.0 million stock options outstanding as of December 31, 2014, 2013 and 2012, respectively.
	Estimates:  The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results may vary from those estimates.
	New Accounting Pronouncements:  In June 2014, the FASB issued new accounting guidance to clarify the treatment of share-based payment awards that require a specific performance target to be achieved in order for employees to be eligible to vest in the awards which include terms that may provide that the performance conditions could be achieved after an employee completes the requisite service period. The guidance requires that a performance target that affects vesting and that could be achieved after the requisite service period be treated as a performance condition. As such, a reporting entity should apply the existing guidance as it relates to awards with performance conditions that affect vesting. The guidance is effective for annual periods beginning after December 15, 2015. Earlier adoption is permitted. Adoption of the guidance is not expected to materially affect the Company's financial condition, results of operations or cash flows.
	In May 2014, the FASB issued new accounting guidance to clarify the principles for revenue recognition. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that principle, the entity should apply the following steps: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. The guidance is effective for fiscal years beginning after December 15, 2016, including interim periods within that reporting period. Early application is not permitted. Entities are permitted to adopt the guidance under one of the following methods: retrospectively to each prior reporting period presented (with certain practical expedients allowed) or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application. If an entity elects this transition method, it must provide disclosures in reporting periods that include the date of initial application of the amount by which each financial statement line item is affected in the current reporting period by application of the guidance as compared to guidance that was in effect before the change, and an explanation for the reasons for significant changes. The Company is currently evaluating the impact of the adoption of the guidance on its financial condition and results of operations.
	In April 2014, the FASB issued new accounting guidance which changes the criteria for reporting discontinued operations and enhances disclosures in this area. Under the new guidance, only disposals representing a strategic shift in operations, such as disposal of a major geographic area or a major line of business, should be presented as discontinued operations. Those strategic shifts should have a major impact on the organization's operations and financial results. In addition, the new guidance requires expanded disclosures about discontinued operations. The guidance is effective for fiscal years beginning on or after December 15, 2014. Adoption of the guidance is not expected to have a material affect on the Company's financial condition, results of operations or cash flows.
	In July 2013, the FASB issued new accounting guidance related to the presentation of unrecognized tax benefits as a reduction to a deferred tax asset for a net operating loss carry-forward, a similar tax loss, or a tax credit carry-forward. However, to the extent a net operating loss carry-forward, a similar tax loss, or a tax credit carry-forward is not available at the reporting date under the tax law of the applicable jurisdiction to settle taxes that would result from the disallowance of the tax position or the entity does not intend to use the deferred tax asset for this purpose (provided that the tax law permits a choice), the unrecognized tax benefit shall be presented in the financial statement as a liability and shall not be combined with deferred tax assets. The guidance was effective for fiscal years beginning after December 15, 2013. The adoption of this new guidance impacted balance sheet classification and footnote disclosure only and did not have a material impact on the Company's financial statements.
	In June 2013, the FASB issued new accounting guidance which amends the criteria for an entity to qualify as an investment company. The guidance clarifies the characteristics of an investment company, provides comprehensive guidance to determine whether an entity is an investment company and sets measurement and disclosure requirements for investment companies. The guidance is effective for interim and annual reporting periods beginning after December 15, 2013. The adoption of this guidance did not materially affect the Company's financial position, results of operations or cash flows.
	In February 2013, the FASB issued new accounting guidance that adds new disclosure requirements for items reclassified out of accumulated other comprehensive income. The Company implemented this new guidance for the reporting period ended March 31, 2013. Other than enhanced disclosure, the adoption of this new guidance did not have a material effect on the Company's financial statements.
	Reclassifications: In the first quarter of 2014, the Company enhanced its operating cash flow presentation within the statements of cash flows to show on single lines the impact of pension and other benefit plan contributions in excess of the related expenses, and the non-cash impact of equity share awards. Previously, the cash flow impact of those items was presented as part of changes in other assets and other liabilities, and changes in other liabilities, respectively. The prior years' presentation was conformed to the current presentation for the following line items within operating cash flows:
	•	Share-based compensation expense
	•	Changes in other assets
	•	Contributions to pension and other benefit plans in excess of current year expense/credit
	•	Changes in other liabilities

Supplemental_Disclosures

Supplemental Disclosures	12 Months Ended
	Dec. 31, 2014
Supplemental Cash Flow Information [Abstract]
Supplemental Disclosures	Supplemental Disclosures
	The following schedule provides additional information concerning acquisitions, interest and income taxes paid:
	(In millions of dollars)	2014			2013			2012
	Assets acquired, excluding cash	$	815		$	217		$	380
	Released from escrow in 2012	—			—			(62		)
	Liabilities assumed	(64		)	(53		)	(42		)
	Contingent/deferred purchase consideration	(197		)	(39		)	(46		)
	Net cash outflow for current year acquisitions	554			125			230
	Purchase of other intangibles	—			2			3
	Net cash outflow for acquisitions	$	554		$	127		$	233
	(In millions of dollars)	2014			2013			2012
	Interest paid	$	172		$	170		$	183
	Income taxes paid, net of refunds	$	426		$	360		$	350
	The Company paid deferred purchase consideration related to prior years' acquisitions of $25 million, $15 million and $59 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	The Company had non-cash issuances of common stock under its share-based payment plan of $108 million, $150 million and $193 million for the years ended December 31, 2014, 2013 and 2012, respectively. The Company recorded stock-based compensation expense related to equity awards of $75 million, $110 million and $152 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	The consolidated statements of cash flows includes the cash flow impact of discontinued operations in each cash flow category, which were insignificant to the overall cash flows of the Company.
	An analysis of the allowance for doubtful accounts is as follows:
	For the Year Ended December 31,
	(In millions of dollars)	2014			2013			2012
	Balance at beginning of year	$	98		$	106		$	105
	Provision charged to operations	20			16			11
	Accounts written-off, net of recoveries	(17		)	(19		)	(12		)
	Effect of exchange rate changes and other	(6		)	(5		)	2
	Balance at end of year	$	95		$	98		$	106

Other_Comprehensive_Income_Los

Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss)
	The changes in the balances of each component of Accumulated Other Comprehensive Income ("AOCI") for the years ended December 31, 2014 and 2013, including amounts reclassified out of AOCI, are as follows:
	(In millions of dollars)	Unrealized Investment Gains				Pension/Post-Retirement Plans Gains (Losses)				Foreign Currency Translation Adjustments			Total
	Balance as of January 1, 2014	$	5			$	(2,682	)		$	56		$	(2,621	)
	Other comprehensive loss before reclassifications	—				(816		)		(515		)	(1,331		)
	Amounts reclassified from accumulated other comprehensive loss	—				105				—			105
	Net current period other comprehensive loss	—				(711		)		(515		)	(1,226		)
	Balance as of December 31, 2014	$	5			$	(3,393	)		$	(459	)	$	(3,847	)
	(In millions of dollars)	Unrealized Investment Gains				Pension/Post-Retirement Plans Gains (Losses)				Foreign Currency Translation Adjustments			Total
	Balance as of January 1, 2013	$	4			$	(3,451	)		$	140		$	(3,307	)
	Other comprehensive income (loss) before reclassifications	1				574				(84		)	491
	Amounts reclassified from accumulated other comprehensive loss	—				195				—			195
	Net current period other comprehensive income (loss)	1				769				(84		)	686
	Balance as of December 31, 2013	$	5			$	(2,682	)		$	56		$	(2,621	)
	The components of other comprehensive income (loss) are as follows:
	For the Year Ended December 31,	2014
	(In millions of dollars)	Pre-Tax			Tax (Credit)			Net of Tax
	Foreign currency translation adjustments	$	(527	)	$	(12	)	$	(515	)
	Pension/post-retirement plans:
	Amortization of prior service credits	(16		)	(5		)	(11		)
	Amortization of net actuarial losses	242			74			168
	Curtailment gain	(65		)	(13		)	(52		)
	Losses included in periodic pension cost	161			56			105
	Net losses arising during period	(1,418		)	(466		)	(952		)
	Foreign currency translation adjustments	180			39			141
	Other adjustments	(8		)	(3		)	(5		)
	Pension/post-retirement plans losses	(1,085		)	(374		)	(711		)
	Other comprehensive loss	$	(1,612	)	$	(386	)	$	(1,226	)
	For the Year Ended December 31,	2013
	(In millions of dollars)	Pre-Tax			Tax (Credit)			Net of Tax
	Foreign currency translation adjustments	$	(86	)	$	(2	)	$	(84	)
	Unrealized investment gains	1			—			1
	Pension/post-retirement plans:
	Amortization of prior service credits	(22		)	(8		)	(14		)
	Amortization of net actuarial losses	317			108			209
	Losses included in periodic pension cost	295			100			195
	Net gains arising during period	898			339			559
	Foreign currency translation adjustments	27			8			19
	Other adjustments	(7		)	(3		)	(4		)
	Pension/post-retirement plans gains	1,213			444			769
	Other comprehensive income	$	1,128		$	442		$	686
	For the Year Ended December 31,	2012
	(In millions of dollars)	Pre-Tax			Tax (Credit)			Net of Tax
	Foreign currency translation adjustments	$	177		$	(5	)	$	182
	Unrealized investment losses	(1		)	1			(2		)
	Pension/post-retirement plans:
	Amortization of prior service credits	(31		)	(12		)	(19		)
	Amortization of net actuarial losses	270			90			180
	Losses included in periodic pension cost	239			78			161
	Net losses arising during period	(648		)	(217		)	(431		)
	Foreign currency translation adjustments	(113		)	(26		)	(87		)
	Other adjustments	75			17			58
	Pension/post-retirement plans losses	(447		)	(148		)	(299		)
	Other comprehensive loss	$	(271	)	$	(152	)	$	(119	)
	The components of accumulated other comprehensive income (loss) are as follows:
	(In millions of dollars)	December 31, 2014				31-Dec-13
	Foreign currency translation adjustments (net of deferred tax (asset) liability of $(5) and $7 in 2014 and 2013, respectively)	$	(459	)		$	56
	Net unrealized investment gains (net of deferred tax liability of $2 in both 2014 and 2013)	5				5
	Net charges related to pension / post-retirement plans (net of deferred tax asset of $1,587 and $1,213 in 2014 and 2013, respectively)	(3,393		)		(2,682		)
		$	(3,847	)		$	(2,621	)

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Acquisitions	Acquisitions
	The Company’s acquisitions have been accounted for as business combinations. Net assets and results of operations are included in the Company’s consolidated financial statements commencing at the respective purchase closing dates. In connection with acquisitions, the Company records the estimated value of the net tangible assets purchased and the value of the identifiable intangible assets purchased, which typically consist of purchased customer lists, trademarks and non-compete agreements. The valuation of purchased intangible assets involves significant estimates and assumptions. Any change in assumptions could affect the carrying value of such intangible assets.
	The Risk and Insurance segment completed fifteen acquisitions during 2014.
	•	January - Marsh & McLennan Agency ("MMA") acquired Barney & Barney, LLC, a San Diego-based insurance broking firm that provides insurance, risk management and employee benefits solutions to businesses and individuals throughout the U.S. and abroad, Great Lakes Employee Benefits Services, Inc., an employee group benefits consulting and brokerage firm based in Michigan, and Bond Network, Inc., a surety bonding agency based in North Carolina.
	•	February - Marsh acquired Central Insurance Services, an independent insurance broker in Scotland that provides insurance broking and risk advisory services to companies of all sizes across industry sectors.
	•	March - MMA acquired Capstone Insurance Services, LLC, an agency that provides property-casualty insurance and risk management solutions to businesses and individuals throughout South Carolina.
	•	May - MMA acquired Kinker-Eveleigh Insurance Agency, an Ohio-based agency specializing in property-casualty and employee benefits solutions, VISICOR, a full-service employee benefits brokerage and consulting firm based in Texas, and Senn Dunn Insurance, a full-service insurance brokerage located in North Carolina.
	•	August - Marsh acquired Seguros Morrice y Urrutia S.A., an insurance broker based in Panama City, Panama.
	•	September - Marsh acquired Kocisko Insurance Brokers, Inc., a full-service commercial insurance brokerage located in Montreal, Quebec.
	•	October - MMA acquired NuWest Insurance Services, Inc., a California-based property-casualty agency.
	•	November - Marsh acquired Torrent Technologies, Inc., a Montana-based flood insurance specialist.
	•	December - Marsh acquired Seafire Insurance Services, LLC, a Kansas-based managing general underwriter, and Trade Insure NV, a leading distributor of credit insurance policies in Belgium, and MMA acquired The Benefit Planning Group, Inc., a North Carolina-based employee benefit consulting firm.
	The Consulting segment completed six acquisitions during 2014.
	•	February - Mercer acquired Transition Assist, a retiree exchange specializing in helping retirees in employer-sponsored plans select Medicare supplemental health care insurance.
	•	September - Oliver Wyman acquired Bonfire Communications, an agency specializing in employee engagement and internal communications based in San Francisco, California.
	•	November - Mercer acquired AUSREM, a remuneration research and workforce consulting specialist based in Australia, and Jeitosa Group International, a global HR business consultancy and IT systems integration firm.
	•	December - Mercer acquired Denarius, a compensation and benefits survey and information products consulting firm based in Chile, and Oliver Wyman acquired OC&C Strategy Consultants (Boston) LLC (part of the OC&C network), a Boston-based consulting firm specializing in the business media, information services and education sectors.
	Total purchase consideration for acquisitions made during 2014 was $772 million, which consisted of cash paid of $575 million and deferred purchase and estimated contingent consideration of $197 million. Contingent consideration arrangements are primarily based on EBITDA and revenue targets over two to four years. The fair value of the contingent consideration was based on projected revenue and earnings of the acquired entities. Estimated fair values of assets acquired and liabilities assumed are subject to adjustment when purchase accounting is finalized. During 2014, the Company also paid $25 million of deferred purchase consideration and $42 million of contingent consideration related to acquisitions made in prior years.
	The following table presents the preliminary allocation of the acquisition cost to the assets acquired and liabilities assumed, based on their fair values:
	(In millions)	2014
	Cash	$	575
	Estimated fair value of deferred/contingent consideration	197
	Total consideration	$	772
	Allocation of purchase price:
	Cash and cash equivalents	$	21
	Accounts receivable, net	12
	Other current assets	1
	Property, plant, and equipment	5
	Intangible assets (primarily customer lists amortized over 10 years)	318
	Goodwill	472
	Other assets	7
	Total assets acquired	836
	Current liabilities	41
	Other liabilities	23
	Total liabilities assumed	64
	Net assets acquired	$	772
	Prior Year Acquisitions
	During 2013, the Risk and Insurance segment completed the following six acquisitions:
	•	June - Marsh acquired Rehder y Asociados Group, an insurance adviser in Peru. The business includes the insurance broker Rehder y Asociados and employee health and benefits specialist, Humanasalud. Marsh also completed the acquisition of Franco & Acra Tecniseguros, an insurance advisor in the Dominican Republic.
	•	July - Guy Carpenter acquired Smith Group, a specialist disability reinsurance risk manager and consultant based in Maine.
	•	September - Marsh purchased an additional stake in Insia a.s., an insurance broker operating in the Czech Republic and Slovakia which, when combined with its prior holdings, gave Marsh a controlling interest. Insia a.s. was previously accounted for under the equity method.
	•	November - MMA acquired Elsey & Associates, a Texas-based provider of surety bonds and insurance coverage to the construction industry.
	•	December - MMA acquired Cambridge Property and Casualty, a Michigan-based company providing insurance and risk management services to high net worth individuals and mid-sized businesses.
	During 2013, the Consulting segment completed the following two acquisitions:
	•	July - Oliver Wyman acquired Corven, a U.K.-based management consultancy firm.
	•	August - Mercer acquired Global Remuneration Solutions, a market leading compensation consulting firm based in South Africa.
	Total purchase consideration for acquisitions made during 2013 was $178 million, which consisted of cash paid of $139 million and deferred purchase and estimated contingent consideration of $39 million. Contingent consideration arrangements are primarily based on EBITDA and revenue targets over two to four years. The fair value of the contingent consideration was based on projected revenue and earnings of the acquired entities. Estimated fair values of assets acquired and liabilities assumed are subject to adjustment when purchase accounting is finalized. During 2013, the Company also paid $15 million of deferred purchase consideration and $17 million of contingent consideration related to acquisitions made in prior years. In addition, the Company paid $2 million to purchase other intangible assets during 2013.
	Pro-Forma Information
	While the Company does not believe its acquisitions in the aggregate are material, the following unaudited pro-forma financial data gives effect to the acquisitions made by the Company during 2014 and 2013. In accordance with accounting guidance related to pro-forma disclosures, the information presented for current year acquisitions is as if they occurred on January 1, 2013 and reflects acquisitions made in 2013 as if they occurred on January 1, 2012. The pro-forma information adjusts for the effects of amortization of acquired intangibles. The unaudited pro-forma financial data is presented for illustrative purposes only and is not necessarily indicative of the operating results that would have been achieved if such acquisitions had occurred on the dates indicated, nor is it necessarily indicative of future consolidated results.
		Years Ended December 31,
	(In millions, except per share data)	2014		2013		2012
	Revenue	$	13,039	$	12,550	$	12,202
	Income from continuing operations	$	1,477	$	1,395	$	1,222
	Net income attributable to the Company	$	1,471	$	1,373	$	1,195
	Basic net income per share:
	– Continuing operations	$	2.65	$	2.49	$	2.2
	– Net income attributable to the Company	$	2.7	$	2.5	$	2.2
	Diluted net income per share:
	– Continuing operations	$	2.62	$	2.45	$	2.16
	– Net income attributable to the Company	$	2.66	$	2.46	$	2.16
	The consolidated statements of income for 2014 include approximately $134 million of revenue and $18 million of operating income related to acquisitions made during 2014.
	Alexander Forbes: In June 2014, Mercer entered into a definitive agreement to acquire a 34% interest in South Africa-based Alexander Forbes Group Holding Limited (“Alexander Forbes”) becoming a strategic shareholder after Alexander Forbes successfully launched an initial public offering. Mercer purchased its stake in Alexander Forbes in two tranches at 7.50 South African Rand per share. In July 2014, the Company purchased 14.9% of Alexander Forbes common shares for approximately $137 million. In October 2014, the Company paid approximately $166 million for the remaining 19.1% of Alexander Forbes common shares.
	The Company’s investment in Alexander Forbes is accounted for using the equity method of accounting and is included in other assets in the consolidated balance sheet. The Company records this investment and its share of Alexander Forbes’ net income or loss on a one quarter lag basis.
	Upon completion of the acquisition, the purchase price of the Alexander Forbes shares exceeded the Company's share of the equity in net assets by approximately $146 million. The majority of this basis difference resulted from the excess of the Company’s purchase price for the Alexander Forbes common stock acquired over the book value of Alexander Forbes’ net assets. Substantially all of this basis difference was allocated, based on our preliminary estimates of the fair values of Alexander Forbes’ assets and liabilities, to the value of investment contracts, customer contracts and relationships acquired and technology related intangible assets, related deferred tax liability and goodwill. The basis difference related to these intangible assets (excluding goodwill) is recorded as additional amortization expense over their estimated lives. The basis difference related to the goodwill will be recognized upon disposition of our investment. The Company is finalizing its purchase price allocation.
Alexander Forbes principally focuses on employee benefits and investment solutions for institutional clients, and financial wellbeing and retail financial solutions for individual clients. Services include retirement funds and investment consulting, actuarial and administration services, employee risk benefits and health-care consulting, multi-manager investments solutions, and personal lines and business insurance.

Discontinued_Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations
	As part of the disposal transactions for Putnam and Kroll, the Company provided certain indemnities, primarily related to pre-transaction tax uncertainties and legal contingencies. In accordance with applicable accounting guidance, liabilities were established related to these indemnities at the time of the sales and reflected as a reduction of the gain on disposal. Discontinued operations includes charges or credits resulting from the settlement or resolution of the indemnified matters, as well as adjustments to the liabilities related to such matters.
	On December 31, 2014, an agreement was reached between Putnam and the Massachusetts Department of Revenue ("DOR") regarding a tax dispute, which was covered under the indemnity agreement discussed above. The December 2014 agreement was subject to certain approvals, which included the State Attorney General and the Commissioner of the DOR. In January 2015, all necessary approvals were received, the agreement was executed and the tax was paid. Concurrently, Putnam and the Company executed a settlement agreement to resolve all remaining matters under the indemnity agreement. The Company recorded a gain, net of federal income taxes, of approximately $28 million in 2014 related to the settlement with Putnam.
	Discontinued operations in 2013 includes estimated costs covered under the indemnity related to the Kroll sale as well as tax indemnities related to the Putnam sale.
	Summarized Statements of Income data for discontinued operations is as follows:
	For the Years Ended December 31,
	(In millions of dollars)	2014		2013			2012
	Income (loss) from discontinued operations, net of tax	$	—	$	—		$	—
	Disposals of discontinued operations	42		(4		)	(2		)
	Income tax (credit) expense	16		(10		)	1
	Disposals of discontinued operations, net of tax	26		6			(3		)
	Discontinued operations, net of tax	$	26	$	6		$	(3	)
	Discontinued operations, net of tax per share
	– Basic	$	0.05	$	0.01		$	—
	– Diluted	$	0.04	$	0.01		$	—

Goodwill_And_Other_Intangibles

Goodwill And Other Intangibles	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill And Other Intangibles	Goodwill and Other Intangibles
	The Company is required to assess goodwill and any indefinite-lived intangible assets for impairment annually, or more frequently if circumstances indicate impairment may have occurred. The Company performs the annual impairment assessment for each of its reporting units during the third quarter of each year. In accordance with applicable accounting guidance, the Company assesses qualitative factors to determine whether it is necessary to perform the two-step goodwill impairment test. The Company considered numerous factors, which included that the fair value of each reporting unit exceeded its carrying value by a substantial margin in its most recent estimate of reporting unit fair values, whether significant acquisitions or dispositions occurred which might alter the fair value of its reporting units, macroeconomic conditions and their potential impact on reporting unit fair values, actual performance compared with budget and prior projections used in its estimation of reporting unit fair values, industry and market conditions, and the year-over-year change in the Company’s share price. The Company completed its qualitative assessment in the third quarter of 2014 and concluded that a two-step goodwill impairment test was not required in 2014 and that goodwill was not impaired.
	Other intangible assets that are not deemed to have an indefinite life are amortized over their estimated lives and reviewed for impairment upon the occurrence of certain triggering events in accordance with applicable accounting literature.
	Changes in the carrying amount of goodwill are as follows:
	(In millions of dollars)	2014			2013
	Balance as of January 1, as reported	$	6,893		$	6,792
	Goodwill acquired	472			113
	Other adjustments(a)	(124		)	(12		)
	Balance at December 31,	$	7,241		$	6,893
	(a)	Primarily due to the impact of foreign exchange in both years.
	The goodwill acquired of $472 million in 2014 (approximately $348 million of which is deductible for tax purposes) is comprised of $445 million related to the Risk and Insurance Services segment and $27 million related to the Consulting segment.
	Goodwill allocable to the Company’s reportable segments is as follows: Risk and Insurance Services, $5 billion and Consulting, $2.2 billion.
	Amortized intangible assets consist primarily of the cost of client lists and trade names acquired. The gross cost and accumulated amortization at December 31, 2014 and 2013 are as follows:
	(In millions of dollars)	2014								2013
		Gross			Accumulated			Net		Gross		Accumulated		Net
		Cost			Amortization			Carrying		Cost		Amortization		Carrying
								Amount						Amount
	Amortized intangibles	$	1,177		$	485		$	692	$	888	$	416	$	472
	The Company recorded an intangible asset impairment charge of $5 million in the third quarter of 2013 in the Risk & Insurance Services segment.
	Aggregate amortization expense was $86 million for the year ended December 31, 2014, and $72 million for the years ended December 31, 2013 and 2012, respectively. The estimated future aggregate amortization expense is as follows:
	For the Years Ending December 31,
	(In millions of dollars)
	2015	$	88
	2016	78
	2017	75
	2018	73
	2019	72
	Subsequent years	306
		$	692

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	For financial reporting purposes, income before income taxes includes the following components:
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013				2012
	Income before income taxes:
	U.S.	$	313			$	407			$	398
	Other	1,744				1,566				1,298
		$	2,057			$	1,973			$	1,696
	The expense (benefit) for income taxes is comprised of:
	Income taxes:
	Current–
	U.S. Federal	$	80			$	102			$	42
	Other national governments	369				264				336
	U.S. state and local	26				45				24
		475				411				402
	Deferred–
	U.S. Federal	27				12				(18		)
	Other national governments	62				149				89
	U.S. state and local	22				22				19
		111				183				90
	Total income taxes	$	586			$	594			$	492
	The significant components of deferred income tax assets and liabilities and their balance sheet classifications are as follows:
	December 31,
	(In millions of dollars)	2014				2013
	Deferred tax assets:
	Accrued expenses not currently deductible	$	572			$	570
	Differences related to non-U.S. operations (a)	119				140
	Accrued retirement benefits U.S.	638				297
	Net operating losses (b)	57				79
	Income currently recognized for tax	75				74
	Foreign tax credit carryforwards	109				157
	Other	84				90
		$	1,654			$	1,407
	Deferred tax liabilities:
	Differences related to non-U.S. operations	$131				$112
	Depreciation and amortization	307				273
	Accrued retirement & postretirement benefits - non-U.S. operations	41				89
	Other	5				5
		$	484			$	479
	(a)	Net of valuation allowances of $15 million in 2014 and $12 million in 2013.
	(b)	Net of valuation allowances of $82 million in 2014 and $70 million in 2013.
	December 31,
	(In millions of dollars)	2014				2013
	Balance sheet classifications:
	Current assets	$	521			$	482
	Other assets	$	876			$	626
	Current liabilities	$	28			$	18
	Other liabilities	$	199			$	162
	U.S. Federal income taxes are not provided on temporary differences with respect to investments in foreign subsidiaries that are essentially permanent in duration, which at December 31, 2014, the Company estimates, amounted to approximately $6.3 billion. The determination of the unrecognized deferred tax liability with respect to these investments is not practicable.
	A reconciliation from the U.S. Federal statutory income tax rate to the Company’s effective income tax rate is shown below.
	For the Years Ended December 31,	2014			2013			2012
	U.S. Federal statutory rate	35	%		35	%		35	%
	U.S. state and local income taxes—net of U.S. Federal income tax benefit	1.7			2.1			1.9
	Differences related to non-U.S. operations	(7.5	)		(6.0	)		(6.1	)
	Other	(0.7	)		(1.0	)		(1.8	)
	Effective tax rate	28.5	%		30.1	%		29	%
	The Company’s consolidated tax rate was 28.5%, 30.1% and 29.0% in 2014, 2013 and 2012, respectively. The tax rate in each year reflects foreign operations, which are generally taxed at rates lower than the U.S. statutory tax rate.
	Valuation allowances had net increases of $15 million, $10 million and $23 million in 2014, 2013 and 2012, respectively. During the respective years, adjustments of the beginning of the year balances of valuation allowances decreased income tax expense by $9 million and $3 million in 2014 and 2013, respectively, and increased income tax expense by $16 million in 2012. Approximately 55% of the Company’s net operating loss carryforwards expire from 2015 through 2034, and others are unlimited. The potential tax benefit from net operating loss carryforwards at the end of 2014 comprised federal, state and local, and non-U.S. tax benefits of $3 million, $40 million and $96 million, respectively, before reduction for valuation allowances. Foreign tax credit carryforwards expire from 2020 through 2022.
	The realization of deferred tax assets depends on generating future taxable income during the periods in which the tax benefits are deductible or creditable. Tax liabilities are determined and assessed jurisdictionally by legal entity or filing group. Certain taxing jurisdictions allow or require combined or consolidated tax filings. The Company assessed the realizability of its deferred tax assets and considered all available evidence, including the existence of a recent history of losses, placing particular weight on evidence that could be objectively verified. A valuation allowance was recorded to reduce deferred tax assets to the amount that the Company believes is more likely than not to be realized.
	Following is a reconciliation of the Company’s total gross unrecognized tax benefits for the years ended December 31, 2014, 2013 and 2012:
	(In millions of dollars)	2014				2013				2012
	Balance at January 1,	$	128			$	117			$	143
	Additions, based on tax positions related to current year	13				16				26
	Additions for tax positions of prior years	3				35				35
Reductions for tax positions of prior years	(29		)		(7		)		(41		)
Settlements	(4		)		(3		)		(6		)
Lapses in statutes of limitation	(14		)		(30		)		(40		)
Balance at December 31,	$	97			$	128			$	117
Of the total unrecognized tax benefits at December 31, 2014, 2013 and 2012, $51 million, $71 million and $96 million, respectively, represent the amount that, if recognized, would favorably affect the effective tax rate in any future periods. The total gross amount of accrued interest and penalties at December 31, 2014, 2013 and 2012, before any applicable federal benefit, was $7 million, $10 million and $13 million, respectively.
As discussed in Note 5, the Company has provided certain indemnities related to contingent tax liabilities as part of the disposals of Putnam and Kroll. At December 31, 2014, 2013 and 2012, $2 million, $2 million and $6 million, respectively, included in the table above, relates to Putnam and Kroll positions included in consolidated Company tax returns. Since the Company remains primarily liable to the taxing authorities for resolution of uncertain tax positions related to consolidated returns, these balances will remain as part of the Company’s consolidated liability for uncertain tax positions. Any future charges or credits related to these matters, including interest accrued, will be recorded in discontinued operations as incurred.
The Company is routinely examined by the jurisdictions in which it has significant operations. In the U.S. federal jurisdiction, the Company participates in the Internal Revenue Service’s (IRS) Compliance Assurance Process (CAP), which is structured to conduct real-time compliance reviews. The IRS is currently examining the Company’s 2013 tax return and performing a pre-filing review of 2014. During 2014, the Company settled its federal tax audit with the IRS for the year 2012, and in 2013 settled the years 2007, and 2009 through 2011. The tax year 2008 was settled in a prior period. New York State and New York City have examinations underway for various entities covering the years 2007 through 2012. Illinois is auditing the Company for years 2009 through 2013. During 2014, California commenced an audit covering the years 2009 through 2013. Outside the U.S., during 2014, examinations commenced in Canada for the year 2012 and in France for years 2011 and 2012. There is an ongoing examination of various subsidiaries for years 2011 and 2012 in the United Kingdom that started in 2013. The Company regularly considers the likelihood of assessments in each of the taxing jurisdictions resulting from examinations. The Company has established liabilities for uncertain tax positions in relation to the potential assessments. The Company believes the resolution of tax matters will not have a material effect on the consolidated financial position of the Company, although a resolution of tax matters could have a material impact on the Company's net income or cash flows and on its effective tax rate in a particular future period. It is reasonably possible that the total amount of unrecognized tax benefits will decrease between zero and approximately $16 million within the next twelve months due to the settlement of audits and the expiration of statutes of limitation.

Retirement_Benefits

Retirement Benefits	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Retirement Benefits	Retirement Benefits
	The Company maintains qualified and non-qualified defined benefit pension plans for its U.S. and non-U.S. eligible employees. The Company’s policy for funding its tax qualified defined benefit retirement plans is to contribute amounts at least sufficient to meet the funding requirements set forth by U.S. law and the laws of the non-U.S. jurisdictions in which the Company offers defined benefit plans.
	Combined U.S. and non-U.S. Plans
	The weighted average actuarial assumptions utilized for the U.S. and significant non-U.S. defined benefit plans and postretirement benefit plans are as follows:
		Pension						Postretirement
		Benefits						Benefits
		2014			2013			2014			2013
	Weighted average assumptions:
	Discount rate (for expense)	4.82	%		4.38	%		4.92	%		4.32	%
	Expected return on plan assets	7.52	%		7.68	%		—			—
	Rate of compensation increase (for expense)	2.64	%		2.43	%		—			—
	Discount rate (for benefit obligation)	3.79	%		4.82	%		4.08	%		5.03	%
	Rate of compensation increase (for benefit obligation)	2.42	%		2.64	%		—			—
	The Company uses actuaries from Mercer, a subsidiary of the Company, to perform valuations of its pension plans. The long-term rate of return on plan assets assumption is determined for each plan based on the facts and circumstances that exist as of the measurement date, and the specific portfolio mix of each plan’s assets. The Company utilizes a model developed by the Mercer actuaries to assist in the determination of this assumption. The model takes into account several factors, including: actual and target portfolio allocation; investment, administrative and trading expenses incurred directly by the plan trust; historical portfolio performance; relevant forward-looking economic analysis; and expected returns, variances and correlations for different asset classes. These measures are used to determine probabilities using standard statistical techniques to calculate a range of expected returns on the portfolio. The Company generally does not adjust the rate of return assumption from year to year if, at the measurement date, it is within the range between the 25th and 75th percentile of the expected long-term annual returns. Historical long-term average asset returns of each plan are also reviewed to determine whether they are consistent and reasonable compared with the rate selected. The expected return on plan assets is determined by applying the assumed long-term rate of return to the market-related value of plan assets. This market-related value recognizes investment gains or losses over a five-year period from the year in which they occur. Investment gains or losses for this purpose are the difference between the expected return calculated using the market-related value of assets and the actual return based on the market value of assets. Since the market-related value of assets recognizes gains or losses over a five-year period, the future market-related value of the assets will be impacted as previously deferred gains or losses are reflected.
	The target asset allocation for the U.S. Plan is 62% equities and equity alternatives and 38% fixed income. At the end of 2014, the actual allocation for the U.S. Plan was 59% equities and equity alternatives and 41% fixed income. The target asset allocation for the U.K. Plans, which comprise approximately 83% of non-U.S. Plan assets, is 50% equities and equity alternatives and 50% fixed income. At the end of 2014, the actual allocation for the U.K. Plans was 43% equities and equity alternatives and 57% fixed income. The assets of the Company's defined benefit plans are diversified and are managed in accordance with applicable laws and with the goal of maximizing the plans' real return within acceptable risk parameters. The Company uses threshold-based portfolio re-balancing to ensure the actual portfolio remains consistent with target asset allocation ranges.
	The discount rate selected for each U.S. plan is based on a model bond portfolio with coupons and redemptions that closely match the expected liability cash flows from the plan. Discount rates for non-U.S. plans are based on appropriate bond indices such as the Markit iBoxx £ Corporates AA 15+ index in the U.K. Projected compensation increases reflect current expectations as to future levels of inflation.
	The components of the net periodic benefit cost for defined benefit and other postretirement plans are as follows:
	Combined U.S. and significant non-U.S. Plans	Pension											Postretirement
	For the Years Ended December 31,	Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Service cost	$	213			$	252			$	240		$	4		$	5		$	5
	Interest cost	641				581				596			11			11			13
	Expected return on plan assets	(990		)		(911		)		(905		)	—			—			—
	Amortization of prior service credit	(16		)		(22		)		(19		)	—			—			(14		)
	Recognized actuarial loss (credit)	243				315				270			(1		)	2			—
	Net periodic benefit cost	$	91			$	215			$	182		$	14		$	18		$	4
	Curtailment gain	(65		)		—				—			—			—			—
	Total cost	$	26			$	215			$	182		$	14		$	18		$	4
	Plan Assets
	For the U.S. Plan, investment allocation decisions are made by a fiduciary committee composed of senior executives appointed by the Company’s Chief Executive Officer. For the non-U.S. plans, investment allocation decisions are made by local fiduciaries, in consultation with the Company for the larger plans. Plan assets are invested in a manner consistent with the fiduciary standards set forth in all relevant laws relating to pensions and trusts in each country. Primary investment objectives are (1) to achieve an investment return that, in combination with current and future contributions, will provide sufficient funds to pay benefits as they become due, and (2) to minimize the risk of large losses. The investment allocations are designed to meet these objectives by broadly diversifying plan assets among numerous asset classes with differing expected returns, volatilities, and correlations.
	The major categories of plan assets include equity securities, equity alternative investments, and fixed income securities. For the U.S. qualified plan, the category ranges are 57-67% for equities and equity alternatives, and 33-43% for fixed income. For the U.K. Plan, the category ranges are 47-53% for equities and equity alternatives, and 47-53% for fixed income. Asset allocation is monitored frequently and re-balancing actions are taken as appropriate. Re-balancing in the U.K. Plan was suspended in 2014 while a contingent guarantee agreement was put in place and the investment strategy of the plan was finalized. After the contingent guarantee agreement was executed in January 2015, re-balancing resumed in February 2015 with target asset allocation of 48% equities and equity alternatives and 52% fixed income.
	Plan investments are exposed to stock market, interest rate, and credit risk. Concentrations of these risks are generally limited due to diversification by investment style within each asset class, diversification by investment manager, diversification by industry sectors and issuers, and the dispersion of investments across many geographic areas.
	Unrecognized Actuarial Gains/Losses
	In accordance with applicable accounting guidance, the funded status of the Company's pension plans is recorded in the consolidated balance sheets and provides for a delayed recognition of actuarial gains or losses arising from changes in the projected benefit obligation due to changes in the assumed discount rates, differences between the actual and expected value of plan assets and other assumption changes. The unrecognized pension plan actuarial gains or losses and prior service costs not yet recognized in net periodic pension cost are recognized in Accumulated Other Comprehensive Income, net of tax. These gains and losses are amortized prospectively out of AOCI over a period that approximates the average remaining service period of active employees, or for plans in which substantially all the participants are inactive, over the remaining life expectancy of the inactive employees.
	U.S. Plans
	The following schedules provide information concerning the Company’s U.S. defined benefit pension plans and postretirement benefit plans:
		U.S. Pension								U.S.  Postretirement
		Benefits								Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Change in benefit obligation:
	Benefit obligation at beginning of year	$	4,827			$	5,197			$	158		$	176
	Service cost	91				104				2			3
	Interest cost	253				229				7			7
	Employee contributions	—				—				13			—
	Plan amendments	—				—				(4		)	—
	Plan combination	—				36				—			—
	Actuarial loss (gain)	955				(547		)		21			(15		)
	Medicare Part D subsidy	—				—				1			1
	Benefits paid	(202		)		(192		)		(21		)	(14		)
	Benefit obligation, December 31	$	5,924			$	4,827			$	177		$	158
	Change in plan assets:
	Fair value of plan assets at beginning of year	$	4,279			$	3,936			$	—		$	—
	Plan combination	—				21				—			—
	Actual return on plan assets	414				488				—			—
	Employer contributions	25				26				13			13
	Employee contributions	—				—				13			—
	Medicare Part D subsidy	—				—				1			1
	Benefits paid	(202		)		(192		)		(21		)	(14		)
	Other	—				—				12			—
	Fair value of plan assets, December 31	$	4,516			$	4,279			$	18		$	—
	Net funded status, December 31	$	(1,408	)		$	(548	)		$	(159	)	$	(158	)
	Amounts recognized in the consolidated balance sheets:
	Current liabilities	$	(25	)		$	(24	)		$	(2	)	$	(8	)
	Noncurrent liabilities	(1,383		)		(524		)		(157		)	(150		)
Net liability recognized, December 31	$	(1,408	)		$	(548	)		$	(159	)	$	(158	)
Amounts recognized in other comprehensive income (loss):
Prior service credit	$	—			$	7			$	4		$	—
Net actuarial (loss) gain	(1,749		)		(974		)		2			13
Total recognized accumulated other comprehensive (loss) income, December 31	$	(1,749	)		$	(967	)		$	6		$	13
Cumulative employer contributions in excess (deficient) of net periodic cost	341				419				(165		)	(171		)
Net amount recognized in consolidated balance sheet	$	(1,408	)		$	(548	)		$	(159	)	$	(158	)
Accumulated benefit obligation at December 31	$	5,825			$	4,753			$	—		$	—
	U.S. Pension								U.S.  Postretirement
	Benefits								Benefits
(In millions of dollars)	2014				2013				2014			2013
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	7			$	23			$	—		$	—
Recognized as component of net periodic benefit cost	(7		)		(16		)		—			—
Plan amendment	—				—				4			—
Prior service credit, December 31	$	—			$	7			$	4		$	—
	U.S. Pension								U.S.  Postretirement
	Benefits								Benefits
(In millions of dollars)	2014				2013				2014			2013
Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(974	)		$	(1,887	)		$	13		$	(2	)
Recognized as component of net periodic benefit cost (credit)	112				208				(2		)	—
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Liability experience	(955		)		541				(21		)	15
Asset experience	68				164				12			—
Total (loss) gain recognized as change in plan assets and benefit obligations	(887		)		705				(9		)	15
Net actuarial (loss) gain, December 31	$	(1,749	)		$	(974	)		$	2		$	13
For the Years Ended December 31,	U.S. Pension											U.S. Postretirement
	Benefits											Benefits
(In millions of dollars)	2014				2013				2012			2014			2013			2012
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	885			$	(696	)		$	346		$	14		$	(5	)	$	24
Estimated amounts that will be amortized from accumulated other comprehensive loss in the next fiscal year:
	U.S. Pension				U.S. Postretirement
	Benefits				Benefits
(In millions of dollars)	2015				2015
Prior service credit	$	—			$	1
Net actuarial loss	181				1
Projected cost	$	181			$	2
The weighted average actuarial assumptions utilized in determining the above amounts for the U.S. defined benefit and other U.S. postretirement plans as of the end of the year are as follows:
	U.S. Pension						U.S. Postretirement Benefits
	Benefits
	2014			2013			2014			2013
Weighted average assumptions:
Discount rate (for expense)	5.3	%		4.45	%		4.99	%		4.25	%
Expected return on plan assets	8.75	%		8.75	%		—			—
Rate of compensation increase (for expense)	2	%		2	%		—			—
Discount rate (for benefit obligation)	4.3	%		5.3	%		4.19	%		5.17	%
Rate of compensation increase (for benefit obligation)	2	%		2	%		—			—
In 2014, the Society of Actuaries in the United States issued a new mortality table (RP-2014) and an updated improvement scale. The Company considered the effect of RP-2014, along with other available information on mortality improvement and industry specific mortality studies, to select its assumptions for measurement of the plans’ benefit obligations at December 31, 2014.
The projected benefit obligation, accumulated benefit obligation and aggregate fair value of plan assets for U.S. pension plans with accumulated benefit obligations in excess of plan assets were $5.9 billion, $5.8 billion and $4.5 billion, respectively, as of December 31, 2014 and $4.8 billion, $4.8 billion and $4.3 billion, respectively, as of December 31, 2013.
The projected benefit obligation and fair value of plan assets for U.S. pension plans with projected benefit obligations in excess of plan assets was $5.9 billion and $4.5 billion, respectively, as of December 31, 2014 and $4.8 billion and $4.3 billion, respectively, as of December 31, 2013.
As of December 31, 2014, the U.S. qualified plan holds 4 million shares of the Company’s common stock which were contributed to the Plan by the Company in 2005. This represented approximately 5.1% of that plan’s assets as of December 31, 2014. In addition, plan assets may be invested in funds managed by Mercer Investments, a subsidiary of the Company.
The components of the net periodic benefit cost for the U.S. defined benefit and other postretirement benefit plans are as follows:
U.S. Plans only	Pension											Postretirement
For the Years Ended December 31,	Benefits											Benefits
(In millions of dollars)	2014				2013				2012			2014			2013			2012
Service cost	$	91			$	104			$	93		$	2		$	3		$	3
Interest cost	253				229				230			7			7			8
Expected return on plan assets	(346		)		(324		)		(322		)	—			—			—
Amortization of prior service credit	(7		)		(16		)		(16		)	—			—			(13		)
Recognized actuarial loss (credit)	112				207				152			(2		)	—			(1		)
Net periodic benefit cost (credit)	$	103			$	200			$	137		$	7		$	10		$	(3	)
In December 2003, the Medicare Prescription Drug, Improvement and Modernization Act of 2003 became law. The net periodic benefit cost for all periods shown above includes the subsidy.
The assumed health care cost trend rate for Medicare eligibles and non-Medicare eligibles is approximately 7.55% in 2014, gradually declining to 4.5% in 2028. Assumed health care cost trend rates have a small effect on the amounts reported for the U.S. health care plans because the Company caps its share of health care trend at 5%. A one percentage point change in assumed health care cost trend rates would have no effect on the total service and interest cost components or the postretirement benefit obligation.
Estimated Future Contributions
The Company expects to fund approximately $25 million for its U.S. non-qualified plans in 2015. The Company’s policy for funding its tax-qualified defined benefit retirement plans is to contribute amounts at least sufficient to meet the funding requirements set forth in the U.S. and applicable foreign law. There is currently no ERISA funding requirement for the U.S. qualified plan for 2015.
Non-U.S. Plans
The following schedules provide information concerning the Company’s non-U.S. defined benefit pension plans and non-U.S. postretirement benefit plans:
	Non-U.S. Pension								Non-U.S.
	Benefits								Postretirement Benefits
(In millions of dollars)	2014				2013				2014			2013
Change in benefit obligation:
Benefit obligation at beginning of year	$	8,711			$	8,579			$	97		$	107
Service cost	122				148				2			2
Interest cost	388				352				4			4
Employee contributions	10				11				—			—
Actuarial loss (gain)	1,619				(53		)		(1		)	(8		)
Plan amendments	13				—				—			—
Effect of settlement	(11		)		(2		)		—			—
Benefits paid	(311		)		(293		)		(3		)	(4		)
Foreign currency changes	(585		)		(31		)		(6		)	(4		)
Other	62				—				—			—
Benefit obligation December 31	$	10,018			$	8,711			$	93		$	97
Change in plan assets:
Fair value of plan assets at beginning of year	$	9,351			$	8,312			$	—		$	—
Actual return on plan assets	1,756				698				—			—
Effect of settlement	(11		)		(2		)		—			—
Company contributions	156				620				3			4
Employee contributions	10				11				—			—
Benefits paid	(311		)		(293		)		(3		)	(4		)
Foreign currency changes	(578		)		5				—			—
Other	37				—				—			—
Fair value of plan assets, December 31	$	10,410			$	9,351			$	—		$	—
Net funded status, December 31	$	392			$	640			$	(93	)	$	(97	)
Amounts recognized in the consolidated balance sheets:
Non-current assets	$	967			$	977			$	—		$	—
Current liabilities	(6		)		(5		)		(4		)	(4		)
Non-current liabilities	(569		)		(332		)		(89		)	(93		)
Net asset (liability) recognized, December 31	$	392			$	640			$	(93	)	$	(97	)
Amounts recognized in other comprehensive (loss) income:
Prior service (cost) credit	$	(2	)		$	85			$	—		$	—
Net actuarial loss	(3,215		)		(3,010		)		(14		)	(16		)
Total recognized accumulated other comprehensive (loss) income, December 31	$	(3,217	)		$	(2,925	)		$	(14	)	$	(16	)
Cumulative employer contributions in excess (deficient) of net periodic cost	3,609				3,565				(79		)	(81		)
Net asset (liability) recognized in consolidated balance sheet, December 31	$	392			$	640			$	(93	)	$	(97	)
Accumulated benefit obligation, December 31	$	9,731			$	8,413			$	—		$	—
	Non-U.S. Pension								Non-U.S.
	Benefits								Postretirement Benefits
(In millions of dollars)	2014				2013				2014			2013
Reconciliation of prior service credit (cost):
Beginning balance	$	85			$	93			$	—		$	—
Recognized as component of net periodic benefit credit	(9		)		(6		)		—			—
Effect of curtailment	(65		)		—				—			—
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	(13		)		—				—			—
Exchange rate adjustments	—				(2		)		—			—
Prior service (cost) credit, December 31	$	(2	)		$	85			$	—		$	—
	Non-U.S. Pension								Non-U.S.
	Benefits								Postretirement Benefits
(In millions of dollars)	2014				2013				2014			2013
Reconciliation of net actuarial (loss) gain:
Beginning balance	$	(3,010	)		$	(3,309	)		$	(16	)	$	(27	)
Recognized as component of net periodic benefit cost	131				108				1			2
Effect of settlement	—				—				—			—
Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
Liability experience	(1,619		)		53				1			8
Asset experience	1,112				111				—			—
Other	(14		)		—				—			—
Total amount recognized as change in plan assets and benefit obligations	(521		)		164				1			8
Exchange rate adjustments	185				27				—			1
Net actuarial loss, December 31	$	(3,215	)		$	(3,010	)		$	(14	)	$	(16	)
For the Years Ended December 31,	Non-U.S. Pension											Non-U.S. Postretirement
	Benefits											Benefits
(In millions of dollars)	2014				2013				2012			2014			2013			2012
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	201			$	(276	)		$	246		$	5		$	(2	)	$	16
Estimated amounts that will be amortized from accumulated other comprehensive income in the next fiscal year:
	Non-U.S. Pension				Non-U.S.
	Benefits				Postretirement Benefits
(In millions of dollars)	2015				2015
Prior service credit	$	(2	)		$	—
Net actuarial loss	140				1
Projected cost	$	138			$	1
The weighted average actuarial assumptions utilized for the non-U.S. defined and postretirement benefit plans as of the end of the year are as follows:
	Non-U.S. Pension						Non-U.S.
	Benefits						Postretirement Benefits
	2014			2013			2014			2013
Weighted average assumptions:
Discount rate (for expense)	4.55	%		4.33	%		4.8	%		4.45	%
Expected return on plan assets	6.95	%		7.17	%		—			—
Rate of compensation increase (for expense)	2.99	%		2.69	%		—			—
Discount rate (for benefit obligation)	3.49	%		4.55	%		3.85	%		4.8	%
Rate of compensation increase (for benefit obligation)	2.67	%		2.99	%		—			—
The non-U.S. defined benefit plans do not have any direct ownership of the Company’s common stock.
The pension plan in the United Kingdom holds a limited partnership interest in the Trident III private equity fund valued at approximately $53 million at December 31, 2014.
The projected benefit obligation, accumulated benefit obligation and fair value of plan assets for the non-U.S. pension plans with accumulated benefit obligations in excess of plan assets were $2.1 billion, $2.0 billion and $1.6 billion, respectively, as of December 31, 2014 and $1.7 billion, $1.5 billion and $1.3 billion, respectively, as of December 31, 2013.
The projected benefit obligation and fair value of plan assets for non-U.S. pension plans with projected benefit obligations in excess of plan assets was $2.2 billion and $1.6 billion, respectively, as of December 31, 2014 and $1.7 billion and $1.3 billion, respectively, as of December 31, 2013.
U.K. Plan Amendment
After completion of a consultation period with affected colleagues, in January 2014, the Company amended its U.K. defined benefit pension plans to close those plans to future benefit accruals effective August 1, 2014 and replaced those plans, along with its existing defined contribution plans, with a new, comprehensive defined contribution arrangement. This change resulted in a curtailment of the U.K. defined benefit plans and, as required under GAAP, the Company re-measured the defined benefit plans’ assets and liabilities at the date the employee consultations concluded and the local operating companies approved the plan amendment, based on assumptions and market conditions at that date. As a result of the re-measurement, the projected benefit obligation ("PBO") increased by approximately $147 million and the funded status decreased by approximately $137 million. The change in the PBO and in the funded status relates primarily to a decrease in the discount rate at the re-measurement date. The net periodic benefit costs recognized in 2014 are the weighted average resulting from the December 31, 2013 measurement and the January 2014 re-measurement.
Components of Net Periodic Benefits Costs
The components of the net periodic benefit cost for the non-U.S. defined benefit and other postretirement benefit plans and the curtailment, settlement and termination expenses are as follows:
For the Years Ended December 31,	Non-U.S. Pension											Non-U.S. Postretirement
	Benefits											Benefits
(In millions of dollars)	2014				2013				2012			2014			2013			2012
Service cost	$	122			$	148			$	147		$	2		$	2		$	2
Interest cost	388				352				366			4			4			5
Expected return on plan assets	(644		)		(587		)		(583		)	—			—			—
Amortization of prior service cost	(9		)		(6		)		(3		)	—			—			(1		)
Recognized actuarial loss	131				108				118			1			2			1
Net periodic benefit (credit) cost	(12		)		15				45			7			8			7
Settlement loss	—				—				1			—			—			—
Curtailment gain	(65		)		—				(1		)	—			—			—
Total (credit) cost	$	(77	)		$	15			$	45		$	7		$	8		$	7
The assumed health care cost trend rate was approximately 5.76% in 2014, gradually declining to 4.92% in 2022. Assumed health care cost trend rates can have a significant effect on the amounts reported for the non-U.S. health care plans. A one percentage point change in assumed health care cost trend rates would have the following effects:
(In millions of dollars)	1 Percentage				1 Percentage
	Point Increase				Point Decrease
Effect on total of service and interest cost components	$	1			$	(1	)
Effect on postretirement benefit obligation	$	9			$	(7	)
Estimated Future Contributions
The Company expects to fund approximately $169 million to its non-U.S. pension plans in 2015. Funding requirements for non-U.S. plans vary by country. Contribution rates are generally based on local funding practices and requirements, which may differ significantly from measurements under U.S. GAAP. Funding amounts may be influenced by future asset performance, the level of discount rates and other variables impacting the assets and/or liabilities of the plan. Discretionary contributions may also be affected by alternative uses of the Company’s cash flows, including dividends, investments and share repurchases.
In the U.K., contributions to defined benefit pension plans are determined through a negotiation process between the Company and the plans' Trustee that typically occurs every three years in conjunction with the actuarial valuation of the plans. This process is governed by U.K. pension regulations. The assumptions that result from the funding negotiations are different from those used for U.S. GAAP and currently result in a lower funded status than under U.S. GAAP. In March 2014, the Company and the Trustee of the U.K. Defined Benefits Plans agreed to a funding deficit recovery plan for the U.K. defined benefit pension plans. The current agreement with the Trustee sets out the annual deficit contributions which would be due based on the deficit at December 31, 2012. The funding level is subject to re-assessment, in most cases on November 1st of each year. If the funding level on November 1st has sufficiently improved, no deficit funding contributions will be required in the following year, and the contribution amount will be deferred. As part of a long-term strategy, which depends on having greater influence over asset allocation and overall investment decisions, the Company has agreed to support annual deficit contributions by the U.K. operating companies under certain circumstances, up to GBP 450 million over a seven-year period.
Estimated Future Benefit Payments
The Plans' estimated future benefit payments for its pension and postretirement benefits (without reduction for Medicare subsidy receipts) are as follows:
For the Years Ended December 31,	Pension								Postretirement
	Benefits								Benefits
(In millions of dollars)	U.S.				Non-U.S.				U.S.			Non-U.S.
2015	$	217			$	264			$	10		$	4
2016	$	232			$	280			$	10		$	4
2017	$	250			$	295			$	10		$	4
2018	$	263			$	312			$	10		$	4
2019	$	275			$	325			$	10		$	4
2020-2024	$	1,581			$	1,890			$	55		$	24
Defined Benefit Plans Fair Value Disclosures
In December 2008, the FASB issued guidance for Employers’ Disclosures About Pension and Other Post Retirement Benefit Plan Assets. The guidance requires fair value plan asset disclosures for an employer’s defined benefit pension and postretirement plans similar to the guidance on Fair Value Measurements as well as (a) how investment allocation decisions are made, (b) the major categories of plan assets, and (c) significant concentrations of risk within plan assets.
The U.S. and non-U.S. plan investments are classified into Level 1, which refers to investments valued using quoted prices from active markets for identical assets; Level 2, which refers to investments not traded on an active market but for which observable market inputs are readily available; and Level 3, which refers to investments valued based on significant unobservable inputs. Assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.
The following table sets forth, by level within the fair value hierarchy, a summary of the U.S. and non-U.S. plans' investments measured at fair value on a recurring basis at December 31, 2014 and 2013:
	Fair Value Measurements at December 31, 2014
Assets (In millions of dollars)	Quoted Prices in				Significant				Significant			Total
	Active Markets				Other				Unobservable
	for Identical				Observable				Inputs
	Assets				Inputs				(Level 3)
	(Level 1)				(Level 2)
Common/Collective trusts	$	172			$	6,766			$	184		$	7,122
Corporate obligations	—				2,938				3			2,941
Corporate stocks	2,087				6				1			2,094
Private equity/partnerships	—				—				727			727
Government securities	—				371				—			371
Real estate	—				6				375			381
Short-term investment funds	724				12				—			736
Company common stock	229				—				—			229
Other investments	16				23				239			278
Total investments	$	3,228			$	10,122			$	1,529		$	14,879
	Fair Value Measurements at December 31, 2013
Assets (In millions of dollars)	Quoted Prices in				Significant				Significant			Total
	Active Markets				Other				Unobservable
	for Identical				Observable				Inputs
	Assets				Inputs				(Level 3)
	(Level 1)				(Level 2)
Common/Collective trusts	$	138			$	5,649			$	151		$	5,938
Corporate obligations	—				2,330				4			2,334
Corporate stocks	2,434				5				1			2,440
Private equity/partnerships	—				2				799			801
Government securities	10				340				2			352
Real estate	—				7				312			319
Short-term investment funds	824				15				—			839
Company common stock	261				—				—			261
Other investments	35				5				238			278
Total investments	$	3,702			$	8,353			$	1,507		$	13,562
In 2014, certain non U.S. government securities that were previously categorized as Level 1 were transferred to Level 2.
The tables below set forth a summary of changes in the fair value of the plans’ Level 3 assets for the years ended December 31, 2014 and December 31, 2013:
Assets (In millions)	Fair Value,				Purchases				Sales			Unrealized			Realized			Exchange			Transfers			Fair
	1-Jan-14											Gain/			Gain/			Rate			in/(out)			Value, December 31, 2014
												(Loss)			(Loss)			Impact			and
																					Other
Private equity/Partnerships	$	799			$	158			$	(185	)	$	(173	)	$	137		$	(12	)	$	3		$	727
Real estate	312				97				(50		)	19			16			(19		)	—			375
Other investments	238				21				(16		)	18			—			(28		)	6			239
Common/Collective trusts	151				—				(1		)	50			—			(16		)	—			184
Corporate stocks	1				—				—			—			—			—			—			1
Corporate obligations	4				3				(1		)	—			—			—			(3		)	3
Government securities	2				—				—			—			—			—			(2		)	—
Total assets	$	1,507			$	279			$	(253	)	$	(86	)	$	153		$	(75	)	$	4		$	1,529
Assets (In millions)	Fair Value,				Purchases				Sales			Unrealized			Realized			Exchange			Transfers			Fair
	1-Jan-13											Gain/			Gain/			Rate			in/(out)			Value,
												(Loss)			(Loss)			Impact			and			31-Dec-13
																					Other
Private equity/Partnerships	$	824			$	146			$	(174	)	$	(155	)	$	150		$	(1	)	$	9		$	799
Real estate	357				21				(95		)	6			26			(3		)	—			312
Other investments	239				18				(13		)	10			—			6			(22		)	238
Common/Collective trusts	—				61				—			(4		)	—			(5		)	99			151
Corporate stocks	9				—				—			—			—			—			(8		)	1
Corporate obligations	1				1				—			—			—			—			2			4
Government securities	—				—				—			(1		)	—			—			3			2
Total assets	$	1,430			$	247			$	(282	)	$	(144	)	$	176		$	(3	)	$	83		$	1,507
The following is a description of the valuation methodologies used for assets measured at fair value:
Company common stock:  Valued at the closing price reported on the New York Stock Exchange.
Common stocks, preferred stocks, convertible equity securities and rights/warrants (included in Corporate stocks):  Valued at the closing price reported on the primary exchange.
Corporate bonds (included in Corporate obligations):  The fair value of corporate bonds is estimated using recently executed transactions, market price quotations (where observable) and bond spreads. The spread data used are for the same maturity as the bond. If the spread data does not reference the issuer, then data that references a comparable issuer are used. When observable price quotations are not available, fair value is determined based on cash flow models.
Commercial paper (included in Corporate obligations):  The fair value of commercial paper is estimated using observable market data such as maturity date, issue date, credit rating, current commercial paper rates and settlement date.
Commercial mortgage-backed and asset-backed securities (included in Corporate obligations):  Fair value is determined using discounted cash flow models. Observable inputs are based on trade and quote activity of bonds with similar features including issuer vintage, purpose of underlying loan (first or second lien), prepayment speeds and credit ratings. The discount rate is the combination of the appropriate rate from the benchmark yield curve and the discount margin based on quoted prices.
Common/Collective trusts:  Valued at the quoted market prices of the investments at year end.
U.S. government bonds (included in Government securities):  The fair value of U.S. government bonds is estimated by pricing models that utilize observable market data including quotes, spreads and data points for yield curves.
U.S. agency securities (included in Government securities):  U.S. agency securities are comprised of two main categories consisting of agency issued debt and mortgage pass-throughs. Agency issued debt securities are valued by benchmarking market-derived prices to quoted market prices and trade data for identical or comparable securities. Mortgage pass-throughs include certain “To-be-announced” (TBA) securities and mortgage pass-through pools. TBA securities are generally valued using quoted market prices or are benchmarked thereto. Fair value of mortgage pass-through pools are model driven with respect to spreads of the comparable TBA security.
Private equity and real estate partnerships:  Investments in private equity and real estate partnerships are valued based on the fair value reported by the manager of the corresponding partnership. The managers provide unaudited quarterly financial statements and audited annual financial statements which set forth the value of the fund. The valuations obtained from the managers are based on various analyses on the underlying holdings in each partnership, including financial valuation models and projections, comparable valuations from the public markets, and precedent private market transactions. Investments are valued in the accompanying financial statements based on the Plan’s beneficial interest in the underlying net assets of the partnership as determined by the partnership agreement.
Insurance group annuity contracts:  The fair values for these investments are based on the current market value of the aggregate accumulated contributions plus interest earned.
Swap assets and liabilities:  Fair values for interest rate swaps, equity index swaps and inflation swaps are estimated using a discounted cash flow pricing model. These models use observable market data such as contractual fixed rate, broker quotes, spot equity price or index value and dividend data. The fair values of credit default swaps are estimated using an income approach model which determines expected cash flows based on default probabilities from the issuer-specific credit spread curve and credit loss recovery rates, both of which are dependent on market quotes.
Real estate investment trusts:  Valued at the closing price reported on an exchange.
Short-term investment funds:  Primarily high-grade money market instruments valued at net asset value at year-end.
Real estate: Valued by investment managers generally using proprietary pricing models.
Registered investment companies:  Valued at the closing price reported on the primary exchange.
Defined Contribution Plans
The Company maintains certain defined contribution plans for its employees, including the Marsh & McLennan Companies 401(k) Savings & Investment Plan (“401(k) Plan”), that are qualified under U.S. tax laws. Under these plans, eligible employees may contribute a percentage of their base salary, subject to certain limitations. For the 401(k) Plan, the Company matches a fixed portion of the employees’ contributions. The 401(k) Plan contains an Employee Stock Ownership Plan feature under U.S. tax law. Approximately $453 million of the 401(k) Plan’s assets at both December 31, 2014 and December 31, 2013 were invested in the Company’s common stock. If a participant does not choose an investment direction for his or her future contributions, they are automatically invested in a BlackRock LifePath Portfolio that most closely matches the participant’s expected retirement year. The cost of these defined contribution plans was $49 million in 2014, $50 million in 2013 and $50 million in 2012. In addition, the Company has a significant defined contribution plan in the U.K. As noted above, effective August 1, 2014, a newly formed defined contribution plan replaced the existing defined contribution and defined benefit plans with regard to future service. The cost of the U.K. defined contribution plan was $65 million, $23 million and $21 million in 2014, 2013 and 2012, respectively.

Stock_Benefit_Plans

Stock Benefit Plans	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Benefit Plans	Stock Benefit Plans
	The Company maintains multiple stock-based payment arrangements under which employees are awarded grants of restricted stock units, stock options and other forms of stock-based payment arrangements.
	Marsh & McLennan Companies, Inc. Incentive and Stock Award Plans
	On May 19, 2011, the Marsh & McLennan Companies, Inc. 2011 Incentive and Stock Award Plan (the “2011 Plan”) was approved by the Company's stockholders. The 2011 Plan replaced the Company's two previous equity incentive plans (the 2000 Senior Executive Incentive and Stock Award Plan and the 2000 Employee Incentive and Stock Award Plan).
	The types of awards permitted under the 2011 Plan include stock options, restricted stock and restricted stock units payable in Company common stock or cash, and other stock-based and performance-based awards. The Compensation Committee of the Board of Directors (the “Compensation Committee”) determines, at its discretion, which affiliates may participate in the 2011 Plan, which eligible employees will receive awards, the types of awards to be received, and the terms and conditions thereof. The right of an employee to receive an award may be subject to performance conditions as specified by the Compensation Committee. The 2011 Plan contains provisions which, in the event of a change in control of the Company, may accelerate the vesting of the awards. The 2011 Plan retains the remaining share authority of the two previous plans as of the date the 2011 Plan was approved by stockholders. Awards relating to not more than approximately 23.2 million shares of common stock, plus shares remaining unused under certain pre-existing plans, may be made over the life of the 2011 Plan.
	Our current practice is to grant non-qualified stock options, restricted stock units and/or performance stock units on an annual basis to senior executives and a limited number of other employees as part of their total compensation. We also grant restricted stock units during the year to new hires or as retention awards for certain employees. We have not granted restricted stock since 2005.
	Stock Options: Options granted under the 2011 Plan may be designated as either incentive stock options or non-qualified stock options. The Compensation Committee determines the terms and conditions of the option, including the time or times at which an option may be exercised, the methods by which such exercise price may be paid, and the form of such payment. Options are generally granted with an exercise price equal to the market value of the Company's common stock on the date of grant. These option awards generally vest 25% per annum and have a contractual term of 10 years.
	The estimated fair value of options granted is calculated using the Black-Scholes option pricing valuation model. This model takes into account several factors and assumptions. The risk-free interest rate is based on the yield on U.S. Treasury zero-coupon issues with a remaining term equal to the expected life assumption at the time of grant. The expected life (estimated period of time outstanding) is estimated using the contractual term of the option and the effects of employees' expected exercise and post-vesting employment termination behavior. The Company uses a blended volatility rate based on the following: (i) volatility derived from daily closing price observations for the 10-year period ended on the valuation date, (ii) implied volatility derived from traded options for the period one week before the valuation date and (iii) average volatility for the 10-year periods ended on 15 anniversaries prior to the valuation date, using daily closing price observations. The expected dividend yield is based on expected dividends for the expected term of the stock options.
	The assumptions used in the Black-Scholes option pricing valuation model for options granted by the Company in 2014, 2013 and 2012 are as follows:
		2014		2013		2012
	Risk-free interest rate	1.88%		1.03%-1.30%		1.26%-1.27%
	Expected life (in years)	6		6		6.5
	Expected volatility	24.20%		23.6%-24.1%		26.2%-26.4%
	Expected dividend yield	2.08%		2.48%-2.54%		2.76%-2.80%
	A summary of the status of the Company’s stock option awards as of December 31, 2014 and changes during the year then ended is presented below:
		Shares			Weighted			Weighted		Aggregate
					Average Exercise			Average		Intrinsic Value
					Price			Remaining		$0
								Contractual
								Term
	Balance at January 1, 2014	22,567,866			$	29.29
	Granted	1,716,637			$	48
	Exercised	(6,060,823	)		$	29.39
	Forfeited	(149,501	)		$	35.76
	Expired	(79,097	)		$	32.98
	Balance at December 31, 2014	17,995,082			$	30.97		5.1 years		$	488,937
	Options vested or expected to vest at December 31, 2014	17,709,073			$	30.9		5.1 years		$	482,479
	Options exercisable at December 31, 2014	12,440,781			$	27.74		3.9 years		$	378,296
	In the above table, forfeited options are unvested options whose requisite service period has not been met. Expired options are vested options that were not exercised. The weighted-average grant-date fair value of the Company's option awards granted during the years ended December 31, 2014, 2013 and 2012 was $9.66, $6.21 and $6.04, respectively. The total intrinsic value of options exercised during the same periods was $174.3 million, $198.1 million and $57.7 million, respectively.
	As of December 31, 2014, there was $16 million of unrecognized compensation cost related to the Company's option awards. The weighted-average period over which that cost is expected to be recognized is approximately 1.05 years. Cash received from the exercise of stock options for the years ended December 31, 2014, 2013 and 2012 was $178.1 million, $281.1 million and $179.3 million, respectively.
	The Company's policy is to issue treasury shares upon option exercises or share unit conversion. The
	Company intends to issue treasury shares as long as an adequate number of those shares is available.
	Restricted Stock Units and Performance Stock Units: Restricted stock units may be awarded under the Company's 2011 Incentive and Stock Award Plan. The Compensation Committee determines the restrictions on such units, when the restrictions lapse, when the units vest and are paid, and under what terms the units are forfeited. The cost of these awards is amortized over the vesting period, which is generally three years. Awards to senior executives and other employees may include three-year performance-based restricted stock units and three-year service-based restricted stock units. The payout of performance stock units (payable in shares of the Company's common stock) may range, generally, from 0-200% of the number of units granted, based on the achievement of objective, pre-determined Company or operating company performance measures, generally, over a three-year performance period. The Company accounts for these awards as performance condition restricted stock units. The performance condition is not considered in the determination of grant date fair value of such awards. Compensation cost is recognized over the performance period based on management's estimate of the number of units expected to vest and is adjusted to reflect the actual number of shares paid out at the end of the three-year performance period. Dividend equivalents are not paid out unless and until such time that the award vests.
	A summary of the status of the Company's restricted stock units and performance stock units ("PSU's") as of December 31, 2014 and changes during the period then ended is presented below:
		Restricted Stock Units					Performance Stock Units
		Shares		Weighted Average			Shares		Weighted Average Grant Date Fair Value
				Grant Date
				Fair Value
	Non-vested balance at January 1, 2014	4,250,809		$	32.04		961,163		$	32.87
	Granted	753,213		$	48.16		231,445		$	48
	Vested	(2,779,606	)	$	31.23		(613,734	)	$	30.73
	Forfeited	(126,686	)	$	34.85		(17,446	)	$	37.45
	Adjustment due to performance	—		$	—		306,580		$	30.72
	Non-vested balance at December 31, 2014	2,097,730		$	38.74		868,008		$	37.56
	The weighted-average grant-date fair value of the Company's restricted stock units granted during the years ended December 31, 2013 and 2012 was $36.70 and $31.96, respectively. The weighted average grant date fair value of the Company's performance stock units granted during the years ended December 31, 2013 and 2012 was $36.54 and $31.89, respectively. The total fair value of the shares distributed during the years ended December 31, 2014, 2013 and 2012 in connection with the Company's restricted stock units and performance stock units was $165.3 million, $205.5 million and $262.6 million, respectively.
	The number of vested performance stock units includes any applicable performance adjustment shares. The adjustment due to performance reflects the incremental portion of the above-target payout at 200% for PSU's awarded in 2011 that vested on the PSU Scheduled Vesting Date in 2014 and PSU's awarded in February 2013 and 2012 that vested during 2014 (either in full or on a pro-rata basis) due to certain types of termination of employment. There is no adjustment due to performance for performance stock units awarded in February 2014 that vested during 2014 due to certain types of termination of employment within the calendar year of grant since the payout of such awards is at 100% of target under the award's terms and conditions.
	Restricted Stock: Restricted shares of the Company's common stock may be awarded under the 2011 Plan and are subject to restrictions on transferability and other restrictions, if any, as the Compensation Committee may impose. The Compensation Committee may also determine when and under what circumstances the restrictions may lapse and whether the participant receives the rights of a stockholder, including, without limitation, the right to vote and receive dividends. Unless the Compensation Committee determines otherwise, restricted stock that is still subject to restrictions is forfeited upon termination of employment. There have been no restricted shares granted since 2005.
	A summary of the status of the Company's restricted stock awards as of December 31, 2014 and changes during the period then ended is presented below:
		Shares			Weighted Average
					Grant Date
					Fair Value
	Non-vested balance at January 1, 2014	7,200			$	46.14
	Granted	—			$	—
	Vested	—			$	—
	Forfeited	—			$	—
	Non-vested balance at December 31, 2014	7,200			$	46.14
	The total fair value of the Company's restricted stock distributed was $1.1 million during the year ended December 31, 2013 and $0.6 million for the year ended December 31, 2012. There were no restricted stock distributions during 2014.
	As of December 31, 2014, there was $55.8 million of unrecognized compensation cost related to the Company's restricted stock, restricted stock units and performance stock unit awards. The weighted-average period over which that cost is expected to be recognized is approximately one year.
	Marsh & McLennan Companies Stock Purchase Plans
	In May 1999, the Company's stockholders approved an employee stock purchase plan (the “1999 Plan”) to replace the 1994 Employee Stock Purchase Plan (the “1994 Plan”), which terminated on September 30, 1999 following its fifth annual offering. Under the current terms of the Plan, shares are purchased four times during the plan year at a price that is 95% of the average market price on each quarterly purchase date. Under the 1999 Plan, after including the available remaining unused shares in the 1994 Plan and reducing the shares available by 10,000,000 consistent with the Company's Board of Directors' action in March 2007, no more than 35,600,000 shares of the Company's common stock may be sold. Employees purchased 608,453 shares during the year ended December 31, 2014 and at December 31, 2014, 2,779,195 shares were available for issuance under the 1999 Plan. Under the 1995 Company Stock Purchase Plan for International Employees (the “International Plan”), after reflecting the additional 5,000,000 shares of common stock for issuance approved by the Company's Board of Directors in July 2002, and the addition of 4,000,000 shares due to a shareholder action in May 2007, no more than 12,000,000 shares of the Company's common stock may be sold. Employees purchased 93,490 shares during the year ended December 31, 2014 and there were 2,893,986 shares available for issuance at December 31, 2014 under the International Plan. The plans are considered non-compensatory.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements
	Fair Value Hierarchy
	The Company has categorized its assets and liabilities that are valued at fair value on a recurring basis into a three-level fair value hierarchy as defined by the accounting literature. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets and liabilities (Level 1) and lowest priority to unobservable inputs (Level 3). In some cases, the inputs used to measure fair value might fall into different levels of the fair value hierarchy. In such cases, the level in the fair value hierarchy, for disclosure purposes, is determined based on the lowest level input that is significant to the fair value measurement. Assets and liabilities recorded in the consolidated balance sheets at fair value are categorized based on the inputs in the valuation techniques as follows:
	Level 1.	Assets and liabilities whose values are based on unadjusted quoted prices for identical assets or liabilities in an active market (examples include active exchange-traded equity securities and money market mutual funds).
	Assets and liabilities utilizing Level 1 inputs include exchange-traded equity securities and mutual funds.
	Level 2.	Assets and liabilities whose values are based on the following:
	a)	Quoted prices for similar assets or liabilities in active markets;
	b)	Quoted prices for identical or similar assets or liabilities in non-active markets (examples include corporate and municipal bonds, which trade infrequently);
	c)	Pricing models whose inputs are observable for substantially the full term of the asset or liability (examples include most over-the-counter derivatives, including interest rate and currency swaps); and
	d)	Pricing models whose inputs are derived principally from or corroborated by observable market data through correlation or other means for substantially the full asset or liability (for example, certain mortgage loans).
	Assets and liabilities utilizing Level 2 inputs include corporate and municipal bonds, senior notes and interest rate swaps.
	Level 3.	Assets and liabilities whose values are based on prices, or valuation techniques that require inputs that are both unobservable and significant to the overall fair value measurement. These inputs reflect management’s own assumptions about the assumptions a market participant would use in pricing the asset or liability (examples include private equity investments, certain commercial mortgage whole loans, and long-dated or complex derivatives including certain foreign exchange options and long-dated options on gas and power).
	Liabilities utilizing Level 3 inputs include liabilities for contingent purchase consideration.
	Valuation Techniques
	Equity Securities and Mutual Funds - Level 1
	Investments for which market quotations are readily available are valued at the sale price on their principal exchange, or official closing bid price for certain markets.
	Interest Rate Swap Derivative - Level 2
	The fair value of interest rate swap derivatives is based on the present value of future cash flows at each valuation date resulting from utilization of the swaps, using a constant discount rate of 1.6% compared to discount rates based on projected future yield curves. The Company settled its interest rate swap positions in July 2014.
	Senior Notes due July 2014 - Level 2
	In the first quarter of 2011, the Company entered into two interest rate swaps to convert interest on a portion of its Senior Notes from a fixed rate to a floating rate. The swaps are designated as fair value hedging instruments. The change in the fair value of the swaps is recorded on the balance sheet. The carrying value of the debt related to these swaps is adjusted by an equal amount. The $250 million of Senior Notes that were tied to the interest rate swaps discussed above matured in July 2014.
	Contingent Purchase Consideration Liability - Level 3
	Purchase consideration for some acquisitions made by the Company includes contingent consideration arrangements. Contingent consideration arrangements are primarily based on meeting EBITDA and revenue targets over periods from two to four years. The fair value of contingent consideration is estimated as the present value of future cash flows resulting from the projected revenue and earnings of the acquired entities.
	The following fair value hierarchy table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis as of December 31, 2014 and 2013:
	(In millions of dollars)	Identical Assets						Observable Inputs						Unobservable				Total
		(Level 1)						(Level 2)						Inputs
														(Level 3)
		12/31/14			12/31/13			12/31/14				12/31/13		12/31/14		12/31/13		12/31/14		12/31/13
	Assets:
	Financial instruments owned:
	Mutual funds(a)	$	150		$	154		$	—			$	—	$	—	$	—	$	150	$	154
	Money market funds(b)	107			45			—				—		—		—		107		45
	Interest rate swap derivatives(c)	—			—			—				3		—		—		—		3
	Total assets measured at fair value	$	257		$	199		$	—			$	3	$	—	$	—	$	257	$	202
	Fiduciary Assets:
	Money market funds	57			—			—				—		—		—		57		—
	Total fiduciary assets measured at fair value	$	57		$	—		$	—			$	—	$	—	$	—	$	57	$	—
	Liabilities:
	Contingent purchase consideration liability(d)	$	—		$	—		$	—			$	—	$	207	$	104	$	207	$	104
	Senior Notes due 2014(e)	—			—			—				253		—		—		—		253
	Total liabilities measured at fair value	$	—		$	—		$	—			$	253	$	207	$	104	$	207	$	357
	(a) Included in other assets in the consolidated balance sheets.
	(b) Included in cash and cash equivalents in the consolidated balance sheets.
	(c) Included in other receivables in the consolidated balance sheets.
	(d) Included in accounts payable and accrued liabilities and other liabilities in the consolidated balance sheets.
	(e) Included in long-term debt in the consolidated balance sheets.
	During the year ended December 31, 2014, there were no assets or liabilities that transferred between any of the levels.
	The table below sets forth a summary of the changes in fair value of the Company’s Level 3 liabilities for the years ended December 31, 2014 and December 31, 2013 that represent contingent purchase consideration related to acquisitions:
	(In millions of dollars)	2014			2013
	Balance at January 1,	$	104		$	63
	Additions	114			26
	Payments	(42		)	(17		)
	Revaluation Impact	31			32
	Balance at December 31,	$	207		$	104
	The fair value of the contingent purchase consideration liability is based on projections of revenue and earnings for the acquired entities that are reassessed on a quarterly basis. As set forth in the table above, based on the Company's ongoing assessment of the fair value of contingent consideration, the Company recorded a net increase in the estimated fair value of such liabilities for prior period acquisitions of $31 million for the year ended December 31, 2014. A 5% increase in the above mentioned projections would increase the liability by approximately $22 million. A 5% decrease in the above mentioned projections would decrease the liability by approximately $26 million.
	Equity Method Investments
	The Company holds investments in certain private companies, public companies and certain private equity investments that are accounted for using the equity method of accounting. The carrying value of these investments amounted to $388 million and $89 million at December 31, 2014 and 2013, respectively. The Company's investments in private equity funds were $61 million and $38 million at December 31, 2014 and December 31, 2013, respectively. The carrying values of these private equity investments approximates fair value. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. The Company records in earnings, investment gains/losses for its proportionate share of the change in fair value of the funds. These investments would be classified as Level 3 in the fair value hierarchy and are included in Other assets in the consolidated balance sheets.
	During 2014, the Company purchased 34% of Alexander Forbes common stock. As of December 31, 2014, the carrying value of the Company’s investment in Alexander Forbes was approximately $282 million. As of December 31, 2014, the market value of the approximately 443 million shares owned by the Company, based on the December 31, 2014 closing share price of 9.5 South African Rand per share, was approximately $362 million. The Company’s investment in Alexander Forbes and its other equity investments in private companies are accounted for using the equity method of accounting and included in revenue in the consolidated income statements and in other assets in the consolidated balance sheets. The Company records its share of income or loss on its equity method investments on a one quarter lag basis since the information is not readily available in time for the Company's Form 10-Q and Form 10-K filings.
	The summarized financial information presented below reflects the aggregated financial information of all significant equity method investees as of and for the twelve months ended September 30 of each year (or portion of those twelve months the company owned its investment), consistent with the Company’s recognition of the results of its equity method investments on a one quarter lag. The investment income information presented below reflects the net realized and unrealized gains/losses, net of expenses, related to the Company's investments in several private equity funds. Certain of the Company’s equity method investments, including Alexander Forbes, have unclassified balance sheets. Therefore, the asset and liability information presented below are not split between current and non-current.
	Below is a summary of the financial information for the Company's significant equity method investees:
	For the twelve months ended September 30,
	(In millions of dollars)		2014			2013			2012
	Revenue		$	239		$	148		$	144
	Net investment income		$	161		$	88		$	(19	)
	Net income		$	216		$	135		$	36
As of September 30,
(In millions of dollars)		2014			2013
Total assets		$	25,497		$	741
Total liabilities		$	24,209		$	136
Non controlling interests		$	14		$	3
The information above includes only two months of income statement activity for Alexander Forbes through September 30, 2014, since the Company purchased its first tranche in July 2014.

LongTerm_Commitments

Long-Term Commitments	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Long-Term Commitments	Long-term Commitments
	The Company leases office facilities, equipment and automobiles under non-cancelable operating leases. These leases expire on varying dates, in some instances contain renewal and expansion options, do not restrict the payment of dividends or the incurrence of debt or additional lease obligations, and contain no significant purchase options. In addition to the base rental costs, occupancy lease agreements generally provide for rent escalations resulting from increased assessments for real estate taxes and other charges. Approximately 98% of the Company’s lease obligations are for the use of office space.
	The consolidated statements of income include net rental costs of $393 million, $403 million and $416 million for 2014, 2013 and 2012, respectively, after deducting rentals from subleases ($12 million in 2014, $13 million in 2013 and $10 million in 2012). These net rental costs exclude rental costs and sublease income for previously accrued restructuring charges related to vacated space.
	At December 31, 2014, the aggregate future minimum rental commitments under all non-cancelable operating lease agreements are as follows:
	For the Years Ended December 31,	Gross		Rentals		Net
		Rental		from		Rental
	(In millions of dollars)	Commitments		Subleases		Commitments
	2015	$	365	$	48	$	317
	2016	$	334	$	48	$	286
	2017	$	293	$	45	$	248
	2018	$	258	$	42	$	216
	2019	$	212	$	35	$	177
	Subsequent years	$	1,010	$	36	$	974
	The Company has entered into agreements, primarily with various service companies, to outsource certain information systems activities and responsibilities and processing activities. Under these agreements, the Company is required to pay minimum annual service charges. Additional fees may be payable depending upon the volume of transactions processed, with all future payments subject to increases for inflation. At December 31, 2014, the aggregate fixed future minimum commitments under these agreements are as follows:
	For the Years Ended December 31,	Future
		Minimum
	(In millions of dollars)	Commitments
	2015	$	195
	2016	113
	2017	64
	Subsequent years	69
		$	441

Debt

Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Debt	Debt
	The Company’s outstanding debt is as follows:
	December 31,
	(In millions of dollars)	2014		2013
	Short-term:
	Current portion of long-term debt	$	11	$	334
	Long-term:
	Senior notes – 5.875% due 2033	297		297
	Senior notes – 5.375% due 2014	—		323
	Senior notes – 5.75% due 2015	—		230
	Senior notes – 2.30% due 2017	249		249
	Senior notes – 9.25% due 2019	—		399
	Senior notes – 4.80% due 2021	497		497
	Senior notes – 2.55% due 2018	249		248
	Senior notes – 4.05% due 2023	248		247
	Senior notes – 3.50% due 2024	595		—
	Senior notes – 2.35% due 2019	300		—
	Senior notes – 3.50% due 2025	498		—
	Mortgage – 5.70% due 2035	403		413
	Term Loan Facility – due 2016	50		50
	Other	1		2
		3,387		2,955
	Less current portion	11		334
		$	3,376	$	2,621
	The senior notes in the table above are publically registered by the Company with no guarantees attached.
	In September 2014, the Company issued $300 million of 2.35% five-year senior notes and $500 million of 3.50% 10.5-year senior notes. In October 2014, a significant portion of the net proceeds of this offering were used to redeem $630 million of debt, including $230 million of 5.75% senior notes due in September 2015 and $400 million of 9.25% senior notes due in 2019. Total cash outflow related to this transaction was approximately $765 million, including a $137 million cost for early redemption, which is reflected as a charge in the consolidated statements of income in the fourth quarter of 2014.
	In May 2014, the Company issued $600 million of 3.50% ten-year senior notes. The net proceeds of this offering were used for general corporate purposes, which included the repayment of $320 million of the existing 5.375% senior notes, which matured on July 15, 2014.
	In September 2013, the Company issued $250 million of 2.55% five-year senior notes and $250 million of 4.05% ten-year senior notes. The net proceeds of this offering were used for general corporate purposes, which included a partial redemption of $250 million of the outstanding principal amount of the existing 5.75% senior notes due 2015. The redemption settled in October 2013 with a total cash outflow of approximately $275 million, including a $24 million cost for early redemption.
	In February 2013, the Company repaid its 4.850% $250 million senior notes.
	On March 27, 2014, the Company and certain of its foreign subsidiaries amended its $1.0 billion facility, as discussed below, into a $1.2 billion multi-currency five-year unsecured revolving credit facility. The interest rate on this facility is based on LIBOR plus a fixed margin which varies with the Company's credit ratings. This facility expires in March 2019 and requires the Company to maintain certain coverage and leverage ratios which are tested quarterly. There were no borrowings outstanding under this facility at December 31, 2014.
	The Company and certain of its foreign subsidiaries previously maintained a $1.0 billion multi-currency five-year revolving credit facility. The facility was previously due to expire in October 2016 and was in effect until March 2014. There were no borrowings outstanding under this facility at the time it was amended.
	In December 2012, the Company closed on a $50 million, three-year term loan facility. The interest rate on this facility at December 31, 2014 was 1.17%, which is based on LIBOR plus a fixed margin which varies with the Company's credit ratings. The facility requires the Company to maintain coverage ratios and leverage ratios consistent with the revolving credit facility discussed above. The Company had $50 million of borrowings under this facility at December 31, 2014.
	Additional credit facilities, guarantees and letters of credit are maintained with various banks, primarily related to operations located outside the United States, aggregating $260 million at December 31, 2014 and $282 million at December 31, 2013. There was $0.6 million outstanding borrowings under these facilities at December 31, 2014 and $1 million outstanding borrowings under these facilities at December 31, 2013.
	Scheduled repayments of long-term debt in 2015 and in the four succeeding years are $10 million, $61 million, $262 million, $262 million and $313 million, respectively.
	Fair value of Short-term and Long-term Debt
	The estimated fair value of the Company’s short-term and long-term debt is provided below. Certain estimates and judgments were required to develop the fair value amounts. The fair value amounts shown below are not necessarily indicative of the amounts that the Company would realize upon disposition, nor do they indicate the Company’s intent or need to dispose of the financial instrument.
		December 31, 2014				December 31, 2013
	(In millions of dollars)	Carrying		Fair		Carrying		Fair
		Amount		Value		Amount		Value
	Short-term debt	$	11	$	11	$	334	$	334
	Long-term debt	$	3,376	$	3,493	$	2,621	$	2,819
	The fair value of the Company’s short-term debt consists primarily of term debt maturing within the next year and its fair value approximates its carrying value. The estimated fair value of a primary portion of the Company's long-term debt is based on discounted future cash flows using current interest rates available for debt with similar terms and remaining maturities. Short- and long-term debt would be classified as Level 2 in the fair value hierarchy.

Integration_and_Restructuring_

Integration and Restructuring Costs	12 Months Ended
	Dec. 31, 2014
Restructuring and Related Activities [Abstract]
Integration and Restructuring Costs	Integration and Restructuring Costs
	In 2014, the Company implemented restructuring actions which resulted in costs totaling $12 million. Restructuring costs consist primarily of severance and benefits, costs for future rent and other real estate costs. These costs were incurred as follows: Risk and Insurance Services—$5 million; Consulting—$1 million; and Corporate—$6 million.
	Details of the restructuring liability activity from January 1, 2013 through December 31, 2014, including actions taken prior to 2014 are as follows:
	(In millions of dollars)	Balance at		Expense		Cash			Other			Balance at		Expense		Cash			Other			Balance at
		1/1/13		Incurred		Paid						12/31/13		Incurred		Paid						12/31/14
	Severance	$	36	$	9	$	(33	)	$	(1	)	$	11	$	4	$	(8	)	$	—		$	7
	Future rent under non-cancelable leases and other costs	134		13		(32		)	(2		)	113		8		(35		)	(1		)	85
	Total	$	170	$	22	$	(65	)	$	(3	)	$	124	$	12	$	(43	)	$	(1	)	$	92
	As of January 1, 2012, the liability balance related to restructuring activity was $181 million. In 2012, the Company accrued $78 million and had cash payments of $88 million related to restructuring activities that resulted in the liability balance at January 1, 2013 reported above.
	The expenses associated with the above initiatives are included in compensation and benefits and other operating expenses in the consolidated statements of income. The liabilities associated with these initiatives are classified on the consolidated balance sheets as accounts payable and accrued liabilities, other liabilities, or accrued compensation and employee benefits, depending on the nature of the items.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Common Stock	Common Stock
	During 2014, the Company repurchased 15.5 million shares of its common stock for total consideration of $800 million. In May 2014, the Board of Directors of the Company authorized share repurchases of up to $2 billion of the Company's common stock. The Company remains authorized to purchase additional shares of its common stock up to a value of $1.3 billion. There is no time limit on the authorization. During 2013, the Company purchased 13.2 million shares of its common stock for total consideration of $550 million.

Claims_Lawsuits_And_Other_Cont

Claims, Lawsuits And Other Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Claims, Lawsuits And Other Contingencies	Claims, Lawsuits and Other Contingencies
	Errors and Omissions Claims
	The Company and its subsidiaries are subject to a significant number of claims, lawsuits and proceedings in the ordinary course of business. Such claims and lawsuits consist principally of alleged errors and omissions in connection with the performance of professional services, including the placement of insurance, the provision of actuarial services for corporate and public sector clients, the provision of investment advice and investment management services to pension plans, the provision of advice relating to pension buy-out transactions and the provision of consulting services relating to the drafting and interpretation of trust deeds and other documentation governing pension plans. Errors and omissions claims may seek damages, including punitive and treble damages, in amounts that could, if awarded, be significant. In establishing liabilities for errors and omissions claims in accordance with FASB ASC Subtopic No. 450-20 (Contingencies-Loss Contingencies), the Company utilizes case level reviews by inside and outside counsel, an internal actuarial analysis and other analysis to estimate potential losses. A liability is established when a loss is both probable and reasonably estimable. The liability is reviewed quarterly and adjusted as developments warrant. In many cases, the Company has not recorded a liability, other than for legal fees to defend the claim, because we are unable, at the present time, to make a determination that a loss is both probable and reasonably estimable.
	To the extent that expected losses exceed our deductible in any policy year, the Company also records an asset for the amount that we expect to recover under any available third-party insurance programs. The Company has varying levels of third-party insurance coverage, with policy limits and coverage terms varying significantly by policy year.
	Governmental Inquiries and Related Claims
	Our activities are regulated under the laws of the United States and its various states, the European Union and its member states, and the other jurisdictions in which the Company operates. In the ordinary course of business the Company is also subject to subpoenas, investigations, lawsuits and/or other regulatory actions undertaken by governmental authorities.
	Other Contingencies-Guarantees
	In connection with its acquisition of U.K.-based Sedgwick Group in 1998, the Company acquired several insurance underwriting businesses that were already in run-off, including River Thames Insurance Company Limited (“River Thames”), which the Company sold in 2001. Sedgwick guaranteed payment of claims on certain policies underwritten through the Institute of London Underwriters (the “ILU”) by River Thames. The policies covered by this guarantee are reinsured up to £40 million by a related party of River Thames. Payment of claims under the reinsurance agreement is collateralized by segregated assets held in a trust. As of December 31, 2014, the reinsurance coverage exceeded the best estimate of the projected liability of the policies covered by the guarantee. To the extent River Thames or the reinsurer is unable to meet its obligations under those policies, a claimant may seek to recover from us under the guarantee.
	From 1980 to 1983, the Company owned indirectly the English & American Insurance Company (“E&A”), which was a member of the ILU. The ILU required the Company to guarantee a portion of E&A's obligations. After E&A became insolvent in 1993, the ILU agreed to discharge the guarantee in exchange for the Company's agreement to post an evergreen letter of credit that is available to pay claims by policyholders on certain E&A policies issued through the ILU and incepting between July 3, 1980 and October 6, 1983. Certain claims have been paid under the letter of credit and we anticipate that additional claimants may seek to recover against the letter of credit.
	Kroll-related Matters
	Under the terms of a stock purchase agreement with Altegrity, Inc. (“Altegrity”) related to Altegrity's purchase of Kroll from the Company in August 2010, a copy of which is attached as an exhibit to the Company's Quarterly Report on Form 10-Q for the period ended June 30, 2010, the Company agreed to provide a limited indemnity to Altegrity with respect to certain Kroll-related litigation and regulatory matters.
	The pending proceedings and other matters described in this Note 15 on Claims, Lawsuits and Other Contingencies may expose the Company or its subsidiaries to liability for significant monetary damages and other forms of relief. Where a loss is both probable and reasonably estimable, the Company establishes liabilities in accordance with FASB ASC Subtopic No. 450-20 (Contingencies-Loss Contingencies). Except as described above, the Company is not able at this time to provide a reasonable estimate of the range of possible loss attributable to these matters or the impact they may have on the Company's consolidated results of operations, financial position or cash flows. This is primarily because these matters are still developing and involve complex issues subject to inherent uncertainty. Adverse determinations in one or more of these matters could have a material impact on the Company's consolidated results of operations, financial condition or cash flows in a future period.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment Information	Segment Information
	The Company is organized based on the types of services provided. Under this organizational structure, the Company’s segments are:
	▪	Risk and Insurance Services, comprising insurance services (Marsh) and reinsurance services (Guy Carpenter); and
	▪	Consulting, comprising Mercer and Oliver Wyman Group
	The accounting policies of the segments are the same as those used for the consolidated financial statements described in Note 1. Segment performance is evaluated based on segment operating income, which includes directly related expenses, and charges or credits related to integration and restructuring but not the Company’s corporate-level expenses. Revenues are attributed to geographic areas on the basis of where the services are performed.
	Selected information about the Company’s segments and geographic areas of operation are as follows:
	For the Year Ended December 31, (In millions of dollars)	Revenue				Operating			Total				Depreciation		Capital
						Income			Assets				and		Expenditures
						(Loss)							Amortization
	2014 –
	Risk and Insurance Services	$	6,931		(a)	$	1,509		$	12,211			$	213	$	173
	Consulting	6,059			(b)	996			5,916				119		92
	Total Segments	12,990				2,505			18,127				332		265
	Corporate / Eliminations	(39		)		(204		)	(287		)	(c)	56		103
	Total Consolidated	$	12,951			$	2,301		$	17,840			$	388	$	368
	2013 –
	Risk and Insurance Services	$	6,596		(a)	$	1,421		$	11,365			$	192	$	158
	Consulting	5,701			(b)	845			5,178				115		155
	Total Segments	12,297				2,266			16,543				307		313
	Corporate / Eliminations	(36		)		(189		)	437			(c)	51		88
	Total Consolidated	$	12,261			$	2,077		$	16,980			$	358	$	401
	2012 –
	Risk and Insurance Services	$	6,350		(a)	$	1,334		$	9,832			$	196	$	131
	Consulting	5,613			(b)	692			5,203				113		117
	Total Segments	11,963				2,026			15,035				309		248
	Corporate / Eliminations	(39		)		(197		)	1,253			(c)	40		72
	Total Consolidated	$	11,924			$	1,829		$	16,288			$	349	$	320
	(a)	Includes inter-segment revenue of $4 million in 2014 and $5 million in both 2013 and 2012, interest income on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively, and equity method income of $9 million, $8 million and $11 million in 2014, 2013 and 2012, respectively.
	(b)	Includes inter-segment revenue of $35 million, $31 million and $34 million in 2014, 2013 and 2012, respectively, interest income on fiduciary funds of $6 million in 2014, $5 million in 2013 and $4 million 2012 and equity method income of $2 million in 2014, and $0 million in both 2013 and 2012.
	(c)	Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations.
	Details of operating segment revenue are as follows:
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013			2012
	Risk and Insurance Services
	Marsh	$	5,774			$	5,461		$	5,265
	Guy Carpenter	1,157				1,135			1,085
	Total Risk and Insurance Services	6,931				6,596			6,350
	Consulting
	Mercer	4,350				4,241			4,147
	Oliver Wyman Group	1,709				1,460			1,466
	Total Consulting	6,059				5,701			5,613
	Total Segments	12,990				12,297			11,963
	Corporate / Eliminations	(39		)		(36		)	(39		)
	Total	$	12,951			$	12,261		$	11,924
	Information by geographic area is as follows:
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013			2012
	Revenue
	United States	$	5,865			$	5,485		$	5,300
	United Kingdom	2,111				1,979			1,960
	Continental Europe	2,077				1,943			1,879
	Asia Pacific	1,420				1,396			1,346
	Other	1,517				1,494			1,478
		12,990				12,297			11,963
	Corporate/Eliminations	(39		)		(36		)	(39		)
		$	12,951			$	12,261		$	11,924
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013			2012
	Fixed Assets, Net
	United States	$	483			$	494		$	494
	United Kingdom	120				121			121
	Continental Europe	60				64			63
	Asia Pacific	62				72			62
	Other	84				77			69
	$	809			$	828		$	809

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data and Supplemental Information Selected Quarterly Financial Data and Supplemental Information (Notes)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Selected Quarterly Financial Data and Supplemental Information
		First			Second			Third			Fourth
		Quarter			Quarter			Quarter			Quarter
	(In millions, except per share figures)
	2014:00:00
	Revenue	$	3,264		$	3,300		$	3,141		$	3,246
	Operating income	$	673		$	647		$	445		$	536
	Income from continuing operations	$	457		$	440		$	305		$	269
	Discontinued operations, net of tax	$	(1	)	$	(2	)	$	(1	)	$	30
	Net income attributable to the Company	$	443		$	431		$	297		$	294
	Basic Per Share Data:
	Continuing operations	$	0.81		$	0.79		$	0.55		$	0.49
	Discontinued operations, net of tax	$	—		$	(0.01	)	$	—		$	0.05
	Net income attributable to the Company	$	0.81		$	0.78		$	0.55		$	0.54
	Diluted Per Share Data:
	Continuing operations	$	0.8		$	0.78		$	0.54		$	0.48
	Discontinued operations, net of tax	$	—		$	(0.01	)	$	—		$	0.06
	Net income attributable to the Company	$	0.8		$	0.77		$	0.54		$	0.54
	Dividends Paid Per Share	$	0.25		$	0.25		$	0.28		$	0.28
	2013:00:00
	Revenue	$	3,126		$	3,088		$	2,932		$	3,115
	Operating income	$	607		$	577		$	404		$	489
	Income from continuing operations	$	412		$	400		$	260		$	307
	Discontinued operations, net of tax	$	12		$	(5	)	$	(1	)	$	—
	Net income attributable to the Company	$	413		$	388		$	253		$	303
	Basic Per Share Data:
	Continuing operations	$	0.73		$	0.71		$	0.46		$	0.55
	Discontinued operations, net of tax	$	0.02		$	—		$	—		$	—
	Net income attributable to the Company	$	0.75		$	0.71		$	0.46		$	0.55
	Diluted Per Share Data:
	Continuing operations	$	0.72		$	0.7		$	0.45		$	0.54
	Discontinued operations, net of tax	$	0.02		$	(0.01	)	$	—		$	—
	Net income attributable to the Company	$	0.74		$	0.69		$	0.45		$	0.54
	Dividends Paid Per Share	$	0.23		$	0.23		$	0.25		$	0.25
	As of February 20th, 2015, there were 6,197 stockholders of record.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations:  Marsh & McLennan Companies, Inc. (the "Company”), a global professional services firm, is organized based on the different services that it offers. Under this organizational structure, the Company’s two business segments are Risk and Insurance Services and Consulting.
	The Risk and Insurance Services segment provides risk management activities and insurance broking, reinsurance broking and insurance program management services for businesses, public entities, insurance companies, associations, professional services organizations, and private clients. The Company conducts business in this segment through Marsh and Guy Carpenter.
	The Company conducts business in its Consulting segment through two main business groups. Mercer provides consulting expertise, advice, services and solutions in the areas of health, retirement, talent and investments. Oliver Wyman Group provides specialized management and economic and brand consulting services.
Principles of Consolidation	Principles of Consolidation:  The accompanying consolidated financial statements include all wholly-owned and majority-owned subsidiaries. All significant inter-company transactions and balances have been eliminated.
Fiduciary Assets And Liabilities	Fiduciary Assets and Liabilities:  In its capacity as an insurance broker or agent, the Company generally collects premiums from insureds and, after deducting its commissions, remits the premiums to the respective insurance underwriters. The Company also collects claims or refunds from underwriters on behalf of insureds. Unremitted insurance premiums and claims proceeds are held by the Company in a fiduciary capacity. Risk and Insurance Services revenue includes interest on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively. The Consulting segment recorded fiduciary interest income of $6 million, $5 million and $4 million in 2014, 2013 and 2012, respectively. Since fiduciary assets are not available for corporate use, they are shown in the consolidated balance sheets as an offset to fiduciary liabilities.
	Net uncollected premiums and claims and the related payables amounted to $7.3 billion and $8.2 billion at December 31, 2014 and 2013, respectively. The Company is not a principal to the contracts under which the right to receive premiums or the right to receive reimbursement of insured losses arises. Net uncollected premiums and claims and the related payables are, therefore, not assets and liabilities of the Company and are not included in the accompanying consolidated balance sheets.
	In certain instances, the Company advances premiums, refunds or claims to insurance underwriters or insureds prior to collection. These advances are made from corporate funds and are reflected in the accompanying consolidated balance sheets as receivables.
	Mercer manages approximately $21 billion of assets in trusts or funds for which Mercer’s management or trustee fee is considered a variable interest. Mercer is not the primary beneficiary of these trusts or funds. Mercer’s only variable interest in any of these trusts or funds is its unpaid fees, if any. Mercer’s maximum exposure to loss of its interests is, therefore, limited to collection of its fees.
Revenue	Revenue:  Risk and Insurance Services revenue includes insurance commissions, fees for services rendered and interest income on certain fiduciary funds. Insurance commissions and fees for risk transfer services generally are recorded as of the effective date of the applicable policies or, in certain cases (primarily in the Company's reinsurance broking operations), as of the effective date or billing date, whichever is later. A reserve for policy cancellation is provided based on historic and current data on cancellations. Consideration for fee arrangements covering multiple insurance placements, the provision of risk management and/or other services are allocated to all deliverables on the basis of their relative selling prices. Fees for non-risk transfer services provided to clients are recognized over the period in which the services are provided, using a proportional performance model. Fees resulting from achievement of certain performance thresholds are recorded when such levels are attained and such fees are not subject to forfeiture.
	Consulting revenue includes fees paid by clients for advice and services and commissions from insurance companies for the placement of individual and group contracts. Fee revenue for engagements where remuneration is based on time plus out-of-pocket expenses is recognized based on the amount of time consulting professionals expend on the engagement. For fixed fee engagements, revenue is recognized using a proportional performance model. Revenue from insurance commissions not subject to a fee arrangement is recorded over the effective period of the applicable policies. Revenue for asset based fees is recognized on an accrual basis by applying the daily/monthly rate as contractually agreed with the client to the applicable net asset value. On a limited number of engagements, performance fees may also be earned for achieving certain prescribed performance criteria. Such fees are recognized when the performance criteria have been achieved and, when required, agreed to by the client. Reimbursable expenses incurred by professional staff in the generation of revenue and sub-advisory fees related to the majority of funds in the investment management business are included in revenue and the related expenses are included in other operating expenses.
Cash and Cash Equivalents	Cash and Cash Equivalents:  Cash and cash equivalents primarily consist of certificates of deposit and time deposits, with original maturities of three months or less, and money market funds. The estimated fair value of the Company's cash and cash equivalents approximates their carrying value. The Company is required to maintain operating funds related to regulatory requirements outside the U.S. or as collateral under captive insurance arrangements. At December 31, 2014, the Company maintained $179 million related to these regulatory requirements.
Fixed Assets	Fixed Assets:  Fixed assets are stated at cost less accumulated depreciation and amortization. Expenditures for improvements are capitalized. Upon sale or retirement, the cost and related accumulated depreciation and amortization are removed from the accounts and any gain or loss is reflected in income. Expenditures for maintenance and repairs are charged to operations as incurred.
	Depreciation of buildings, building improvements, furniture, and equipment is provided on a straight-line basis over the estimated useful lives of these assets. Furniture and equipment is depreciated over periods ranging from three to ten years. Leasehold improvements are amortized on a straight-line basis over the periods covered by the applicable leases or the estimated useful life of the improvement, whichever is less. Buildings are depreciated over periods ranging from thirty to forty years. The Company periodically reviews long-lived assets for impairment whenever events or changes indicate that the carrying value of assets may not be recoverable.
Investments	Investments:  The Company holds investments in private companies and private equity funds.
	Investments in private equity funds are accounted for under the equity method of accounting using a consistently applied three-month lag period adjusted for any known significant changes from the lag period to the reporting date of the Company. The underlying private equity funds follow investment company accounting, where investments within the fund are carried at fair value. The Company records in earnings, investment gains/losses for its proportionate share of the change in fair value of the funds. Investments using the equity method of accounting are included in other assets in the consolidated balance sheets.
	As part of the sale of MMC Capital in 2005, the Company retained the rights to receive certain performance fees related to the Trident II and Trident III private equity partnerships. The Company recognizes performance fee income when such fees are no longer subject to forfeiture, which may take a number of years to resolve. This income is based on the investment performance over the life of each investment in the private equity fund, and future declines in the fund performance from current levels may result in forfeiture of such revenue. Since Trident II fully harvested all its portfolio investments and made final distributions to its partners in 2013, the Company no longer holds any rights to Trident II performance fees. In 2014, the Company recorded investment income of $37 million compared to $69 million in 2013 and $24 million in 2012. The Company recorded Investment income related to its general partner carried interest from Trident III no longer subject to clawback of $31 million and $41 million in 2014 and 2013, respectively. In 2013, the Company recorded $15 million of general partner carried interest from Trident II. There was no carried interest recorded in 2012. The Company also recorded net investment income gains of $6 million, $13 million and $33 million in 2014, 2013 and 2012, respectively, primarily related to its equity method investments. The Company had deferred performance fees of approximately $16 million and $38 million, at December 31, 2014 and 2013, respectively, related to Trident III. Recognition of these deferred performance fees will only occur as the Trident III investments are harvested and the performance fees are no longer subject to clawback. The timing of recognition of the remaining deferred performance fees is unknown and is not controlled by the Company.
	Subsequent Event
	On February 24, 2015, Mercer announced that it had made an investment to purchase approximately 10% of Benefitfocus (NASDAQ: BNFT), a benefits technology provider, with the option to increase its ownership over time.
Goodwill and Intangible Assets	Goodwill and Other Intangible Assets:  Goodwill represents acquisition costs in excess of the fair value of net assets acquired. Goodwill is reviewed at least annually for impairment. The Company performs an annual impairment test for each of its reporting units during the third quarter of each year. When a step 1 test is performed, fair values of the reporting units are estimated using either a market approach or a discounted cash flow model. Carrying values for the reporting units are based on balances at the prior quarter end and include directly identified assets and liabilities as well as an allocation of those assets and liabilities not recorded at the reporting unit level. As discussed in Note 6, the Company may elect to assess qualitative factors to determine if a step 1 test is necessary. Other intangible assets, which primarily consist of customer lists, that are not deemed to have an indefinite life are amortized over their estimated lives and reviewed for impairment upon the occurrence of certain triggering events in accordance with applicable accounting literature.
Capitalized Software Costs	Capitalized Software Costs:  The Company capitalizes certain costs to develop, purchase or modify software for the internal use of the Company. These costs are amortized on a straight-line basis over periods ranging from 3 to 10 years. Costs incurred during the preliminary project stage and post implementation stage, are expensed as incurred. Costs incurred during the application development stage are capitalized. Costs related to updates and enhancements are only capitalized if they will result in additional functionality. Capitalized computer software costs of $501 million and $399 million, net of accumulated amortization of $837 million and $748 million at December 31, 2014 and 2013, respectively, are included in other assets in the consolidated balance sheets.
Legal and Other Loss Contingencies	Legal and Other Loss Contingencies:  The Company and its subsidiaries are subject to a significant number of claims, lawsuits and proceedings including claims for errors and omissions ("E&O"). GAAP requires that a liability be recorded when a loss is both probable and reasonably estimable. Significant management judgment is required to apply this guidance. The Company utilizes case level reviews by inside and outside counsel, an internal actuarial analysis and other analysis to estimate potential losses. The liability is reviewed quarterly and adjusted as developments warrant. In many cases, the Company has not recorded a liability, other than for legal fees to defend the claim, because we are unable, at the present time, to make a determination that a loss is both probable and reasonably estimable. Given the unpredictability of E&O claims and of litigation that could flow from them, it is possible that an adverse outcome in a particular matter could have a material adverse effect on the Company’s businesses, results of operations, financial condition or cash flow in a given quarterly or annual period.
	In addition, to the extent that insurance coverage is available, significant management judgment is required to determine the amount of recoveries that are probable of collection under the Company’s various insurance programs.
	The legal and other contingent liabilities described above are not discounted.
Income Taxes	Income Taxes:  The Company's effective tax rate reflects its income, statutory tax rates and tax planning in the various jurisdictions in which it operates. Significant judgment is required in determining the annual effective tax rate and in evaluating uncertain tax positions and the ability to realize deferred tax assets.
	The Company reports a liability for unrecognized tax benefits resulting from uncertain tax positions taken or expected to be taken in a tax return. The evaluation of a tax position is a two-step process. The first step involves recognition. The Company determines whether it is more likely than not that a tax position will be sustained upon tax examination, including resolution of any related appeals or litigation, based on only the technical merits of the position. The technical merits of a tax position derive from both statutory and judicial authority (legislation and statutes, legislative intent, regulations, rulings, and case law) and their applicability to the facts and circumstances of the tax position. If a tax position does not meet the more likely than not recognition threshold, the benefit of that position is not recognized in the financial statements. The second step is measurement. A tax position that meets the more likely than not recognition threshold is measured to determine the amount of benefit to recognize in the financial statements. The tax position is measured as the largest amount of benefit that is greater than 50 percent likely of being realized upon ultimate resolution with a taxing authority. Uncertain tax positions are evaluated based upon the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based upon new information may lead to changes in recognition, de-recognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. The Company recognizes in income tax expense, interest and penalties, if any, related to unrecognized tax benefits.
	Tax law requires items be included in the Company's tax returns at different times than the items are reflected in the financial statements. As a result, the annual tax expense reflected in the consolidated statements of income is different than that reported in the income tax returns. Some of these differences are permanent, such as expenses that are not deductible in the returns, and some differences are temporary and reverse over time, such as depreciation expense. Temporary differences create deferred tax assets and liabilities. Deferred tax assets generally represent items that can be used as a tax deduction or credit in tax returns in future years for which benefit has already been recorded in the financial statements. Valuation allowances are established for deferred tax assets when it is estimated that future taxable income will be insufficient to use a deduction or credit in that jurisdiction. Deferred tax liabilities generally represent tax expense recognized in the financial statements for which payment has been deferred, or expense for which a deduction has been taken already in the tax return but the expense has not yet been recognized in the financial statements.
Derivative Instruments	Derivative Instruments:  All derivatives, whether designated in hedging relationships or not, are recorded on the balance sheet at fair value. If the derivative is designated as a fair value hedge, the changes in the fair value of the derivative and of the hedged item attributable to the hedged risk are recognized in earnings. If the derivative is designated as a cash flow hedge, the effective portions of changes in the fair value of the derivative are recorded in other comprehensive income and are recognized in the income statement when the hedged item affects earnings. Changes in the fair value attributable to the ineffective portion of cash flow hedges are recognized in earnings.
Concentrations Of Credit Risk	Concentrations of Credit Risk:  Financial instruments which potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents, commissions and fees receivable and insurance recoverable. The Company maintains a policy providing for the diversification of cash and cash equivalent investments and places its investments in a large number of high quality financial institutions to limit the amount of credit risk exposure. Concentrations of credit risk with respect to receivables are generally limited due to the large number of clients and markets in which the Company does business, as well as the dispersion across many geographic areas.
Per Share Data	Per Share Data: Basic net income per share attributable to the Company and income from continuing operations per share are calculated by dividing the respective after-tax income attributable to common shares by the weighted average number of outstanding shares of the Company’s common stock.
	Diluted net income per share attributable to the Company and income from continuing operations per share are calculated by dividing the respective after-tax income attributable to common shares by the weighted average number of outstanding shares of the Company’s common stock, which have been adjusted for the dilutive effect of potentially issuable common shares.
Estimates	Estimates:  The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results may vary from those estimates.
New Accounting Pronouncements	New Accounting Pronouncements:  In June 2014, the FASB issued new accounting guidance to clarify the treatment of share-based payment awards that require a specific performance target to be achieved in order for employees to be eligible to vest in the awards which include terms that may provide that the performance conditions could be achieved after an employee completes the requisite service period. The guidance requires that a performance target that affects vesting and that could be achieved after the requisite service period be treated as a performance condition. As such, a reporting entity should apply the existing guidance as it relates to awards with performance conditions that affect vesting. The guidance is effective for annual periods beginning after December 15, 2015. Earlier adoption is permitted. Adoption of the guidance is not expected to materially affect the Company's financial condition, results of operations or cash flows.
	In May 2014, the FASB issued new accounting guidance to clarify the principles for revenue recognition. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that principle, the entity should apply the following steps: identify the contract(s) with the customer, identify the performance obligations in the contract(s), determine the transaction price, allocate the transaction price to the performance obligations in the contract and recognize revenue when (or as) the entity satisfies a performance obligation. The guidance is effective for fiscal years beginning after December 15, 2016, including interim periods within that reporting period. Early application is not permitted. Entities are permitted to adopt the guidance under one of the following methods: retrospectively to each prior reporting period presented (with certain practical expedients allowed) or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application. If an entity elects this transition method, it must provide disclosures in reporting periods that include the date of initial application of the amount by which each financial statement line item is affected in the current reporting period by application of the guidance as compared to guidance that was in effect before the change, and an explanation for the reasons for significant changes. The Company is currently evaluating the impact of the adoption of the guidance on its financial condition and results of operations.
	In April 2014, the FASB issued new accounting guidance which changes the criteria for reporting discontinued operations and enhances disclosures in this area. Under the new guidance, only disposals representing a strategic shift in operations, such as disposal of a major geographic area or a major line of business, should be presented as discontinued operations. Those strategic shifts should have a major impact on the organization's operations and financial results. In addition, the new guidance requires expanded disclosures about discontinued operations. The guidance is effective for fiscal years beginning on or after December 15, 2014. Adoption of the guidance is not expected to have a material affect on the Company's financial condition, results of operations or cash flows.
	In July 2013, the FASB issued new accounting guidance related to the presentation of unrecognized tax benefits as a reduction to a deferred tax asset for a net operating loss carry-forward, a similar tax loss, or a tax credit carry-forward. However, to the extent a net operating loss carry-forward, a similar tax loss, or a tax credit carry-forward is not available at the reporting date under the tax law of the applicable jurisdiction to settle taxes that would result from the disallowance of the tax position or the entity does not intend to use the deferred tax asset for this purpose (provided that the tax law permits a choice), the unrecognized tax benefit shall be presented in the financial statement as a liability and shall not be combined with deferred tax assets. The guidance was effective for fiscal years beginning after December 15, 2013. The adoption of this new guidance impacted balance sheet classification and footnote disclosure only and did not have a material impact on the Company's financial statements.
	In June 2013, the FASB issued new accounting guidance which amends the criteria for an entity to qualify as an investment company. The guidance clarifies the characteristics of an investment company, provides comprehensive guidance to determine whether an entity is an investment company and sets measurement and disclosure requirements for investment companies. The guidance is effective for interim and annual reporting periods beginning after December 15, 2013. The adoption of this guidance did not materially affect the Company's financial position, results of operations or cash flows.
	In February 2013, the FASB issued new accounting guidance that adds new disclosure requirements for items reclassified out of accumulated other comprehensive income. The Company implemented this new guidance for the reporting period ended March 31, 2013. Other than enhanced disclosure, the adoption of this new guidance did not have a material effect on the Company's financial statements.
Reclassifications	Reclassifications: In the first quarter of 2014, the Company enhanced its operating cash flow presentation within the statements of cash flows to show on single lines the impact of pension and other benefit plan contributions in excess of the related expenses, and the non-cash impact of equity share awards. Previously, the cash flow impact of those items was presented as part of changes in other assets and other liabilities, and changes in other liabilities, respectively. The prior years' presentation was conformed to the current presentation for the following line items within operating cash flows:
	•	Share-based compensation expense
	•	Changes in other assets
	•	Contributions to pension and other benefit plans in excess of current year expense/credit
	•	Changes in other liabilities

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Components of Fixed Assets	The components of fixed assets are as follows:
	December 31,
	(In millions of dollars)		2014				2013
	Furniture and equipment		$	1,193			$	1,201
	Land and buildings		401				408
	Leasehold and building improvements		854				816
			2,448				2,425
	Less-accumulated depreciation and amortization		(1,639		)		(1,597		)
			$	809			$	828
Diluted Earnings per Share for Continuing Operations	Reconciliations of the applicable income components used for diluted EPS - Continuing Operations and basic weighted average common shares outstanding to diluted weighted average common shares outstanding are presented below. The reconciling items related to the calculation of diluted weighted average common shares outstanding are the same for net income attributable to the Company.
	Basic and Diluted EPS Calculation - Continuing Operations
	(In millions, except per share figures)	2014				2013				2012
	Net income from continuing operations	$	1,471			$	1,379			$	1,204
	Less: Net income attributable to non-controlling interests	32				28				25
		$	1,439			$	1,351			$	1,179
	Basic weighted average common shares outstanding	545				549				544
	Dilutive effect of potentially issuable common shares	8				9				8
	Diluted weighted average common shares outstanding	553				558				552
	Average stock price used to calculate common stock equivalents	$	51.15			$	40.97			$	33.1

Supplemental_Disclosures_Table

Supplemental Disclosures (Tables)	12 Months Ended
	Dec. 31, 2014
Supplemental Cash Flow Information [Abstract]
Additional Information Concerning Acquisitions, Interest and Income Taxes Paid	The following schedule provides additional information concerning acquisitions, interest and income taxes paid:
	(In millions of dollars)	2014			2013			2012
	Assets acquired, excluding cash	$	815		$	217		$	380
	Released from escrow in 2012	—			—			(62		)
	Liabilities assumed	(64		)	(53		)	(42		)
	Contingent/deferred purchase consideration	(197		)	(39		)	(46		)
	Net cash outflow for current year acquisitions	554			125			230
	Purchase of other intangibles	—			2			3
	Net cash outflow for acquisitions	$	554		$	127		$	233
	(In millions of dollars)	2014			2013			2012
	Interest paid	$	172		$	170		$	183
	Income taxes paid, net of refunds	$	426		$	360		$	350
Analysis of Allowance For Doubtful Accounts	An analysis of the allowance for doubtful accounts is as follows:
	For the Year Ended December 31,
	(In millions of dollars)	2014			2013			2012
	Balance at beginning of year	$	98		$	106		$	105
	Provision charged to operations	20			16			11
	Accounts written-off, net of recoveries	(17		)	(19		)	(12		)
	Effect of exchange rate changes and other	(6		)	(5		)	2
	Balance at end of year	$	95		$	98		$	106

Other_Comprehensive_Income_Los1

Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The components of accumulated other comprehensive income (loss) are as follows:
	(In millions of dollars)	December 31, 2014				31-Dec-13
	Foreign currency translation adjustments (net of deferred tax (asset) liability of $(5) and $7 in 2014 and 2013, respectively)	$	(459	)		$	56
	Net unrealized investment gains (net of deferred tax liability of $2 in both 2014 and 2013)	5				5
	Net charges related to pension / post-retirement plans (net of deferred tax asset of $1,587 and $1,213 in 2014 and 2013, respectively)	(3,393		)		(2,682		)
		$	(3,847	)		$	(2,621	)
	The changes in the balances of each component of Accumulated Other Comprehensive Income ("AOCI") for the years ended December 31, 2014 and 2013, including amounts reclassified out of AOCI, are as follows:
	(In millions of dollars)	Unrealized Investment Gains				Pension/Post-Retirement Plans Gains (Losses)				Foreign Currency Translation Adjustments			Total
	Balance as of January 1, 2014	$	5			$	(2,682	)		$	56		$	(2,621	)
	Other comprehensive loss before reclassifications	—				(816		)		(515		)	(1,331		)
	Amounts reclassified from accumulated other comprehensive loss	—				105				—			105
	Net current period other comprehensive loss	—				(711		)		(515		)	(1,226		)
	Balance as of December 31, 2014	$	5			$	(3,393	)		$	(459	)	$	(3,847	)
	(In millions of dollars)	Unrealized Investment Gains				Pension/Post-Retirement Plans Gains (Losses)				Foreign Currency Translation Adjustments			Total
	Balance as of January 1, 2013	$	4			$	(3,451	)		$	140		$	(3,307	)
	Other comprehensive income (loss) before reclassifications	1				574				(84		)	491
	Amounts reclassified from accumulated other comprehensive loss	—				195				—			195
	Net current period other comprehensive income (loss)	1				769				(84		)	686
	Balance as of December 31, 2013	$	5			$	(2,682	)		$	56		$	(2,621	)
Schedule of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss) are as follows:
	For the Year Ended December 31,	2014
	(In millions of dollars)	Pre-Tax			Tax (Credit)			Net of Tax
	Foreign currency translation adjustments	$	(527	)	$	(12	)	$	(515	)
	Pension/post-retirement plans:
	Amortization of prior service credits	(16		)	(5		)	(11		)
	Amortization of net actuarial losses	242			74			168
	Curtailment gain	(65		)	(13		)	(52		)
	Losses included in periodic pension cost	161			56			105
	Net losses arising during period	(1,418		)	(466		)	(952		)
	Foreign currency translation adjustments	180			39			141
	Other adjustments	(8		)	(3		)	(5		)
	Pension/post-retirement plans losses	(1,085		)	(374		)	(711		)
	Other comprehensive loss	$	(1,612	)	$	(386	)	$	(1,226	)
	For the Year Ended December 31,	2013
	(In millions of dollars)	Pre-Tax			Tax (Credit)			Net of Tax
	Foreign currency translation adjustments	$	(86	)	$	(2	)	$	(84	)
	Unrealized investment gains	1			—			1
	Pension/post-retirement plans:
	Amortization of prior service credits	(22		)	(8		)	(14		)
	Amortization of net actuarial losses	317			108			209
	Losses included in periodic pension cost	295			100			195
	Net gains arising during period	898			339			559
	Foreign currency translation adjustments	27			8			19
	Other adjustments	(7		)	(3		)	(4		)
	Pension/post-retirement plans gains	1,213			444			769
	Other comprehensive income	$	1,128		$	442		$	686
	For the Year Ended December 31,	2012
	(In millions of dollars)	Pre-Tax			Tax (Credit)			Net of Tax
	Foreign currency translation adjustments	$	177		$	(5	)	$	182
	Unrealized investment losses	(1		)	1			(2		)
	Pension/post-retirement plans:
	Amortization of prior service credits	(31		)	(12		)	(19		)
	Amortization of net actuarial losses	270			90			180
	Losses included in periodic pension cost	239			78			161
	Net losses arising during period	(648		)	(217		)	(431		)
	Foreign currency translation adjustments	(113		)	(26		)	(87		)
	Other adjustments	75			17			58
	Pension/post-retirement plans losses	(447		)	(148		)	(299		)
	Other comprehensive loss	$	(271	)	$	(152	)	$	(119	)

Acquisitions_Tables

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Allocation Of Acquisition Costs	The following table presents the preliminary allocation of the acquisition cost to the assets acquired and liabilities assumed, based on their fair values:
	(In millions)	2014
	Cash	$	575
	Estimated fair value of deferred/contingent consideration	197
	Total consideration	$	772
	Allocation of purchase price:
	Cash and cash equivalents	$	21
	Accounts receivable, net	12
	Other current assets	1
	Property, plant, and equipment	5
	Intangible assets (primarily customer lists amortized over 10 years)	318
	Goodwill	472
	Other assets	7
	Total assets acquired	836
	Current liabilities	41
	Other liabilities	23
	Total liabilities assumed	64
	Net assets acquired	$	772
Pro-Forma Information	The unaudited pro-forma financial data is presented for illustrative purposes only and is not necessarily indicative of the operating results that would have been achieved if such acquisitions had occurred on the dates indicated, nor is it necessarily indicative of future consolidated results.
		Years Ended December 31,
	(In millions, except per share data)	2014		2013		2012
	Revenue	$	13,039	$	12,550	$	12,202
	Income from continuing operations	$	1,477	$	1,395	$	1,222
	Net income attributable to the Company	$	1,471	$	1,373	$	1,195
	Basic net income per share:
	– Continuing operations	$	2.65	$	2.49	$	2.2
	– Net income attributable to the Company	$	2.7	$	2.5	$	2.2
	Diluted net income per share:
	– Continuing operations	$	2.62	$	2.45	$	2.16
	– Net income attributable to the Company	$	2.66	$	2.46	$	2.16

Discontinued_Operations_Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]
Income Statement Data	Summarized Statements of Income data for discontinued operations is as follows:
	For the Years Ended December 31,
	(In millions of dollars)	2014		2013			2012
	Income (loss) from discontinued operations, net of tax	$	—	$	—		$	—
	Disposals of discontinued operations	42		(4		)	(2		)
	Income tax (credit) expense	16		(10		)	1
	Disposals of discontinued operations, net of tax	26		6			(3		)
	Discontinued operations, net of tax	$	26	$	6		$	(3	)
	Discontinued operations, net of tax per share
	– Basic	$	0.05	$	0.01		$	—
	– Diluted	$	0.04	$	0.01		$	—

Goodwill_And_Other_Intangibles1

Goodwill And Other Intangibles (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Changes in the Carrying Amount of Goodwill	Changes in the carrying amount of goodwill are as follows:
	(In millions of dollars)	2014			2013
	Balance as of January 1, as reported	$	6,893		$	6,792
	Goodwill acquired	472			113
	Other adjustments(a)	(124		)	(12		)
	Balance at December 31,	$	7,241		$	6,893
	(a)	Primarily due to the impact of foreign exchange in both years.
Amortized Intangible Assets	The gross cost and accumulated amortization at December 31, 2014 and 2013 are as follows:
	(In millions of dollars)	2014								2013
		Gross			Accumulated			Net		Gross		Accumulated		Net
		Cost			Amortization			Carrying		Cost		Amortization		Carrying
								Amount						Amount
	Amortized intangibles	$	1,177		$	485		$	692	$	888	$	416	$	472
Estimated Future Aggregate Amortization Expense	The estimated future aggregate amortization expense is as follows:
	For the Years Ending December 31,
	(In millions of dollars)
	2015	$	88
	2016	78
	2017	75
	2018	73
	2019	72
	Subsequent years	306
		$	692

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Taxes on income	For financial reporting purposes, income before income taxes includes the following components:
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013				2012
	Income before income taxes:
	U.S.	$	313			$	407			$	398
	Other	1,744				1,566				1,298
		$	2,057			$	1,973			$	1,696
	The expense (benefit) for income taxes is comprised of:
	Income taxes:
	Current–
	U.S. Federal	$	80			$	102			$	42
	Other national governments	369				264				336
	U.S. state and local	26				45				24
		475				411				402
	Deferred–
	U.S. Federal	27				12				(18		)
	Other national governments	62				149				89
	U.S. state and local	22				22				19
		111				183				90
	Total income taxes	$	586			$	594			$	492
Deferred income tax assets and liabilities	The significant components of deferred income tax assets and liabilities and their balance sheet classifications are as follows:
	December 31,
	(In millions of dollars)	2014				2013
	Deferred tax assets:
	Accrued expenses not currently deductible	$	572			$	570
	Differences related to non-U.S. operations (a)	119				140
	Accrued retirement benefits U.S.	638				297
	Net operating losses (b)	57				79
	Income currently recognized for tax	75				74
	Foreign tax credit carryforwards	109				157
	Other	84				90
		$	1,654			$	1,407
	Deferred tax liabilities:
	Differences related to non-U.S. operations	$131				$112
	Depreciation and amortization	307				273
	Accrued retirement & postretirement benefits - non-U.S. operations	41				89
	Other	5				5
		$	484			$	479
	(a)	Net of valuation allowances of $15 million in 2014 and $12 million in 2013.
	(b)	Net of valuation allowances of $82 million in 2014 and $70 million in 2013.
	December 31,
	(In millions of dollars)	2014				2013
	Balance sheet classifications:
	Current assets	$	521			$	482
	Other assets	$	876			$	626
	Current liabilities	$	28			$	18
	Other liabilities	$	199			$	162
U.S. Federal statutory income tax rate	A reconciliation from the U.S. Federal statutory income tax rate to the Company’s effective income tax rate is shown below.
	For the Years Ended December 31,	2014			2013			2012
	U.S. Federal statutory rate	35	%		35	%		35	%
	U.S. state and local income taxes—net of U.S. Federal income tax benefit	1.7			2.1			1.9
	Differences related to non-U.S. operations	(7.5	)		(6.0	)		(6.1	)
	Other	(0.7	)		(1.0	)		(1.8	)
	Effective tax rate	28.5	%		30.1	%		29	%
Unrecognized tax benefits	Following is a reconciliation of the Company’s total gross unrecognized tax benefits for the years ended December 31, 2014, 2013 and 2012:
	(In millions of dollars)	2014				2013				2012
	Balance at January 1,	$	128			$	117			$	143
	Additions, based on tax positions related to current year	13				16				26
	Additions for tax positions of prior years	3				35				35
	Reductions for tax positions of prior years	(29		)		(7		)		(41		)
	Settlements	(4		)		(3		)		(6		)
	Lapses in statutes of limitation	(14		)		(30		)		(40		)
	Balance at December 31,	$	97			$	128			$	117

Retirement_Benefits_Tables

Retirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Schedule of weighted average actuarial assumptions utilized defined benefit plans	The weighted average actuarial assumptions utilized for the non-U.S. defined and postretirement benefit plans as of the end of the year are as follows:
		Non-U.S. Pension						Non-U.S.
		Benefits						Postretirement Benefits
		2014			2013			2014			2013
	Weighted average assumptions:
	Discount rate (for expense)	4.55	%		4.33	%		4.8	%		4.45	%
	Expected return on plan assets	6.95	%		7.17	%		—			—
	Rate of compensation increase (for expense)	2.99	%		2.69	%		—			—
	Discount rate (for benefit obligation)	3.49	%		4.55	%		3.85	%		4.8	%
	Rate of compensation increase (for benefit obligation)	2.67	%		2.99	%		—			—
	The weighted average actuarial assumptions utilized in determining the above amounts for the U.S. defined benefit and other U.S. postretirement plans as of the end of the year are as follows:
		U.S. Pension						U.S. Postretirement Benefits
		Benefits
		2014			2013			2014			2013
	Weighted average assumptions:
	Discount rate (for expense)	5.3	%		4.45	%		4.99	%		4.25	%
	Expected return on plan assets	8.75	%		8.75	%		—			—
	Rate of compensation increase (for expense)	2	%		2	%		—			—
	Discount rate (for benefit obligation)	4.3	%		5.3	%		4.19	%		5.17	%
	Rate of compensation increase (for benefit obligation)	2	%		2	%		—			—
	The weighted average actuarial assumptions utilized for the U.S. and significant non-U.S. defined benefit plans and postretirement benefit plans are as follows:
		Pension						Postretirement
		Benefits						Benefits
		2014			2013			2014			2013
	Weighted average assumptions:
	Discount rate (for expense)	4.82	%		4.38	%		4.92	%		4.32	%
	Expected return on plan assets	7.52	%		7.68	%		—			—
	Rate of compensation increase (for expense)	2.64	%		2.43	%		—			—
	Discount rate (for benefit obligation)	3.79	%		4.82	%		4.08	%		5.03	%
	Rate of compensation increase (for benefit obligation)	2.42	%		2.64	%		—			—
Schedule of components of net periodic benefit cost for U.S. defined benefit and other postretirement benefit plans	The components of the net periodic benefit cost for the U.S. defined benefit and other postretirement benefit plans are as follows:
	U.S. Plans only	Pension											Postretirement
	For the Years Ended December 31,	Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Service cost	$	91			$	104			$	93		$	2		$	3		$	3
	Interest cost	253				229				230			7			7			8
	Expected return on plan assets	(346		)		(324		)		(322		)	—			—			—
	Amortization of prior service credit	(7		)		(16		)		(16		)	—			—			(13		)
	Recognized actuarial loss (credit)	112				207				152			(2		)	—			(1		)
	Net periodic benefit cost (credit)	$	103			$	200			$	137		$	7		$	10		$	(3	)
	The components of the net periodic benefit cost for the non-U.S. defined benefit and other postretirement benefit plans and the curtailment, settlement and termination expenses are as follows:
	For the Years Ended December 31,	Non-U.S. Pension											Non-U.S. Postretirement
		Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Service cost	$	122			$	148			$	147		$	2		$	2		$	2
	Interest cost	388				352				366			4			4			5
	Expected return on plan assets	(644		)		(587		)		(583		)	—			—			—
	Amortization of prior service cost	(9		)		(6		)		(3		)	—			—			(1		)
	Recognized actuarial loss	131				108				118			1			2			1
	Net periodic benefit (credit) cost	(12		)		15				45			7			8			7
	Settlement loss	—				—				1			—			—			—
	Curtailment gain	(65		)		—				(1		)	—			—			—
	Total (credit) cost	$	(77	)		$	15			$	45		$	7		$	8		$	7
	The components of the net periodic benefit cost for defined benefit and other postretirement plans are as follows:
	Combined U.S. and significant non-U.S. Plans	Pension											Postretirement
	For the Years Ended December 31,	Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Service cost	$	213			$	252			$	240		$	4		$	5		$	5
	Interest cost	641				581				596			11			11			13
	Expected return on plan assets	(990		)		(911		)		(905		)	—			—			—
	Amortization of prior service credit	(16		)		(22		)		(19		)	—			—			(14		)
	Recognized actuarial loss (credit)	243				315				270			(1		)	2			—
	Net periodic benefit cost	$	91			$	215			$	182		$	14		$	18		$	4
	Curtailment gain	(65		)		—				—			—			—			—
	Total cost	$	26			$	215			$	182		$	14		$	18		$	4
Schedule of MMC's defined benefit plans and postretirement plans	The following schedules provide information concerning the Company’s non-U.S. defined benefit pension plans and non-U.S. postretirement benefit plans:
		Non-U.S. Pension								Non-U.S.
		Benefits								Postretirement Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Change in benefit obligation:
	Benefit obligation at beginning of year	$	8,711			$	8,579			$	97		$	107
	Service cost	122				148				2			2
	Interest cost	388				352				4			4
	Employee contributions	10				11				—			—
	Actuarial loss (gain)	1,619				(53		)		(1		)	(8		)
	Plan amendments	13				—				—			—
	Effect of settlement	(11		)		(2		)		—			—
	Benefits paid	(311		)		(293		)		(3		)	(4		)
	Foreign currency changes	(585		)		(31		)		(6		)	(4		)
	Other	62				—				—			—
	Benefit obligation December 31	$	10,018			$	8,711			$	93		$	97
	Change in plan assets:
	Fair value of plan assets at beginning of year	$	9,351			$	8,312			$	—		$	—
	Actual return on plan assets	1,756				698				—			—
	Effect of settlement	(11		)		(2		)		—			—
	Company contributions	156				620				3			4
	Employee contributions	10				11				—			—
	Benefits paid	(311		)		(293		)		(3		)	(4		)
	Foreign currency changes	(578		)		5				—			—
	Other	37				—				—			—
	Fair value of plan assets, December 31	$	10,410			$	9,351			$	—		$	—
	Net funded status, December 31	$	392			$	640			$	(93	)	$	(97	)
	Amounts recognized in the consolidated balance sheets:
	Non-current assets	$	967			$	977			$	—		$	—
	Current liabilities	(6		)		(5		)		(4		)	(4		)
	Non-current liabilities	(569		)		(332		)		(89		)	(93		)
	Net asset (liability) recognized, December 31	$	392			$	640			$	(93	)	$	(97	)
	Amounts recognized in other comprehensive (loss) income:
	Prior service (cost) credit	$	(2	)		$	85			$	—		$	—
	Net actuarial loss	(3,215		)		(3,010		)		(14		)	(16		)
	Total recognized accumulated other comprehensive (loss) income, December 31	$	(3,217	)		$	(2,925	)		$	(14	)	$	(16	)
	Cumulative employer contributions in excess (deficient) of net periodic cost	3,609				3,565				(79		)	(81		)
	Net asset (liability) recognized in consolidated balance sheet, December 31	$	392			$	640			$	(93	)	$	(97	)
	Accumulated benefit obligation, December 31	$	9,731			$	8,413			$	—		$	—
		Non-U.S. Pension								Non-U.S.
		Benefits								Postretirement Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of prior service credit (cost):
	Beginning balance	$	85			$	93			$	—		$	—
	Recognized as component of net periodic benefit credit	(9		)		(6		)		—			—
	Effect of curtailment	(65		)		—				—			—
	Changes in plan assets and benefit obligations recognized in other comprehensive income:
	Plan amendments	(13		)		—				—			—
	Exchange rate adjustments	—				(2		)		—			—
	Prior service (cost) credit, December 31	$	(2	)		$	85			$	—		$	—
		Non-U.S. Pension								Non-U.S.
		Benefits								Postretirement Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of net actuarial (loss) gain:
	Beginning balance	$	(3,010	)		$	(3,309	)		$	(16	)	$	(27	)
	Recognized as component of net periodic benefit cost	131				108				1			2
	Effect of settlement	—				—				—			—
	Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
	Liability experience	(1,619		)		53				1			8
	Asset experience	1,112				111				—			—
	Other	(14		)		—				—			—
	Total amount recognized as change in plan assets and benefit obligations	(521		)		164				1			8
	Exchange rate adjustments	185				27				—			1
	Net actuarial loss, December 31	$	(3,215	)		$	(3,010	)		$	(14	)	$	(16	)
For the Years Ended December 31,	Non-U.S. Pension											Non-U.S. Postretirement
	Benefits											Benefits
(In millions of dollars)	2014				2013				2012			2014			2013			2012
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	201			$	(276	)		$	246		$	5		$	(2	)	$	16
The following schedules provide information concerning the Company’s U.S. defined benefit pension plans and postretirement benefit plans:
	U.S. Pension								U.S.  Postretirement
	Benefits								Benefits
(In millions of dollars)	2014				2013				2014			2013
Change in benefit obligation:
Benefit obligation at beginning of year	$	4,827			$	5,197			$	158		$	176
Service cost	91				104				2			3
Interest cost	253				229				7			7
Employee contributions	—				—				13			—
Plan amendments	—				—				(4		)	—
Plan combination	—				36				—			—
Actuarial loss (gain)	955				(547		)		21			(15		)
Medicare Part D subsidy	—				—				1			1
Benefits paid	(202		)		(192		)		(21		)	(14		)
Benefit obligation, December 31	$	5,924			$	4,827			$	177		$	158
Change in plan assets:
Fair value of plan assets at beginning of year	$	4,279			$	3,936			$	—		$	—
Plan combination	—				21				—			—
Actual return on plan assets	414				488				—			—
Employer contributions	25				26				13			13
Employee contributions	—				—				13			—
Medicare Part D subsidy	—				—				1			1
Benefits paid	(202		)		(192		)		(21		)	(14		)
Other	—				—				12			—
Fair value of plan assets, December 31	$	4,516			$	4,279			$	18		$	—
Net funded status, December 31	$	(1,408	)		$	(548	)		$	(159	)	$	(158	)
Amounts recognized in the consolidated balance sheets:
Current liabilities	$	(25	)		$	(24	)		$	(2	)	$	(8	)
Noncurrent liabilities	(1,383		)		(524		)		(157		)	(150		)
Net liability recognized, December 31	$	(1,408	)		$	(548	)		$	(159	)	$	(158	)
Amounts recognized in other comprehensive income (loss):
Prior service credit	$	—			$	7			$	4		$	—
Net actuarial (loss) gain	(1,749		)		(974		)		2			13
Total recognized accumulated other comprehensive (loss) income, December 31	$	(1,749	)		$	(967	)		$	6		$	13
Cumulative employer contributions in excess (deficient) of net periodic cost	341				419				(165		)	(171		)
Net amount recognized in consolidated balance sheet	$	(1,408	)		$	(548	)		$	(159	)	$	(158	)
Accumulated benefit obligation at December 31	$	5,825			$	4,753			$	—		$	—
	U.S. Pension								U.S.  Postretirement
	Benefits								Benefits
(In millions of dollars)	2014				2013				2014			2013
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	7			$	23			$	—		$	—
Recognized as component of net periodic benefit cost	(7		)		(16		)		—			—
Plan amendment	—				—				4			—
Prior service credit, December 31	$	—			$	7			$	4		$	—
Schedule of total recognized in net periodic benefit cost and other comprehensive income (loss)
		U.S. Pension								U.S.  Postretirement
		Benefits								Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
	Beginning balance	$	(974	)		$	(1,887	)		$	13		$	(2	)
	Recognized as component of net periodic benefit cost (credit)	112				208				(2		)	—
	Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
	Liability experience	(955		)		541				(21		)	15
	Asset experience	68				164				12			—
	Total (loss) gain recognized as change in plan assets and benefit obligations	(887		)		705				(9		)	15
	Net actuarial (loss) gain, December 31	$	(1,749	)		$	(974	)		$	2		$	13
		Non-U.S. Pension								Non-U.S.
		Benefits								Postretirement Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of net actuarial (loss) gain:
	Beginning balance	$	(3,010	)		$	(3,309	)		$	(16	)	$	(27	)
	Recognized as component of net periodic benefit cost	131				108				1			2
	Effect of settlement	—				—				—			—
	Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
	Liability experience	(1,619		)		53				1			8
	Asset experience	1,112				111				—			—
	Other	(14		)		—				—			—
	Total amount recognized as change in plan assets and benefit obligations	(521		)		164				1			8
	Exchange rate adjustments	185				27				—			1
	Net actuarial loss, December 31	$	(3,215	)		$	(3,010	)		$	(14	)	$	(16	)
		Non-U.S. Pension								Non-U.S.
		Benefits								Postretirement Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of prior service credit (cost):
	Beginning balance	$	85			$	93			$	—		$	—
	Recognized as component of net periodic benefit credit	(9		)		(6		)		—			—
	Effect of curtailment	(65		)		—				—			—
	Changes in plan assets and benefit obligations recognized in other comprehensive income:
	Plan amendments	(13		)		—				—			—
	Exchange rate adjustments	—				(2		)		—			—
	Prior service (cost) credit, December 31	$	(2	)		$	85			$	—		$	—
		U.S. Pension								U.S.  Postretirement
		Benefits								Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
	Beginning balance	$	7			$	23			$	—		$	—
	Recognized as component of net periodic benefit cost	(7		)		(16		)		—			—
	Plan amendment	—				—				4			—
	Prior service credit, December 31	$	—			$	7			$	4		$	—
		U.S. Pension								U.S.  Postretirement
		Benefits								Benefits
	(In millions of dollars)	2014				2013				2014			2013
	Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
	Beginning balance	$	(974	)		$	(1,887	)		$	13		$	(2	)
	Recognized as component of net periodic benefit cost (credit)	112				208				(2		)	—
	Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
	Liability experience	(955		)		541				(21		)	15
	Asset experience	68				164				12			—
	Total (loss) gain recognized as change in plan assets and benefit obligations	(887		)		705				(9		)	15
	Net actuarial (loss) gain, December 31	$	(1,749	)		$	(974	)		$	2		$	13
	For the Years Ended December 31,	U.S. Pension											U.S. Postretirement
		Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	885			$	(696	)		$	346		$	14		$	(5	)	$	24
Schedule of amounts recognized in other comprehensive income (loss)
	For the Years Ended December 31,	Non-U.S. Pension											Non-U.S. Postretirement
		Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	201			$	(276	)		$	246		$	5		$	(2	)	$	16
	For the Years Ended December 31,	U.S. Pension											U.S. Postretirement
		Benefits											Benefits
	(In millions of dollars)	2014				2013				2012			2014			2013			2012
	Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	885			$	(696	)		$	346		$	14		$	(5	)	$	24
Schedule of estimated amounts that will be amortized from accumulated other comprehensive in the next fiscal year	Estimated amounts that will be amortized from accumulated other comprehensive income in the next fiscal year:
		Non-U.S. Pension				Non-U.S.
		Benefits				Postretirement Benefits
	(In millions of dollars)	2015				2015
	Prior service credit	$	(2	)		$	—
	Net actuarial loss	140				1
	Projected cost	$	138			$	1
	Estimated amounts that will be amortized from accumulated other comprehensive loss in the next fiscal year:
		U.S. Pension				U.S. Postretirement
		Benefits				Benefits
	(In millions of dollars)	2015				2015
	Prior service credit	$	—			$	1
	Net actuarial loss	181				1
	Projected cost	$	181			$	2
Schedule of change in assumed health care cost trend rates	A one percentage point change in assumed health care cost trend rates would have no effect on the total service and interest cost components or the postretirement benefit obligation.
	A one percentage point change in assumed health care cost trend rates would have the following effects:
	(In millions of dollars)	1 Percentage				1 Percentage
		Point Increase				Point Decrease
	Effect on total of service and interest cost components	$	1			$	(1	)
	Effect on postretirement benefit obligation	$	9			$	(7	)
Schedule of estimated future benefit payments for its pension and postretirement benefits	The Plans' estimated future benefit payments for its pension and postretirement benefits (without reduction for Medicare subsidy receipts) are as follows:
	For the Years Ended December 31,	Pension								Postretirement
		Benefits								Benefits
	(In millions of dollars)	U.S.				Non-U.S.				U.S.			Non-U.S.
	2015	$	217			$	264			$	10		$	4
	2016	$	232			$	280			$	10		$	4
	2017	$	250			$	295			$	10		$	4
	2018	$	263			$	312			$	10		$	4
	2019	$	275			$	325			$	10		$	4
	2020-2024	$	1,581			$	1,890			$	55		$	24
Summary of the U.S. and non-U.S. plans investments measured at fair value on a recurring basis	The following table sets forth, by level within the fair value hierarchy, a summary of the U.S. and non-U.S. plans' investments measured at fair value on a recurring basis at December 31, 2014 and 2013:
		Fair Value Measurements at December 31, 2014
	Assets (In millions of dollars)	Quoted Prices in				Significant				Significant			Total
		Active Markets				Other				Unobservable
		for Identical				Observable				Inputs
		Assets				Inputs				(Level 3)
		(Level 1)				(Level 2)
	Common/Collective trusts	$	172			$	6,766			$	184		$	7,122
	Corporate obligations	—				2,938				3			2,941
	Corporate stocks	2,087				6				1			2,094
	Private equity/partnerships	—				—				727			727
	Government securities	—				371				—			371
	Real estate	—				6				375			381
	Short-term investment funds	724				12				—			736
	Company common stock	229				—				—			229
	Other investments	16				23				239			278
	Total investments	$	3,228			$	10,122			$	1,529		$	14,879
		Fair Value Measurements at December 31, 2013
	Assets (In millions of dollars)	Quoted Prices in				Significant				Significant			Total
		Active Markets				Other				Unobservable
		for Identical				Observable				Inputs
		Assets				Inputs				(Level 3)
		(Level 1)				(Level 2)
	Common/Collective trusts	$	138			$	5,649			$	151		$	5,938
	Corporate obligations	—				2,330				4			2,334
	Corporate stocks	2,434				5				1			2,440
	Private equity/partnerships	—				2				799			801
	Government securities	10				340				2			352
	Real estate	—				7				312			319
	Short-term investment funds	824				15				—			839
	Company common stock	261				—				—			261
	Other investments	35				5				238			278
	Total investments	$	3,702			$	8,353			$	1,507		$	13,562
Summary of changes in the fair value of the plans' Level 3 assets	The tables below set forth a summary of changes in the fair value of the plans’ Level 3 assets for the years ended December 31, 2014 and December 31, 2013:
	Assets (In millions)	Fair Value,				Purchases				Sales			Unrealized			Realized			Exchange			Transfers			Fair
		1-Jan-14											Gain/			Gain/			Rate			in/(out)			Value, December 31, 2014
													(Loss)			(Loss)			Impact			and
																						Other
	Private equity/Partnerships	$	799			$	158			$	(185	)	$	(173	)	$	137		$	(12	)	$	3		$	727
	Real estate	312				97				(50		)	19			16			(19		)	—			375
	Other investments	238				21				(16		)	18			—			(28		)	6			239
	Common/Collective trusts	151				—				(1		)	50			—			(16		)	—			184
	Corporate stocks	1				—				—			—			—			—			—			1
	Corporate obligations	4				3				(1		)	—			—			—			(3		)	3
	Government securities	2				—				—			—			—			—			(2		)	—
	Total assets	$	1,507			$	279			$	(253	)	$	(86	)	$	153		$	(75	)	$	4		$	1,529
	Assets (In millions)	Fair Value,				Purchases				Sales			Unrealized			Realized			Exchange			Transfers			Fair
		1-Jan-13											Gain/			Gain/			Rate			in/(out)			Value,
													(Loss)			(Loss)			Impact			and			31-Dec-13
																						Other
	Private equity/Partnerships	$	824			$	146			$	(174	)	$	(155	)	$	150		$	(1	)	$	9		$	799
	Real estate	357				21				(95		)	6			26			(3		)	—			312
	Other investments	239				18				(13		)	10			—			6			(22		)	238
	Common/Collective trusts	—				61				—			(4		)	—			(5		)	99			151
	Corporate stocks	9				—				—			—			—			—			(8		)	1
	Corporate obligations	1				1				—			—			—			—			2			4
	Government securities	—				—				—			(1		)	—			—			3			2
	Total assets	$	1,430			$	247			$	(282	)	$	(144	)	$	176		$	(3	)	$	83		$	1,507

Stock_Benefit_Plans_Tables

Stock Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Option pricing valuation model for options granted	The assumptions used in the Black-Scholes option pricing valuation model for options granted by the Company in 2014, 2013 and 2012 are as follows:
		2014		2013		2012
	Risk-free interest rate	1.88%		1.03%-1.30%		1.26%-1.27%
	Expected life (in years)	6		6		6.5
	Expected volatility	24.20%		23.6%-24.1%		26.2%-26.4%
	Expected dividend yield	2.08%		2.48%-2.54%		2.76%-2.80%
Summary of the status of MMC's stock option awards	A summary of the status of the Company’s stock option awards as of December 31, 2014 and changes during the year then ended is presented below:
		Shares			Weighted			Weighted		Aggregate
					Average Exercise			Average		Intrinsic Value
					Price			Remaining		$0
								Contractual
								Term
	Balance at January 1, 2014	22,567,866			$	29.29
	Granted	1,716,637			$	48
	Exercised	(6,060,823	)		$	29.39
	Forfeited	(149,501	)		$	35.76
	Expired	(79,097	)		$	32.98
	Balance at December 31, 2014	17,995,082			$	30.97		5.1 years		$	488,937
	Options vested or expected to vest at December 31, 2014	17,709,073			$	30.9		5.1 years		$	482,479
	Options exercisable at December 31, 2014	12,440,781			$	27.74		3.9 years		$	378,296
Summary of restricted stock units and performance stock units	A summary of the status of the Company's restricted stock units and performance stock units ("PSU's") as of December 31, 2014 and changes during the period then ended is presented below:
		Restricted Stock Units					Performance Stock Units
		Shares		Weighted Average			Shares		Weighted Average Grant Date Fair Value
				Grant Date
				Fair Value
	Non-vested balance at January 1, 2014	4,250,809		$	32.04		961,163		$	32.87
	Granted	753,213		$	48.16		231,445		$	48
	Vested	(2,779,606	)	$	31.23		(613,734	)	$	30.73
	Forfeited	(126,686	)	$	34.85		(17,446	)	$	37.45
	Adjustment due to performance	—		$	—		306,580		$	30.72
	Non-vested balance at December 31, 2014	2,097,730		$	38.74		868,008		$	37.56
Summary of restricted stock awards activity	A summary of the status of the Company's restricted stock awards as of December 31, 2014 and changes during the period then ended is presented below:
		Shares			Weighted Average
					Grant Date
					Fair Value
	Non-vested balance at January 1, 2014	7,200			$	46.14
	Granted	—			$	—
	Vested	—			$	—
	Forfeited	—			$	—
	Non-vested balance at December 31, 2014	7,200			$	46.14

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following fair value hierarchy table presents information about the Company’s assets and liabilities measured at fair value on a recurring basis as of December 31, 2014 and 2013:
	(In millions of dollars)	Identical Assets						Observable Inputs						Unobservable				Total
		(Level 1)						(Level 2)						Inputs
														(Level 3)
		12/31/14			12/31/13			12/31/14				12/31/13		12/31/14		12/31/13		12/31/14		12/31/13
	Assets:
	Financial instruments owned:
	Mutual funds(a)	$	150		$	154		$	—			$	—	$	—	$	—	$	150	$	154
	Money market funds(b)	107			45			—				—		—		—		107		45
	Interest rate swap derivatives(c)	—			—			—				3		—		—		—		3
	Total assets measured at fair value	$	257		$	199		$	—			$	3	$	—	$	—	$	257	$	202
	Fiduciary Assets:
	Money market funds	57			—			—				—		—		—		57		—
	Total fiduciary assets measured at fair value	$	57		$	—		$	—			$	—	$	—	$	—	$	57	$	—
	Liabilities:
	Contingent purchase consideration liability(d)	$	—		$	—		$	—			$	—	$	207	$	104	$	207	$	104
	Senior Notes due 2014(e)	—			—			—				253		—		—		—		253
	Total liabilities measured at fair value	$	—		$	—		$	—			$	253	$	207	$	104	$	207	$	357
	(a) Included in other assets in the consolidated balance sheets.
	(b) Included in cash and cash equivalents in the consolidated balance sheets.
	(c) Included in other receivables in the consolidated balance sheets.
	(d) Included in accounts payable and accrued liabilities and other liabilities in the consolidated balance sheets.
	(e) Included in long-term debt in the consolidated balance sheets.
Changes in Fair Value of Level 3 Liabilities Representing Acquisition Related Contingent Consideration	The table below sets forth a summary of the changes in fair value of the Company’s Level 3 liabilities for the years ended December 31, 2014 and December 31, 2013 that represent contingent purchase consideration related to acquisitions:
	(In millions of dollars)	2014			2013
	Balance at January 1,	$	104		$	63
	Additions	114			26
	Payments	(42		)	(17		)
	Revaluation Impact	31			32
	Balance at December 31,	$	207		$	104
Equity Method Investments Summarized Financial Information	Below is a summary of the financial information for the Company's significant equity method investees:
	For the twelve months ended September 30,
	(In millions of dollars)		2014			2013			2012
	Revenue		$	239		$	148		$	144
	Net investment income		$	161		$	88		$	(19	)
	Net income		$	216		$	135		$	36
	As of September 30,
	(In millions of dollars)		2014			2013
	Total assets		$	25,497		$	741
	Total liabilities		$	24,209		$	136
	Non controlling interests		$	14		$	3

LongTerm_Commitments_Tables

Long-Term Commitments (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	At December 31, 2014, the aggregate future minimum rental commitments under all non-cancelable operating lease agreements are as follows:
	For the Years Ended December 31,	Gross		Rentals		Net
		Rental		from		Rental
	(In millions of dollars)	Commitments		Subleases		Commitments
	2015	$	365	$	48	$	317
	2016	$	334	$	48	$	286
	2017	$	293	$	45	$	248
	2018	$	258	$	42	$	216
	2019	$	212	$	35	$	177
	Subsequent years	$	1,010	$	36	$	974
Future Minimum Rental Commitments	At December 31, 2014, the aggregate fixed future minimum commitments under these agreements are as follows:
	For the Years Ended December 31,	Future
		Minimum
	(In millions of dollars)	Commitments
	2015	$	195
	2016	113
	2017	64
	Subsequent years	69
		$	441

Debt_Tables

Debt (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of Outstanding Debt	The Company’s outstanding debt is as follows:
	December 31,
	(In millions of dollars)	2014		2013
	Short-term:
	Current portion of long-term debt	$	11	$	334
	Long-term:
	Senior notes – 5.875% due 2033	297		297
	Senior notes – 5.375% due 2014	—		323
	Senior notes – 5.75% due 2015	—		230
	Senior notes – 2.30% due 2017	249		249
	Senior notes – 9.25% due 2019	—		399
	Senior notes – 4.80% due 2021	497		497
	Senior notes – 2.55% due 2018	249		248
	Senior notes – 4.05% due 2023	248		247
	Senior notes – 3.50% due 2024	595		—
	Senior notes – 2.35% due 2019	300		—
	Senior notes – 3.50% due 2025	498		—
	Mortgage – 5.70% due 2035	403		413
	Term Loan Facility – due 2016	50		50
	Other	1		2
		3,387		2,955
	Less current portion	11		334
		$	3,376	$	2,621
Estimated Fair Value Of Significant Financial Instruments	The fair value amounts shown below are not necessarily indicative of the amounts that the Company would realize upon disposition, nor do they indicate the Company’s intent or need to dispose of the financial instrument.
		December 31, 2014				December 31, 2013
	(In millions of dollars)	Carrying		Fair		Carrying		Fair
		Amount		Value		Amount		Value
	Short-term debt	$	11	$	11	$	334	$	334
	Long-term debt	$	3,376	$	3,493	$	2,621	$	2,819

Integration_and_Restructuring_1

Integration and Restructuring Costs (Tables)	12 Months Ended
	Dec. 31, 2014
Restructuring and Related Activities [Abstract]
Restructuring Activities	Details of the restructuring liability activity from January 1, 2013 through December 31, 2014, including actions taken prior to 2014 are as follows:
	(In millions of dollars)	Balance at		Expense		Cash			Other			Balance at		Expense		Cash			Other			Balance at
		1/1/13		Incurred		Paid						12/31/13		Incurred		Paid						12/31/14
	Severance	$	36	$	9	$	(33	)	$	(1	)	$	11	$	4	$	(8	)	$	—		$	7
	Future rent under non-cancelable leases and other costs	134		13		(32		)	(2		)	113		8		(35		)	(1		)	85
	Total	$	170	$	22	$	(65	)	$	(3	)	$	124	$	12	$	(43	)	$	(1	)	$	92

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Selected Information And Details For MMC's Operating Segments	Selected information about the Company’s segments and geographic areas of operation are as follows:
	For the Year Ended December 31, (In millions of dollars)	Revenue				Operating			Total				Depreciation		Capital
						Income			Assets				and		Expenditures
						(Loss)							Amortization
	2014 –
	Risk and Insurance Services	$	6,931		(a)	$	1,509		$	12,211			$	213	$	173
	Consulting	6,059			(b)	996			5,916				119		92
	Total Segments	12,990				2,505			18,127				332		265
	Corporate / Eliminations	(39		)		(204		)	(287		)	(c)	56		103
	Total Consolidated	$	12,951			$	2,301		$	17,840			$	388	$	368
	2013 –
	Risk and Insurance Services	$	6,596		(a)	$	1,421		$	11,365			$	192	$	158
	Consulting	5,701			(b)	845			5,178				115		155
	Total Segments	12,297				2,266			16,543				307		313
	Corporate / Eliminations	(36		)		(189		)	437			(c)	51		88
	Total Consolidated	$	12,261			$	2,077		$	16,980			$	358	$	401
	2012 –
	Risk and Insurance Services	$	6,350		(a)	$	1,334		$	9,832			$	196	$	131
	Consulting	5,613			(b)	692			5,203				113		117
	Total Segments	11,963				2,026			15,035				309		248
	Corporate / Eliminations	(39		)		(197		)	1,253			(c)	40		72
	Total Consolidated	$	11,924			$	1,829		$	16,288			$	349	$	320
	(a)	Includes inter-segment revenue of $4 million in 2014 and $5 million in both 2013 and 2012, interest income on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively, and equity method income of $9 million, $8 million and $11 million in 2014, 2013 and 2012, respectively.
	(b)	Includes inter-segment revenue of $35 million, $31 million and $34 million in 2014, 2013 and 2012, respectively, interest income on fiduciary funds of $6 million in 2014, $5 million in 2013 and $4 million 2012 and equity method income of $2 million in 2014, and $0 million in both 2013 and 2012.
	(c)	Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations.
Details of Operating Segment Revenue	Details of operating segment revenue are as follows:
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013			2012
	Risk and Insurance Services
	Marsh	$	5,774			$	5,461		$	5,265
	Guy Carpenter	1,157				1,135			1,085
	Total Risk and Insurance Services	6,931				6,596			6,350
	Consulting
	Mercer	4,350				4,241			4,147
	Oliver Wyman Group	1,709				1,460			1,466
	Total Consulting	6,059				5,701			5,613
	Total Segments	12,990				12,297			11,963
	Corporate / Eliminations	(39		)		(36		)	(39		)
	Total	$	12,951			$	12,261		$	11,924
Information by Geographic Area	Information by geographic area is as follows:
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013			2012
	Revenue
	United States	$	5,865			$	5,485		$	5,300
	United Kingdom	2,111				1,979			1,960
	Continental Europe	2,077				1,943			1,879
	Asia Pacific	1,420				1,396			1,346
	Other	1,517				1,494			1,478
		12,990				12,297			11,963
	Corporate/Eliminations	(39		)		(36		)	(39		)
		$	12,951			$	12,261		$	11,924
	For the Years Ended December 31,
	(In millions of dollars)	2014				2013			2012
	Fixed Assets, Net
	United States	$	483			$	494		$	494
	United Kingdom	120				121			121
	Continental Europe	60				64			63
	Asia Pacific	62				72			62
	Other	84				77			69
		$	809			$	828		$	809

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data and Supplemental Information (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Schedule of Quarterly Financial Data and Supplemental Information
		First			Second			Third			Fourth
		Quarter			Quarter			Quarter			Quarter
	(In millions, except per share figures)
	2014:00:00
	Revenue	$	3,264		$	3,300		$	3,141		$	3,246
	Operating income	$	673		$	647		$	445		$	536
	Income from continuing operations	$	457		$	440		$	305		$	269
	Discontinued operations, net of tax	$	(1	)	$	(2	)	$	(1	)	$	30
	Net income attributable to the Company	$	443		$	431		$	297		$	294
	Basic Per Share Data:
	Continuing operations	$	0.81		$	0.79		$	0.55		$	0.49
	Discontinued operations, net of tax	$	—		$	(0.01	)	$	—		$	0.05
	Net income attributable to the Company	$	0.81		$	0.78		$	0.55		$	0.54
	Diluted Per Share Data:
	Continuing operations	$	0.8		$	0.78		$	0.54		$	0.48
	Discontinued operations, net of tax	$	—		$	(0.01	)	$	—		$	0.06
	Net income attributable to the Company	$	0.8		$	0.77		$	0.54		$	0.54
	Dividends Paid Per Share	$	0.25		$	0.25		$	0.28		$	0.28
	2013:00:00
	Revenue	$	3,126		$	3,088		$	2,932		$	3,115
	Operating income	$	607		$	577		$	404		$	489
	Income from continuing operations	$	412		$	400		$	260		$	307
	Discontinued operations, net of tax	$	12		$	(5	)	$	(1	)	$	—
	Net income attributable to the Company	$	413		$	388		$	253		$	303
	Basic Per Share Data:
	Continuing operations	$	0.73		$	0.71		$	0.46		$	0.55
	Discontinued operations, net of tax	$	0.02		$	—		$	—		$	—
	Net income attributable to the Company	$	0.75		$	0.71		$	0.46		$	0.55
	Diluted Per Share Data:
	Continuing operations	$	0.72		$	0.7		$	0.45		$	0.54
	Discontinued operations, net of tax	$	0.02		$	(0.01	)	$	—		$	—
	Net income attributable to the Company	$	0.74		$	0.69		$	0.45		$	0.54
	Dividends Paid Per Share	$	0.23		$	0.23		$	0.25		$	0.25

Summary_of_Significant_Account3

Summary of Significant Accounting Policies (Narratives) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 24, 2015
	segment
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Indefinite-lived intangible assets	$0	$0
Schedule of Equity Method Investments [Line Items]
Number of business segments	2
Uncollected Premiums and Claims, Net of Related Payables	7,300,000,000	8,200,000,000
Restricted Cash and Cash Equivalents	179,000,000
Investment income	37,000,000	69,000,000	24,000,000
Capitalized computer software costs	501,000,000	399,000,000
Capitalized computer software costs accumulated amortization	837,000,000	748,000,000
Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Investment securities	6,000,000	13,000,000	33,000,000
Trident III [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees	31,000,000	41,000,000
Trident III [Member] | Performance Fees [Member]
Schedule of Equity Method Investments [Line Items]
Deferred Revenue	16,000,000	38,000,000
Trident II [Member] | Private Equity Funds [Member]
Schedule of Equity Method Investments [Line Items]
Recognition of deferred performance fees		15,000,000	0
Mercer Consulting Group [Member]
Schedule of Equity Method Investments [Line Items]
Assets in trusts or funds for management or trustee fee	21,000,000,000
Minimum [Member] | Software Development [Member]
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	3 years
Maximum [Member] | Software Development [Member]
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	10 years
Risk and Insurance Services Segment [Member]
Schedule of Equity Method Investments [Line Items]
Interest Income, Fiduciary Assets	24,000,000	27,000,000	39,000,000
Consulting Segment [Member]
Schedule of Equity Method Investments [Line Items]
Number of business groups	2
Interest Income, Fiduciary Assets	$6,000,000	$5,000,000	$4,000,000
Subsequent Event [Member] | Benefitfocus [Member]
Schedule of Equity Method Investments [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired				10.00%

Summary_of_Significant_Account4

Summary of Significant Accounting Policies (Components of Fixed Assets and Capitalized Software Costs) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,448	$2,425
Less-accumulated depreciation and amortization	-1,639	-1,597
Property, plant and equipment, net	809	828	809
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,193	1,201
Land and Building [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	401	408
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	854	$816
Minimum [Member] | Software Development [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Minimum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Minimum [Member] | Building [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	30 years
Maximum [Member] | Software Development [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Maximum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Maximum [Member] | Building [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	40 years

Summary_of_Significant_Account5

Summary of Significant Accounting Policies (Basic and Diluted EPS Calculation for Continuing Operations) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Income from continuing operations	$269	$305	$440	$457	$307	$260	$400	$412	$1,471	$1,379	$1,204
Less: Net income attributable to non-controlling interests									32	28	25
Net income attributable to the Company									$1,439	$1,351	$1,179
Basic weighted average common shares outstanding (in shares)									545,000,000	549,000,000	544,000,000
Dilutive effect of potentially issuable common shares (in shares)									8,000,000	9,000,000	8,000,000
Dilutive weighted average common shares outstanding (in shares)									553,000,000	558,000,000	552,000,000
Average stock price used to calculate common stock equivalents (in dollars per share)									$51.15	$40.97	$33.10
Stock options outstanding (in shares)	17,995,082				22,567,866				17,995,082	22,567,866	32,045,793

Supplemental_Disclosures_Narra

Supplemental Disclosures (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Abstract]
Deferred purchase consideration from prior years' acquisitions	$25	$15	$59
Non-cash issuance of common stock	108	150	193
Stock-based compensation expense, equity awards	$75	$110	$152

Supplemental_Disclosures_Sched

Supplemental Disclosures (Schedule of Supplemental Cash Flow Disclosures) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Abstract]
Assets acquired, excluding cash	$815	$217	$380
Released from escrow in 2012	0	0	-62
Liabilities assumed	-64	-53	-42
Contingent/deferred purchase consideration	-197	-39	-46
Net cash outflow for current year acquisitions	554	125	230
Purchase of other intangibles	0	2	3
Net cash outflow for acquisitions	554	127	233
Interest paid	172	170	183
Income taxes paid, net of refunds	$426	$360	$350

Supplemental_Disclosures_Sched1

Supplemental Disclosures (Schedule of Analysis for Doubtful Accounts) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$98	$106	$105
Provision charged to operations	20	16	11
Accounts written-off, net of recoveries	-17	-19	-12
Effect of exchange rate changes and other	-6	-5	2
Balance at end of year	$95	$98	$106

Other_Comprehensive_Income_Los2

Other Comprehensive Income (Loss) (Components of Accumulated Other Comprehensive Income Loss) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of January 1, 2014	($2,621)	($3,307)
Other comprehensive loss before reclassifications	-1,331	491
Amounts reclassified from accumulated other comprehensive loss	105	195
Other comprehensive (loss) income, net of tax	-1,226	686	-119
Balance as of December 31, 2014	-3,847	-2,621	-3,307
Foreign currency translation adjustments (net of deferred tax (asset) liability of $(5) and $7 in 2014 and 2013, respectively)	-459	56
Net unrealized investment gains (net of deferred tax liability of $2 in both 2014 and 2013)	5	5
Net charges related to pension / post-retirement plans (net of deferred tax asset of $1,587 and $1,213 in 2014 and 2013, respectively)	-3,393	-2,682
Accumulated other comprehensive income (loss)	-3,847	-2,621	-3,307
Foreign currency translation adjustments, tax portion	-5	7
Net unrealized investment gains, tax portion	2	2
Net charges related to pension/ retiree plans, tax portion	1,587	1,213
Unrealized Investment Gains [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of January 1, 2014	5	4
Other comprehensive loss before reclassifications	0	1
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive (loss) income, net of tax	0	1
Balance as of December 31, 2014	5	5
Accumulated other comprehensive income (loss)	5	5
Pension/Post-Retirement Plans Gains (Losses) [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of January 1, 2014	-2,682	-3,451
Other comprehensive loss before reclassifications	-816	574
Amounts reclassified from accumulated other comprehensive loss	105	195
Other comprehensive (loss) income, net of tax	-711	769
Balance as of December 31, 2014	-3,393	-2,682
Accumulated other comprehensive income (loss)	-3,393	-2,682
Foreign Currency Translation Adjustments [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance as of January 1, 2014	56	140
Other comprehensive loss before reclassifications	-515	-84
Amounts reclassified from accumulated other comprehensive loss	0	0
Other comprehensive (loss) income, net of tax	-515	-84
Balance as of December 31, 2014	-459	56
Accumulated other comprehensive income (loss)	($459)	$56

Other_Comprehensive_Income_Los3

Other Comprehensive Income (Loss) (Schedule Of Components Of Other Comprehensive Income (Loss)) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Foreign currency translation adjustments, Pre-Tax	($527)	($86)	$177
Foreign currency translation adjustments, Tax (Credit)	-12	-2	-5
Foreign currency translation adjustments, Net of Tax	-515	-84	182
Unrealized investment gains (losses), Pre-Tax	0	1	-1
Unrealized investment gains (losses), Tax (Credit)		0	1
Unrealized investment gains (losses), Net of Tax		1	-2
Pension/post-retirement plans:
Amortization of prior service credits, Pre-Tax	-16	-22	-31
Amortization of prior service credits, Tax (Credit)	-5	-8	-12
Amortization of prior service credits, Net of Tax	-11	-14	-19
Net actuarial losses, Pre-Tax	242	317	270
Net actuarial losses, Tax (Credit)	74	108	90
Net actuarial losses, Net of Tax	168	209	180
Curtailment gain, Pre-Tax	-65
Curtailment gain, Tax (Credit)	-13
Curtailment gain, Net of Tax	-52
Losses included in periodic pension cost, Pre-Tax	161	295	239
Losses including in periodic pension cost, Tax (Credit)	56	100	78
Losses including in periodic pension cost, Net of Tax	105	195	161
Net gain (loss) arising during period, Pre-Tax	-1,418	898	-648
Net gain (loss) arising during period, Tax (Credit)	-466	339	-217
Net gain (loss) arising during period, Net of Tax	-952	559	-431
Foreign currency translation adjustments, Pre-Tax	180	27	-113
Foreign currency translation adjustments, Tax (Credit)	39	8	-26
Foreign currency translation adjustments, Net of Tax	141	19	-87
Other adjustments. Pre-Tax	-8	-7	75
Other adjustments, Tax (Credit)	-3	-3	17
Other adjustments, Net of Tax	-5	-4	58
Pension/post-retirement plans gains (losses), Pre-Tax	-1,085	1,213	-447
Pension/post-retirement plans gains (losses), Tax (Credit)	-374	444	-148
Pension/post-retirement plans gains (losses), Net of Tax	-711	769	-299
Other comprehensive (loss) income, before tax	-1,612	1,128	-271
Other comprehensive income (loss), Tax (Credit)	-386	442	-152
Other comprehensive (loss) income, net of tax	($1,226)	$686	($119)

Acquisitions_Narrative_Details

Acquisitions (Narrative) (Details)

12 Months Ended

0 Months Ended

1 Months Ended

In Millions, except Per Share data, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2014

Oct. 01, 2014

Jul. 24, 2014

Jun. 30, 2014

Jul. 24, 2014

Oct. 31, 2014

USD ($)

USD ($)

USD ($)

Current Fiscal Period Acquisitions [Member]

Prior Fiscal Periods Acquisitions [Member]

Prior Fiscal Periods Acquisitions [Member]

Risk and Insurance Services Segment [Member]

Consulting Segment [Member]

Maximum [Member]

Maximum [Member]

Minimum [Member]

Minimum [Member]

Marsh Insurance Group [Member]

Mercer Consulting Group [Member]

Mercer Consulting Group [Member]

Mercer Consulting Group [Member]

Mercer Consulting Group [Member]

Mercer Consulting Group [Member]

Other Assets [Member]

Other Assets [Member]

USD ($)

USD ($)

USD ($)

acquisition

acquisition

acquisition

acquisition

Alexander Forbes Group Holdings Limited [Member]

Alexander Forbes Group Holdings Limited [Member]

Alexander Forbes Group Holdings Limited [Member]

Alexander Forbes Group Holdings Limited [Member]

Mercer Consulting Group [Member]

Mercer Consulting Group [Member]

USD ($)

ZAR

Alexander Forbes Group Holdings Limited [Member]

Alexander Forbes Group Holdings Limited [Member]

USD ($)

USD ($)

Business Acquisition [Line Items]

Number of acquisitions made

15

6

6

2

Total Consideration

$772

$178

$772

$137

$166

Cash

575

139

575

Estimated fair value of deferred/contingent consideration

197

39

46

197

Revenue target period (in years)

4 years

4 years

2 years

2 years

Deferred purchase consideration from prior years' acquisitions

25

15

59

25

Deferred purchase consideration

15

Contingent payments for acquisitions

42

17

Purchase of other intangibles

0

2

3

Revenue related to acquisitions

134

Net operating income related to acquisitions

18

Percentage of interest acquired

34.00%

Share price

7.5

Business combination remaining interest acquired

19.10%

14.90%

Value of carrying amount that exceeds share in equity method investment

$146

Acquisitions_Allocation_Of_Acq

Acquisitions (Allocation Of Acquisition Costs) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Intangible asset useful life	10 years
Cash	$575	$139
Estimated fair value of deferred/contingent consideration	197	39	46
Total consideration	772	178
Allocation of purchase price:
Goodwill	7,241	6,893	6,792
Current Fiscal Period Acquisitions [Member]
Business Acquisition [Line Items]
Cash	575
Estimated fair value of deferred/contingent consideration	197
Total consideration	772
Allocation of purchase price:
Cash and cash equivalents	21
Accounts receivable, net	12
Other current assets	1
Property, plant, and equipment	5
Intangible assets (primarily customer lists amortized over 10 years)	318
Goodwill	472
Other assets	7
Total assets acquired	836
Current liabilities	41
Other liabilities	23
Total liabilities assumed	64
Net assets acquired	$772

Acquisitions_ProForma_Informat

Acquisitions (Pro-Forma Information) (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Combinations [Abstract]
Revenue	$13,039	$12,550	$12,202
Income from continuing operations	1,477	1,395	1,222
Net income attributable to the Company	$1,471	$1,373	$1,195
Basic net income per share:
Basic net income per share - Continuing operations (in dollars per share)	$2.65	$2.49	$2.20
Basic net income per share - Net income attributable to the Company (in dollars per share)	$2.70	$2.50	$2.20
Diluted net income per share:
Diluted net income per share - Continuing operations (in dollars per share)	$2.62	$2.45	$2.16
Diluted net income per share - Net income attributable to the Company (in dollars per share)	$2.66	$2.46	$2.16

Discontinued_Operations_Income

Discontinued Operations (Income Statement Data) (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax	$0	$0	$0
Disposals of discontinued operations	42	-4	-2
Income tax (credit) expense	16	-10	1
Disposals of discontinued operations, net of tax	26	6	-3
Discontinued operations, net of tax	26	6	-3
Discontinued operations, net of tax per share
- Basic (in dollars per share)	$0.05	$0.01	$0
- Diluted (in dollars per share)	$0.04	$0.01	$0
Putnam [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (Loss) Related to Settlement of Indemnity Agreement, Net of Tax	$28

Goodwill_And_Other_Intangibles2

Goodwill And Other Intangibles (Narrative) (Details) (USD $)	12 Months Ended			3 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012
Segment Reporting Information [Line Items]
Goodwill acquired	$472	$113
Goodwill expected to be tax deductible	348
Goodwill	7,241	6,893	6,792
Intangible asset impairment charge	0	5	8
Finite lived assets amortization expense	86	72	72
Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Goodwill acquired	445
Goodwill	5,000
Intangible asset impairment charge				5
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Goodwill acquired	27
Goodwill	$2,200

Goodwill_And_Other_Intangibles3

Goodwill And Other Intangibles (Changes In The Carrying Amount Of Goodwill) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Goodwill [Roll Forward]
Balance as of January 1, as reported	$6,893		$6,792
Goodwill acquired	472		113
Other adjustments	-124	[1]	-12	[1]
Balance at December 31,	$7,241		$6,893
[1]	due to the impact of foreign exchange in both years.

Goodwill_And_Other_Intangibles4

Goodwill And Other Intangibles (Amortized Intangible Assets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Goodwill and Intangible Assets Disclosure [Abstract]
Gross Cost	$1,177	$888
Accumulated Amortization	485	416
Net Carrying Amount	$692	$472

Goodwill_And_Other_Intangibles5

Goodwill And Other Intangibles (Estimated Future Aggregate Amortization Expense) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Goodwill and Intangible Assets Disclosure [Abstract]
2015	$88
2016	78
2017	75
2018	73
2019	72
Subsequent years	306
Net Carrying Amount	$692	$472

Income_Taxes_Narrative_Details

Income Taxes (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Investments in foreign subsidiaries	$6,300,000,000
Effective tax rate	28.50%	30.10%	29.00%
Valuation allowances	15,000,000	10,000,000	23,000,000
Valuation allowances, beginning balance	-9,000,000	-3,000,000	16,000,000
Net Operating Loss Carryforwards	55.00%
Unrecognized tax benefits that would impact effective tax rate	51,000,000	71,000,000	96,000,000
Accrued interest and penalties	7,000,000	10,000,000	13,000,000
Unrecognized tax benefits minimum	0
Unrecognized tax benefits maximum	16,000,000
Tax Credit Carryforward [Line Items]
Putnam and issues included in consolidated mmc tax returns	97,000,000	128,000,000	117,000,000	143,000,000
Federal [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	3,000,000
State and Local [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	40,000,000
Non-U.S. [Member]
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	96,000,000
Putnam and Kroll [Member]
Tax Credit Carryforward [Line Items]
Putnam and issues included in consolidated mmc tax returns	$2,000,000	$2,000,000	$6,000,000

Income_Taxes_Taxes_on_Income_D

Income Taxes (Taxes on Income) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
U.S., income before income taxes	$313	$407	$398
Other, income before income taxes	1,744	1,566	1,298
Total income before income taxes	2,057	1,973	1,696
U.S. Federal, current	80	102	42
Other national governments, current	369	264	336
U.S. state and local, current	26	45	24
Total current income taxes	475	411	402
U.S. Federal, deferred	27	12	-18
Other national governments, deferred	62	149	89
U.S. state and local, deferred	22	22	19
Total deferred income taxes	111	183	90
Total income taxes	$586	$594	$492

Income_Taxes_Deferred_Income_T

Income Taxes (Deferred Income Tax Assets And Liabilities) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Deferred tax assets:
Accrued expenses not currently deductible	$572		$570
Differences related to non-U.S. operations	119	[1]	140	[1]
Net operating losses	57	[2]	79	[2]
Income currently recognized for tax	75		74
Foreign tax credit carryforwards	109		157
Other	84		90
Total, deferred tax assets	1,654		1,407
Deferred tax liabilities:
Differences related to non-U.S. operations	131		112
Depreciation and amortization	307		273
Accrued retirement & postretirement benefits - non-U.S. operations	41		89
Other	5		5
Total, deferred tax liabilities	484		479
Deferred Tax Assets Related To Non U S Operations [Member]
Deferred tax liabilities:
Valuation allowance, amount	15		12
Deferred Tax Assets Related To US Pension [Member]
Deferred tax assets:
Accrued retirement & postretirement benefits	638		297
Deferred Tax Assets Related To Net Operating Loss [Member]
Deferred tax liabilities:
Valuation allowance, amount	82		70
Current Assets [Member]
Balance sheet classifications:
Current assets	521		482
Other Noncurrent Assets [Member]
Balance sheet classifications:
Other assets	876		626
Other Current Liabilities [Member]
Balance sheet classifications:
Current liabilities	28		18
Other Noncurrent Liabilities [Member]
Balance sheet classifications:
Other liabilities	$199		$162
[1]	Net of valuation allowances of $15 million in 2014 and $12 million in 2013.
[2]	Net of valuation allowances of $82 million in 2014 and $70 million in 2013.

Income_Taxes_US_Federal_Statut

Income Taxes (U.S. Federal Statutory Rate) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
U.S. Federal statutory rate	35.00%	35.00%	35.00%
U.S. state and local income taxes—net of U.S. Federal income tax benefit	1.70%	2.10%	1.90%
Differences related to non-U.S. operations	-7.50%	-6.00%	-6.10%
Other	-0.70%	-1.00%	-1.80%
Effective tax rate	28.50%	30.10%	29.00%

Income_Taxes_Unrecognized_Tax_

Income Taxes (Unrecognized Tax Benefits) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1,	$128	$117	$143
Additions, based on tax positions related to current year	13	16	26
Additions for tax positions of prior years	3	35	35
Reductions for tax positions of prior years	-29	-7	-41
Settlements	-4	-3	-6
Lapses in statutes of limitation	-14	-30	-40
Balance at December 31,	$97	$128	$117

Retirement_Benefits_Narrative_

Retirement Benefits (Narrative) (Details)

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

Share data in Millions, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Jan. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Feb. 26, 2015

Feb. 26, 2015

Dec. 31, 2014

USD ($)

USD ($)

U.S. Pension Benefits [Member]

U.S. Pension Benefits [Member]

U.S. Pension Benefits [Member]

Non- U.S Pension Benefits [Member]

Non- U.S Pension Benefits [Member]

Non- U.S Pension Benefits [Member]

Non- U.S Pension Benefits [Member]

U.S. Postretirement Benefits [Member]

U.S. Postretirement Benefits [Member]

U.S. Postretirement Benefits [Member]

United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

United States Nonqualified Pension Plan of US Entity, Defined Benefit [Member]

Equity Funds [Member]

Fixed Income Funds [Member]

United Kingdom [Member]

United Kingdom [Member]

United Kingdom [Member]

United Kingdom [Member]

United Kingdom [Member]

United States [Member]

United States [Member]

United States [Member]

Geographic Concentration Risk [Member]

Trident III [Member]

Subsequent Event [Member]

Subsequent Event [Member]

Pension Plan Funded Status Guarantee [Member]

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]

United States Pension and Postretirement Benefit Plan of US Entity, Defined Benefit [Member]

USD ($)

USD ($)

USD ($)

Equity Funds [Member]

Fixed Income Funds [Member]

USD ($)

USD ($)

USD ($)

United Kingdom [Member]

United Kingdom [Member]

United Kingdom [Member]

United Kingdom [Member]

Non- U.S Pension Benefits [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

Equity Funds [Member]

Equity Funds [Member]

Fixed Income Funds [Member]

GBP (£)

Non- U.S Pension Benefits [Member]

Non- U.S Pension Benefits [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]

USD ($)

Defined Benefit Plan Disclosure [Line Items]

Market-related plan assets value, recognition period of investment gains and losses

5 years

Actual plan asset allocations

59.00%

41.00%

43.00%

57.00%

Target asset allocation percentage for non-U.S. plan assets, U.K. Plans

83.00%

Defined benefit plan target allocation percentage of assets equity alternative investments range, minimum

57.00%

33.00%

47.00%

47.00%

Defined benefit plan target allocation percentage of assets equity alternative investments range, maximum

67.00%

43.00%

53.00%

53.00%

Target asset allocation

62.00%

38.00%

50.00%

50.00%

48.00%

52.00%

The projected benefit obligation with accumulated benefit obligations

$5,900,000,000

$4,800,000,000

$2,100,000,000

$1,700,000,000

Accumulated benefit obligation

5,800,000,000

4,800,000,000

2,000,000,000

1,500,000,000

Aggregate fair value of plan assets

4,500,000,000

4,300,000,000

1,600,000,000

1,300,000,000

Plans with benefit obligations in excess of plan assets, aggregate benefit obligation

5,900,000,000

4,800,000,000

2,200,000,000

1,700,000,000

Plans with benefit obligations in excess of plan assets, aggregate fair value of plan assets

4,500,000,000

4,300,000,000

1,600,000,000

1,300,000,000

Number of shares the U.S. qualified plan holds contributed by the Company

4

Shares of common stock representing percentage of plan assets

5.00%

Health care cost trend rate assumed for next fiscal year

5.76%

7.55%

Ultimate assumed health care cost trend rate

4.92%

4.50%

Assumed health care cost trend rate

5.00%

Estimated future contributions (less than $1 million for U.S. tax-qualified plan)

169,000,000

25,000,000

Limited partnership interest in private equity fund

14,879,000,000

13,562,000,000

4,516,000,000

4,279,000,000

3,936,000,000

10,410,000,000

9,351,000,000

8,312,000,000

18,000,000

0

0

53,000,000

Effect of plan amendment on projected benefit obligation

147,000,000

Effect of plan amendment on funded status

137,000,000

Maximum exposure under guarantee

450,000,000

Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]

ESOP invested in MMC common stock

453,000,000

453,000,000

The cost of defined contribution plans

$65,000,000

$23,000,000

$21,000,000

$49,000,000

$50,000,000

$50,000,000

Guarantor Obligation, Period

7 years

Retirement_Benefits_Weighted_A

Retirement Benefits (Weighted Average Actuarial Assumptions Utilized) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Pension Plan [Member]
Weighted average assumptions:
Discount rate (for expense)	4.82%	4.38%
Expected return on plan assets	7.52%	7.68%
Rate of compensation increase (for expense)	2.64%	2.43%
Discount rate (for benefit obligation)	3.79%	4.82%
Rate of compensation increase (for benefit obligation)	2.42%	2.64%
U.S. Pension Benefits [Member]
Weighted average assumptions:
Discount rate (for expense)	5.30%	4.45%
Expected return on plan assets	8.75%	8.75%
Rate of compensation increase (for expense)	2.00%	2.00%
Discount rate (for benefit obligation)	4.30%	5.30%
Rate of compensation increase (for benefit obligation)	2.00%	2.00%
Non- U.S Pension Benefits [Member]
Weighted average assumptions:
Discount rate (for expense)	4.55%	4.33%
Expected return on plan assets	6.95%	7.17%
Rate of compensation increase (for expense)	2.99%	2.69%
Discount rate (for benefit obligation)	3.49%	4.55%
Rate of compensation increase (for benefit obligation)	2.67%	2.99%
Postretirement Benefits [Member]
Weighted average assumptions:
Discount rate (for expense)	4.92%	4.32%
Expected return on plan assets	0.00%	0.00%
Rate of compensation increase (for expense)	0.00%	0.00%
Discount rate (for benefit obligation)	4.08%	5.03%
Rate of compensation increase (for benefit obligation)	0.00%	0.00%
U.S. Postretirement Benefits [Member]
Weighted average assumptions:
Discount rate (for expense)	4.99%	4.25%
Expected return on plan assets	0.00%	0.00%
Rate of compensation increase (for expense)	0.00%	0.00%
Discount rate (for benefit obligation)	4.19%	5.17%
Rate of compensation increase (for benefit obligation)	0.00%	0.00%
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Weighted average assumptions:
Discount rate (for expense)	4.80%	4.45%
Expected return on plan assets	0.00%	0.00%
Rate of compensation increase (for expense)	0.00%	0.00%
Discount rate (for benefit obligation)	3.85%	4.80%
Rate of compensation increase (for benefit obligation)	0.00%	0.00%

Retirement_Benefits_Components

Retirement Benefits (Components of the Net Periodic Benefit Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$213	$252	$240
Interest cost	641	581	596
Expected return on plan assets	-990	-911	-905
Amortization of prior service credit	-16	-22	-19
Recognized actuarial loss (credit)	243	315	270
Net periodic benefit (credit) cost	91	215	182
Curtailment gain	-65	0	0
Total (credit) cost	26	215	182
U.S. Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	91	104	93
Interest cost	253	229	230
Expected return on plan assets	-346	-324	-322
Amortization of prior service credit	-7	-16	-16
Recognized actuarial loss (credit)	112	207	152
Net periodic benefit (credit) cost	103	200	137
Non- U.S Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	122	148	147
Interest cost	388	352	366
Expected return on plan assets	-644	-587	-583
Amortization of prior service credit	-9	-6	-3
Recognized actuarial loss (credit)	131	108	118
Net periodic benefit (credit) cost	-12	15	45
Settlement loss	0	0	1
Curtailment gain	-65	0	-1
Total (credit) cost	-77	15	45
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5	5
Interest cost	11	11	13
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	-14
Recognized actuarial loss (credit)	-1	2	0
Net periodic benefit (credit) cost	14	18	4
Curtailment gain	0	0	0
Total (credit) cost	14	18	4
U.S. Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	3	3
Interest cost	7	7	8
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	-13
Recognized actuarial loss (credit)	-2	0	-1
Net periodic benefit (credit) cost	7	10	-3
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	2	2
Interest cost	4	4	5
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	-1
Recognized actuarial loss (credit)	1	2	1
Net periodic benefit (credit) cost	7	8	7
Settlement loss	0	0	0
Curtailment gain	0	0	0
Total (credit) cost	$7	$8	$7

Retirement_Benefits_Effects_of

Retirement Benefits (Effects of One Percentage Point Change in Assumed Health Care Cost Trend Rates) (Details) (Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Foreign Pension and Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Effect on total of service and interest cost components 1 Percentage Point Increase	$1
Effect on total of service and interest cost components 1 Percentage Point Decrease	-1
Effect on postretirement benefit obligation 1 Percentage Point Increase	9
Effect on postretirement benefit obligation 1 Percentage Point Decrease	($7)

Retirement_Benefits_Schedules_

Retirement Benefits (Schedules Providing Information Concerning MMC's Defined Benefit Pension Plans and Postretirement Benefit Plans) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at end of year	$14,879	$13,562
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	967	979
Noncurrent liabilities	-2,244	-1,150
U.S. Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	4,827	5,197
Service cost	91	104	93
Interest cost	253	229	230
Plan amendments	0	0
Defined Benefit Plan, Plan Combination	0	36
Actuarial (gain) loss	955	-547
Medicare Part D subsidy	0	0
Benefits paid	-202	-192
Benefit obligation at end of year	5,924	4,827	5,197
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	4,279	3,936
Plan Combination	0	21
Actual return on plan assets	414	488
Employer/Company contributions	25	26
Medicare Part D subsidy	0	0
Employee contributions	0	0
Benefits paid	-202	-192
Other	0	0
Fair value of plan assets at end of year	4,516	4,279	3,936
Funded status	-1,408	-548
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	-25	-24
Noncurrent liabilities	-1,383	-524
Net asset (liability) recognized in consolidated balance sheet, December 31	-1,408	-548
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	7
Net actuarial (loss) gain	-1,749	-974
Total recognized accumulated other comprehensive (loss) income, December 31	-1,749	-967
Cumulative employer contributions in excess (deficient) of net periodic cost	341	419
Net asset (liability) recognized in consolidated balance sheet, December 31	-1,408	-548
Accumulated benefit obligation at December 31	5,825	4,753
U.S. Postretirement Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	158	176
Service cost	2	3	3
Interest cost	7	7	8
Plan amendments	-4	0
Defined Benefit Plan, Plan Combination	0	0
Actuarial (gain) loss	21	-15
Medicare Part D subsidy	1	1
Benefits paid	-21	-14
Benefit obligation at end of year	177	158	176
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Plan Combination	0	0
Actual return on plan assets	0	0
Employer/Company contributions	13	13
Medicare Part D subsidy	1	1
Employee contributions	13	0
Benefits paid	-21	-14
Other	12	0
Fair value of plan assets at end of year	18	0	0
Funded status	-159	-158
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Current liabilities	-2	-8
Noncurrent liabilities	-157	-150
Net asset (liability) recognized in consolidated balance sheet, December 31	-159	-158
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	4	0
Net actuarial (loss) gain	2	13
Total recognized accumulated other comprehensive (loss) income, December 31	6	13
Cumulative employer contributions in excess (deficient) of net periodic cost	-165	-171
Net asset (liability) recognized in consolidated balance sheet, December 31	-159	-158
Accumulated benefit obligation at December 31	0	0
Non- U.S Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	8,711	8,579
Service cost	122	148	147
Interest cost	388	352	366
Plan amendments	-13	0
Actuarial (gain) loss	1,619	-53
Effect of settlement	-11	-2
Benefits paid	-311	-293
Foreign currency charges	-585	-31
Other	62	0
Benefit obligation at end of year	10,018	8,711	8,579
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	9,351	8,312
Actual return on plan assets	1,756	698
Effect of settlement	-11	-2
Employer/Company contributions	156	620
Employee contributions	10	11
Benefits paid	-311	-293
Foreign currency changes	-578	5
Other	37	0
Fair value of plan assets at end of year	10,410	9,351	8,312
Funded status	392	640
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	967	977
Current liabilities	-6	-5
Noncurrent liabilities	-569	-332
Net asset (liability) recognized in consolidated balance sheet, December 31	392	640
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	-2	85
Net actuarial (loss) gain	-3,215	-3,010
Total recognized accumulated other comprehensive (loss) income, December 31	-3,217	-2,925
Cumulative employer contributions in excess (deficient) of net periodic cost	3,609	3,565
Net asset (liability) recognized in consolidated balance sheet, December 31	392	640
Accumulated benefit obligation at December 31	9,731	8,413
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	97	107
Service cost	2	2	2
Interest cost	4	4	5
Plan amendments	0	0
Actuarial (gain) loss	-1	-8
Effect of settlement	0	0
Benefits paid	-3	-4
Foreign currency charges	-6	-4
Other	0	0
Benefit obligation at end of year	93	97	107
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Effect of settlement	0	0
Employer/Company contributions	3	4
Employee contributions	0	0
Benefits paid	-3	-4
Foreign currency changes	0	0
Other	0	0
Fair value of plan assets at end of year	0	0	0
Funded status	-93	-97
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent assets	0	0
Current liabilities	-4	-4
Noncurrent liabilities	-89	-93
Net asset (liability) recognized in consolidated balance sheet, December 31	-93	-97
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Prior service credit	0	0
Net actuarial (loss) gain	-14	-16
Total recognized accumulated other comprehensive (loss) income, December 31	-14	-16
Cumulative employer contributions in excess (deficient) of net periodic cost	-79	-81
Net asset (liability) recognized in consolidated balance sheet, December 31	-93	-97
Accumulated benefit obligation at December 31	$0	$0

Retirement_Benefits_Reconcilia

Retirement Benefits (Reconciliation of Prior Service Credit) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Effect of curtailment	$65
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | U.S. Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	7	23
Recognized as component of net periodic benefit cost	-7	-16
Plan amendments	0	0
Ending balance, December 31	0	7
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Non- U.S Pension Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	85	93
Recognized as component of net periodic benefit cost	-9	-6
Effect of curtailment	-65	0
Plan amendments	-13	0
Exchange rate adjustments	0	-2
Ending balance, December 31	-2	85
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | U.S. Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0
Recognized as component of net periodic benefit cost	0	0
Plan amendments	4	0
Ending balance, December 31	4	0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0
Recognized as component of net periodic benefit cost	0	0
Effect of curtailment	0	0
Plan amendments	0	0
Exchange rate adjustments	0	0
Ending balance, December 31	$0	$0

Retirement_Benefits_Reconcilia1

Retirement Benefits (Reconciliation of Net Actuarial Loss) (Details) (Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
U.S. Pension Benefits [Member]
Reconciliation of unrecognized net actuarial gain loss [Roll Forward]
Beginning balance	($974)	($1,887)
Recognized as component of net periodic benefit cost	112	208
Liability experience	-955	541
Asset experience	68	164
Total amount recognized as change in plan assets and benefit obligations	-887	705
Ending balance, December 31	-1,749	-974
Non- U.S Pension Benefits [Member]
Reconciliation of unrecognized net actuarial gain loss [Roll Forward]
Beginning balance	-3,010	-3,309
Recognized as component of net periodic benefit cost	131	108
Effect of settlement	0	0
Liability experience	-1,619	53
Asset experience	1,112	111
Plan amendments	-14	0
Total amount recognized as change in plan assets and benefit obligations	-521	164
Exchange rate adjustments	185	27
Ending balance, December 31	-3,215	-3,010
U.S. Postretirement Benefits [Member]
Reconciliation of unrecognized net actuarial gain loss [Roll Forward]
Beginning balance	13	-2
Recognized as component of net periodic benefit cost	-2	0
Liability experience	-21	15
Asset experience	12	0
Total amount recognized as change in plan assets and benefit obligations	-9	15
Ending balance, December 31	2	13
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Reconciliation of unrecognized net actuarial gain loss [Roll Forward]
Beginning balance	-16	-27
Recognized as component of net periodic benefit cost	1	2
Effect of settlement	0	0
Liability experience	1	8
Asset experience	0	0
Plan amendments	0	0
Total amount recognized as change in plan assets and benefit obligations	1	8
Exchange rate adjustments	0	1
Ending balance, December 31	($14)	($16)

Retirement_Benefits_Schedule_o

Retirement Benefits (Schedule of Total Recognized in Net Periodic Benefit Cost and Other Comprehensive Income (Loss) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
U.S. Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$885	($696)	$346
Non- U.S Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Total recognized in net periodic benefit cost and other comprehensive loss (income)	201	-276	246
U.S. Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Total recognized in net periodic benefit cost and other comprehensive loss (income)	14	-5	24
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$5	($2)	$16

Retirement_Benefits_Schedule_o1

Retirement Benefits (Schedule of Estimated Amounts That Will Be Amortized from Accumulated Other Comprehensive in the Next Fiscal Year) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
U.S. Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$0
Net actuarial loss	181
Projected cost	181
Non- U.S Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	-2
Net actuarial loss	140
Projected cost	138
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	0
Net actuarial loss	1
Projected cost	1
U.S. Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	1
Net actuarial loss	1
Projected cost	$2

Retirement_Benefits_Schedule_o2

Retirement Benefits (Schedule of Estimated Future Benefit Payments for Pension and Postretirement Benefits) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
U.S. Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2015	$217
2016	232
2017	250
2018	263
2019	275
2020-2024	1,581
Non- U.S Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2015	264
2016	280
2017	295
2018	312
2019	325
2020-2024	1,890
U.S. Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2015	10
2016	10
2017	10
2018	10
2019	10
2020-2024	55
Foreign Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
2015	4
2016	4
2017	4
2018	4
2019	4
2020-2024	$24

Retirement_Benefits_Summary_of

Retirement Benefits (Summary of the U.S. and Non-U.S. Plan Investments Measured At Fair Value on a Recurring Basis) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$14,879	$13,562
Transfers from Level 1 to Level 2		0
Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,228	3,702
Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,122	8,353
Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,529	1,507
Common/Collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,122	5,938
Common/Collective trusts [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	172	138
Common/Collective trusts [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,766	5,649
Common/Collective trusts [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	184	151
Corporate Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,941	2,334
Corporate Obligations [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate Obligations [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,938	2,330
Corporate Obligations [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Corporate stocks [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,094	2,440
Corporate stocks [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,087	2,434
Corporate stocks [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	5
Corporate stocks [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Private equity/partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	727	801
Private equity/partnerships [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity/partnerships [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
Private equity/partnerships [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	727	799
Government Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	371	352
Government Securities [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	10
Government Securities [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	371	340
Government Securities [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	381	319
Real Estate [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Real Estate [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	7
Real Estate [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	375	312
Short-term investment funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	736	839
Short-term investment funds [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	724	824
Short-term investment funds [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	15
Short-term investment funds [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Company common stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	229	261
Company common stock [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	229	261
Company common stock [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Company common stock [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	278	278
Other Investments [Member] | Identical Assets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	35
Other Investments [Member] | Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23	5
Other Investments [Member] | Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$239	$238

Retirement_Benefits_Summary_of1

Retirement Benefits (Summary of changes in the fair value of the plans’ Level 3 assets) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	$1,507	$1,430
Purchases	279	247
Sales	-253	-282
Unrealized Gain/(Loss)	-86	-144
Realized Gain/(Loss)	153	176
Exchange Rate Impact	-75	3
Transfers in/(out) and Other	4	83
Fair Value, Asset Value, Ending	1,529	1,507
Private equity/partnerships [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	799	824
Purchases	158	146
Sales	-185	-174
Unrealized Gain/(Loss)	-173	-155
Realized Gain/(Loss)	137	150
Exchange Rate Impact	-12	-1
Transfers in/(out) and Other	3	9
Fair Value, Asset Value, Ending	727	799
Real Estate [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	312	357
Purchases	97	21
Sales	-50	-95
Unrealized Gain/(Loss)	19	6
Realized Gain/(Loss)	16	26
Exchange Rate Impact	-19	-3
Transfers in/(out) and Other	0	0
Fair Value, Asset Value, Ending	375	312
Other Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	238	239
Purchases	21	18
Sales	-16	-13
Unrealized Gain/(Loss)	18	10
Realized Gain/(Loss)	0	0
Exchange Rate Impact	-28	6
Transfers in/(out) and Other	6	-22
Fair Value, Asset Value, Ending	239	238
Common/Collective trusts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	151	0
Purchases	0	61
Sales	-1	0
Unrealized Gain/(Loss)	50	-4
Realized Gain/(Loss)	0	0
Exchange Rate Impact	-16	-5
Transfers in/(out) and Other	0	99
Fair Value, Asset Value, Ending	184	151
Corporate stocks [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	1	9
Purchases	0	0
Sales	0	0
Unrealized Gain/(Loss)	0	0
Realized Gain/(Loss)	0	0
Exchange Rate Impact	0	0
Transfers in/(out) and Other	0	-8
Fair Value, Asset Value, Ending	1	1
Corporate Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	4	1
Purchases	3	1
Sales	-1	0
Unrealized Gain/(Loss)	0	0
Realized Gain/(Loss)	0	0
Exchange Rate Impact	0	0
Transfers in/(out) and Other	-3	2
Fair Value, Asset Value, Ending	3	4
Government Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset Value, Beginning	2	0
Purchases	0	0
Sales	0	0
Unrealized Gain/(Loss)	0	-1
Realized Gain/(Loss)	0	0
Exchange Rate Impact	0	0
Transfers in/(out) and Other	-2	3
Fair Value, Asset Value, Ending	$0	$2

Stock_Benefit_Plans_Narrative_

Stock Benefit Plans (Narrative) (Details) (USD $)	12 Months Ended			1 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	31-May-07	Jul. 31, 2002	19-May-11
						equity_incentive_plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options, weighted-average grant-date fair value (in dollars per share)	$9.66	$6.21	$6.04
Total intrinsic value of options exercised	$174.30	$198.10	$57.70
Cash received from the exercise of stock options	178.1	281.1	179.3
Performance-based restricted stock unit payable range maximum	200.00%
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value (in dollars per share)	$0
Total fair value of deferred stock units		1.1	0.6
Restricted Stock [Member] | Certain grants in 2005 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, years	3 years
Restricted Stock Units And Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, years	3 years
Total fair value of deferred stock units	165.3	205.5	262.6
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value (in dollars per share)	$48.16	$36.70	$31.96
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based restricted stock unit payable range minimum	0.00%
Performance-based restricted stock unit payable range maximum	200.00%
Performance period	3 years
Weighted-average grant-date fair value (in dollars per share)	$48	$36.54	$31.89
Target payout percentage	100.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Daily closing price observation period	10 years
Average Volatility Periods Ending On Fifteen Anniversaries Prior To Valuation Date	10 years
Unrecognized compensation cost related to option awards	16
Weighted-average period over which that cost is expected to be recognized, years	1 year 0 months 19 days
Restricted Stock Units, Restricted Stock and Performance Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period over which that cost is expected to be recognized, years	1 year
Unrecognized compensation cost	$55.80
Award Plans 2000 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of previous equity plans replaced						2
Incentive and Stock Award Plan 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards authorized (not more than 23.2 million shares for the 2011 Plan and 35.6 million shares for the 1999 Plan)	23,200,000
Incentive and Stock Award Plan 2011 [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting percentage	25.00%
Contractual term of stock option awards	10 years
Employee Stock Purchase Plan 1999 [Member] | Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards authorized (not more than 23.2 million shares for the 2011 Plan and 35.6 million shares for the 1999 Plan)	35,600,000
Number of share purchase times per plan year	4
Stock option price percent	95.00%
Reduction in the shares available	10,000,000
Shares purchased by employees	608,453
Shares available for issuance under the plan	2,779,195
International Plan [Member] | Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards authorized (not more than 23.2 million shares for the 2011 Plan and 35.6 million shares for the 1999 Plan)	12,000,000
Shares purchased by employees	93,490
Shares available for issuance under the plan	2,893,986
Shares due to shareholder action				4,000,000	5,000,000

Stock_Benefit_Plans_BlackSchol

Stock Benefit Plans (Black-Scholes Option Pricing Valuation Model For Options) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, Range Minimum		1.03%	1.26%
Risk-free interest rate	1.88%
Risk-free interest rate, Range Maximum		1.30%	1.27%
Expected life (in years)	6 years	6 years	6 years 6 months
Expected volatility, Range Minimum		23.60%	26.20%
Expected volatility	24.20%
Expected volatility, Range Maximum		24.10%	26.40%
Expected dividend yield	2.08%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield		2.48%	2.76%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield		2.54%	2.80%

Stock_Benefit_Plans_The_Status

Stock Benefit Plans (The Status Of Stock Option Awards) (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2012
Options, Outstanding [Roll Forward]
Number of options outstanding at January 1, 2014 (in shares)	22,567,866	32,045,793
Number of Options Granted (in shares)	1,716,637
Number of Options Exercised (in shares)	-6,060,823
Number of Options Forfeited (in shares)	-149,501
Number of options Expired (in shares)	-79,097
Number of options outstanding at December, 31 2014 (in shares)	17,995,082	32,045,793
Options vested or expected to vest at December 31, 2014 (in shares)	17,709,073
Options exercisable at December 31, 2014 (in shares)	12,440,781
Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Weighted Average Exercise Price per option at January 1, 2014	$29.29
Weighted Average Exercise Price per option Granted	$48
Weighted Average Exercise Price per option Exercised	$29.39
Weighted Average Exercise Price per Option Forfeited	$35.76
Weighted Average Exercise Price per Option Expired	$32.98
Weighted Average Exercise Price per option at December 31, 2014	$30.97
Weighted Average Exercise Price per option expected to vest	$30.90
Weighted Average Exercise Price per option exercisable at closing	$27.74
Weighted Average Remaining Contractual Term at closing, years	5 years 1 month 6 days
Weighted Average Remaining Contractual Term expected to vest, years	5 years 1 month 6 days
Weighted Average Remaining Contractual Term exercisable at closing, years	3 years 10 months 24 days
Aggregate Intrinsic Value at closing	$488,937
Aggregate Intrinsic Value expected to vest	482,479
Aggregate Intrinsic Value exercisable at closing	$378,296

Stock_Benefit_Plans_Summary_Of

Stock Benefit Plans (Summary Of The Status Of Restricted Stock Unit And Performance Unit Awards) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Restricted Stock Units and Performance Stock Units [Roll Forward]
Non-vested balance at January 1, 2014 (in shares)	4,250,809
Granted (in shares)	753,213
Vested (in shares)	-2,779,606
Forfeited (in shares)	-126,686
Adjustment due to performance (in shares)	0
Non-vested balance at December 31, 2014 (in shares)	2,097,730	4,250,809
Weighted Average Grant Date Fair Value [Roll Forward]
Non-vested balance at January 1, 2014 (in dollars per share)	$32.04
Granted (in dollars per share)	$48.16	$36.70	$31.96
Vested (in dollars per share)	$31.23
Forfeited (in dollars per share)	$34.85
Adjustment due to performance (in dollars per share)	$0
Non-vested balance at December 31, 2014 (in dollars per share)	$38.74	$32.04
Performance Shares [Member]
Restricted Stock Units and Performance Stock Units [Roll Forward]
Non-vested balance at January 1, 2014 (in shares)	961,163
Granted (in shares)	231,445
Vested (in shares)	-613,734
Forfeited (in shares)	-17,446
Adjustment due to performance (in shares)	306,580
Non-vested balance at December 31, 2014 (in shares)	868,008	961,163
Weighted Average Grant Date Fair Value [Roll Forward]
Non-vested balance at January 1, 2014 (in dollars per share)	$32.87
Granted (in dollars per share)	$48	$36.54	$31.89
Vested (in dollars per share)	$30.73
Forfeited (in dollars per share)	$37.45
Adjustment due to performance (in dollars per share)	$30.72
Non-vested balance at December 31, 2014 (in dollars per share)	$37.56	$32.87

Stock_Benefit_Plans_Status_Of_

Stock Benefit Plans (Status Of Restricted Stock Awards) (Details) (Restricted Stock [Member], USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, Grants In Period, Fair Value		$1.10	$0.60
Restricted Stock [Roll Forward]
Non-vested balance at January 1, 2014 (in shares)	7,200
Granted (in shares)	0
Vested (in shares)	0
Forfeited (in shares)	0
Non-vested balance at December 31, 2014 (in shares)	7,200	7,200
Weighted Average Grant Date Fair Value [Roll Forward]
Non-vested balance at January 1, 2014 (in dollars per share)	$46.14
Granted (in dollars per share)	$0
Vested (in dollars per share)	$0
Forfeited (in dollars per share)	$0
Non-vested balance at December 31, 2014 (in dollars per share)	$46.14	$46.14

Fair_Value_Measurements_Narrat

Fair Value Measurements (Narrative) (Details)

12 Months Ended

12 Months Ended

Share data in Millions, except Per Share data, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Mar. 31, 2011

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Jun. 30, 2014

USD ($)

USD ($)

interest_rate_swap

Senior Debt Obligations Due 2014 [Member]

Interest Rate Swap [Member]

Minimum [Member]

Minimum [Member]

Maximum [Member]

Maximum [Member]

Private Equity Funds [Member]

Private Equity Funds [Member]

Alexander Forbes Group Holdings Limited [Member]

Alexander Forbes Group Holdings Limited [Member]

Mercer Consulting Group [Member]

USD ($)

USD ($)

USD ($)

USD ($)

ZAR

Alexander Forbes Group Holdings Limited [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Fair value interest rate swap discount rate

1.60%

Number of interest rate swaps to convert fixed interest rate on notes to floating rate

2

Fair value measurement on senior notes subject to interest rate swaps

$250,000,000

Revenue target period (in years)

2 years

2 years

4 years

4 years

Assets transferred between Level 1 and Level 2

0

Adjustments to acquisition related contingent consideration liability

31,000,000

Increase in fair value of contingent consideration due to 5% increase in projections

22,000,000

Decrease in fair value of contingent consideration due to 5% decrease in projections

26,000,000

Carrying value of equity method investments

388,000,000

89,000,000

61,000,000

38,000,000

282,000,000

Percentage of interest acquired

34.00%

Equity investment shares owned by the Company

443

443

Share price

9.5

Market value of equity method investment

362,000,000

Fair_Value_Measurements_Assets

Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (Fair Value, Measurements, Recurring [Member], USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Financial instruments owned:
Total assets measured at fair value	$257		$202
Fiduciary Assets:
Fiduciary assets	57		0
Liabilities:
Total liabilities measured at fair value	207		357
Identical Assets (Level 1) [Member]
Financial instruments owned:
Total assets measured at fair value	257		199
Fiduciary Assets:
Fiduciary assets	57		0
Liabilities:
Total liabilities measured at fair value	0		0
Observable Inputs (Level 2) [Member]
Financial instruments owned:
Total assets measured at fair value	0		3
Fiduciary Assets:
Fiduciary assets	0		0
Liabilities:
Total liabilities measured at fair value	0		253
Unobservable Inputs (Level 3) [Member]
Fiduciary Assets:
Fiduciary assets	0
Liabilities:
Total liabilities measured at fair value	207		104
Other Assets [Member]
Financial instruments owned:
Mutual funds	150	[1]	154	[1]
Other Assets [Member] | Identical Assets (Level 1) [Member]
Financial instruments owned:
Mutual funds	150	[1]	154	[1]
Cash and Cash Equivalents [Member]
Financial instruments owned:
Money market funds	107	[2]	45	[2]
Cash and Cash Equivalents [Member] | Identical Assets (Level 1) [Member]
Financial instruments owned:
Money market funds	107	[2]	45	[2]
Other Receivables [Member]
Financial instruments owned:
Interest rate swap derivatives	0	[3]	3
Other Receivables [Member] | Observable Inputs (Level 2) [Member]
Financial instruments owned:
Interest rate swap derivatives	0	[3]	3	[3]
Accounts Payable and Accrued Liabilities and Other Liabilities [Member]
Liabilities:
Contingent purchase consideration liability	207	[4]	104	[4]
Accounts Payable and Accrued Liabilities and Other Liabilities [Member] | Unobservable Inputs (Level 3) [Member]
Liabilities:
Contingent purchase consideration liability	207	[4]	104	[4]
Money Market Funds [Member]
Fiduciary Assets:
Fiduciary assets	57		0
Money Market Funds [Member] | Identical Assets (Level 1) [Member]
Fiduciary Assets:
Fiduciary assets	57		0
Senior Debt Obligations Due 2014 [Member] | Long-term Debt [Member]
Liabilities:
Senior Notes due 2014	0	[5]	253	[5]
Senior Debt Obligations Due 2014 [Member] | Long-term Debt [Member] | Observable Inputs (Level 2) [Member]
Liabilities:
Senior Notes due 2014	$0	[5]	$253	[5]
[1]	Included in other assets in the consolidated balance sheets.
[2]	Included in cash and cash equivalents in the consolidated balance sheets.
[3]	Included in other receivables in the consolidated balance sheets.
[4]	Included in accounts payable and accrued liabilities and other liabilities in the consolidated balance sheets
[5]	Included in long-term debt in the consolidated balance sheets.

Fair_Value_Measurements_Change

Fair Value Measurements (Changes In Fair Value Of Level 3 Liabilities Representing Acquisition Related Contingent Consideration) (Details) (Contingent Consideration [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Contingent Consideration [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at January 1,	$104	$63
Additions	114	26
Payments	-42	-17
Revaluation Impact	31	32
Balance at December 31,	$207	$104

Fair_Value_Measurements_Equity

Fair Value Measurements - Equity Method Investments, Summarized Financial Information (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Equity Method Investments and Joint Ventures [Abstract]
Revenue	$239	$148	$144
Net investment income	161	88	-19
Net income	216	135	36
Total assets	25,497	741
Total liabilities	24,209	136
Non controlling interests	$14	$3

LongTerm_Commitments_Narrative

Long-Term Commitments (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Net rental costs	$393	$403	$416
Rentals from subleases	$12	$13	$10
Office Building [Member] | Lease Concentration Risk [Member] | Lease Obligations [Member]
Concentration Risk [Line Items]
Lease obligations for office space	98.00%

Longterm_Commitments_Operating

Long-term Commitments (Operating Lease Agreements) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Gross Rental Commitments
2015, Net Rental Commitments	$365
2016, Gross Rental Commitments	334
2017, Gross Rental Commitments	293
2018, Gross Rental Commitments	258
2019, Gross Rental Commitments	212
Subsequent years, Gross Rental Commitments	1,010
Rental from Subleases
2015, Rentals from Subleases	48
2016, Rentals from Subleases	48
2017, Rentals from Subleases	45
2018, Rentals from Subleases	42
2019, Rentals from Subleases	35
Subsequent years, Rentals from Subleases	36
Net Rental Commitments
2015, Net Rental Commitments	317
2016, Net Rental Commitments	286
2017, Net Rental Commitments	248
2018, Net Rental Commitments	216
2019, Net Rental Commitments	177
Subsequent years, Net Rental Commitments	$974

Longterm_Commitments_Future_Mi

Long-term Commitments (Future Minimum Commitments) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Commitments and Contingencies Disclosure [Abstract]
2015	$195
2016	113
2017	64
Subsequent years	69
Total	$441

Debt_Narrative_Details

Debt (Narrative) (Details) (USD $)

12 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

3 Months Ended

1 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Sep. 30, 2014

Sep. 30, 2013

Mar. 28, 2014

Mar. 27, 2014

Dec. 31, 2012

31-May-14

Oct. 31, 2014

Dec. 31, 2014

31-May-14

Oct. 31, 2013

Feb. 28, 2013

Debt Instrument [Line Items]

Repayments of long-term debt

$331,000,000

$260,000,000

$259,000,000

Cost for early redemption

137,000,000

24,000,000

0

2.35% Senior Debt Obligations Due 2019 [Member]

Debt Instrument [Line Items]

Debt instrument, amount

300,000,000

Interest rate

2.35%

2.35%

2.35%

Debt instrument, term

5 years

Senior Debt Obligations Due 2018 [Member]

Debt Instrument [Line Items]

Debt instrument, amount

250,000,000

Interest rate

2.55%

2.55%

2.55%

Debt instrument, term

5 years

Other Debt Facilities [Member]

Debt Instrument [Line Items]

Revolving credit facility, amount outstanding

600,000

1,000,000

600,000

Debt instrument, unused borrowing capacity

260,000,000

282,000,000

260,000,000

Senior Debt Obligations Due 2021 [Member]

Debt Instrument [Line Items]

Interest rate

4.80%

4.80%

Revolving Credit Facility [Member]

Debt Instrument [Line Items]

Debt instrument, term

5 years

Line of Credit Facility, Current Borrowing Capacity

1,000,000,000

Revolving credit facility, borrowing capacity

1,000,000,000

Revolving credit facility, amount outstanding

0

0

Amended Revolving Credit Facility March 28, 2014 [Member]

Debt Instrument [Line Items]

Debt instrument, term

5 years

Revolving credit facility, borrowing capacity

1,200,000,000

Revolving credit facility, amount outstanding

0

0

Senior Debt Obligations Due 2015 [Member]

Debt Instrument [Line Items]

Interest rate

5.75%

5.75%

5.75%

Senior Debt Obligations Due 2014 [Member]

Debt Instrument [Line Items]

Interest rate

5.38%

5.38%

5.38%

5.38%

Senior Debt Obligations Due 2017 [Member]

Debt Instrument [Line Items]

Interest rate

2.30%

2.30%

Three-Year Delayed Draw Term Loan Facility [Member]

Debt Instrument [Line Items]

Debt instrument, term

3 years

Revolving credit facility, borrowing capacity

50,000,000

50,000,000

Revolving credit facility, amount outstanding

50,000,000

50,000,000

Interest rate on credit facility

1.17%

1.17%

Senior Debt Obligations Due 2023 [Member]

Debt Instrument [Line Items]

Debt instrument, amount

250,000,000

Interest rate

4.05%

4.05%

4.05%

Debt instrument, term

10 years

3.50% Senior Debt Obligations Due 2025 [Member]

Debt Instrument [Line Items]

Debt instrument, amount

500,000,000

Interest rate

3.50%

3.50%

3.50%

Debt instrument, term

10 years 6 months

Senior Debt Obligations Due 2024 [Member] [Domain]

Debt Instrument [Line Items]

Debt instrument, amount

600,000,000

600,000,000

Interest rate

3.50%

3.50%

3.50%

3.50%

Debt instrument, term

10 years

Senior Debt Obligations Due 2019 [Member]

Debt Instrument [Line Items]

Interest rate

9.25%

Partial redemption of notes

400,000,000

Senior Debt Obligations Due 2015 and Senior Debt Obligations Due 2019 [Member]

Debt Instrument [Line Items]

Partial redemption of notes

630,000,000

Repayments of long-term debt

765,000,000

Cost for early redemption

137,000,000

Senior Debt Obligations Due 2014 [Member]

Debt Instrument [Line Items]

Repayments of long-term debt

320,000,000

Senior Debt Obligations Due 2015 [Member]

Debt Instrument [Line Items]

Interest rate

5.75%

Partial redemption of notes

250,000,000

230,000,000

Repayments of long-term debt

275,000,000

Cost for early redemption

24,000,000

Senior Debt Obligations Due 2013 [Member]

Debt Instrument [Line Items]

Interest rate

4.85%

Revised Purchase Price Allocation For Principal Amount Outstanding of Notes

$250,000,000

Debt_Schedule_Of_Outstanding_D

Debt (Schedule Of Outstanding Debt) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	31-May-14	Sep. 30, 2013	Sep. 30, 2014
In Millions, unless otherwise specified
Debt Instrument [Line Items]
Current portion of long-term debt	$11	$334
Long-term debt, current and noncurrent	3,387	2,955
Long-term debt and capital lease obligations	3,376	2,621
Senior Debt Obligations Due 2033 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	297	297
Interest rate	5.88%
Senior Debt Obligations Due 2014 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	0	323
Interest rate	5.38%		5.38%
Senior Debt Obligations Due 2015 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	0	230
Interest rate	5.75%			5.75%
Senior Debt Obligations Due 2017 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	249	249
Interest rate	2.30%
Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	0	399
Interest rate	9.25%
Senior Debt Obligations Due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	497	497
Interest rate	4.80%
Senior Debt Obligations Due 2018 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	249	248
Interest rate	2.55%			2.55%
Senior Debt Obligations Due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	248	247
Interest rate	4.05%			4.05%
Senior Debt Obligations Due 2024 [Member] [Domain]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	595	0
Interest rate	3.50%		3.50%
2.35% Senior Debt Obligations Due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	300	0
Interest rate	2.35%				2.35%
3.50% Senior Debt Obligations Due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	498	0
Interest rate	3.50%				3.50%
Mortgage Due 2035 [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	403	413
Interest rate	5.70%
Three-Year Delayed Draw Term Loan Facility [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	50	50
Other Debt Instruments [Member]
Debt Instrument [Line Items]
Long-term debt, current and noncurrent	$1	$2

Debt_Scheduled_Repayments_Deta

Debt (Scheduled Repayments) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Maturities of Long-term Debt [Abstract]
Repayments of principal in 2014	$10
Repayments of principal in 2015	61
Repayments of principal in 2016	262
Repayments of principal in 2017	262
Repayments of principal in 2018	$313

Debt_Estimated_Fair_Value_Of_S

Debt (Estimated Fair Value Of Significant Financial Instruments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	$11	$334
Long-term debt	3,493	2,819
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	11	334
Long-term debt	$3,376	$2,621

Integration_and_Restructuring_2

Integration and Restructuring Costs (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	$12	$22	$78
Liability balance	92	124	170	181
Payments for restructuring	43	65	88
Risk and Insurance Services Segment [Member] | Acquisition Related [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	5
Consulting Segment [Member] | Acquisition Related [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	1
Corporate Segment [Member] | Acquisition Related [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring actions costs	$6

Integration_and_Restructuring_3

Integration and Restructuring Costs (Restructuring Activities) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$124	$170	$181
Expense Incurred	12	22	78
Cash Paid	-43	-65	-88
Other	-1	-3
Liability at end of period	92	124	170
Severance [Member]
Restructuring Reserve [Roll Forward]
Liability at beginning of period	11	36
Expense Incurred	4	9
Cash Paid	-8	-33
Other		-1
Liability at end of period	7	11
Other Restructuring [Member]
Restructuring Reserve [Roll Forward]
Liability at beginning of period	113	134
Expense Incurred	8	13
Cash Paid	-35	-32
Other	-1	-2
Liability at end of period	$85	$113

Common_Stock_Details

Common Stock (Details) (USD $)	12 Months Ended
Share data in Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	31-May-14
Equity, Class of Treasury Stock [Line Items]
Payments for repurchase of common stock	$800,000,000	$550,000,000	$230,000,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased	15.5	13.2
Stock repurchase program, authorized amount				2,000,000,000
Stock repurchase program, remaining authorized repurchase amount	$1,300,000,000

Claims_Lawsuits_And_Other_Cont1

Claims, Lawsuits And Other Contingencies Claims, Lawsuits And Other Contingencies (Details) (Other Contingencies-Guarantees [Member], GBP £)	Dec. 31, 2014
In Millions, unless otherwise specified
Other Contingencies-Guarantees [Member]
Loss Contingencies [Line Items]
Amount reinsured by third party	£ 40

Segment_Information_Details_Fo

Segment Information (Details For MMC's Operating Segments) (Details) (USD $)	3 Months Ended										12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$3,246		$3,141	$3,300	$3,264	$3,115		$2,932	$3,088	$3,126	$12,951		$12,261		$11,924
Operating Income (Loss)	536		445	647	673	489		404	577	607	2,301		2,077		1,829
Total Assets	17,840					16,980					17,840		16,980		16,288
Depreciation and Amortization											388		358		349
Capital Expenditures											368		401		320
Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets											24		27		39
Consulting Segment [Member]
Segment Reporting Information [Line Items]
Interest Income, Fiduciary Assets											6		5		4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											12,990		12,297		11,963
Operating Income (Loss)											2,505		2,266		2,026
Total Assets	18,127					16,543					18,127		16,543		15,035
Depreciation and Amortization											332		307		309
Capital Expenditures											265		313		248
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue											6,931	[1]	6,596	[1]	6,350	[1]
Operating Income (Loss)											1,509		1,421		1,334
Total Assets	12,211					11,365					12,211		11,365		9,832
Depreciation and Amortization											213		192		196
Capital Expenditures											173		158		131
Interest Income, Fiduciary Assets											24		27		39
Equity method income											9		8		11
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue											6,059	[2]	5,701	[2]	5,613	[2]
Operating Income (Loss)											996		845		692
Total Assets	5,916					5,178					5,916		5,178		5,203
Depreciation and Amortization											119		115		113
Capital Expenditures											92		155		117
Interest Income, Fiduciary Assets											6		5		4
Equity method income											2		0		0
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue											-39		-36		-39
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue											4		5		5
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue											35		31		34
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)											-204		-189		-197
Total Assets	-287	[3]				437	[3]				-287	[3]	437	[3]	1,253	[3]
Depreciation and Amortization											56		51		40
Capital Expenditures											$103		$88		$72
[1]	Includes inter-segment revenue of $4 million in 2014 and $5 million in both 2013 and 2012, interest income on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively, and equity method income of $9 million, $8 million and $11 million in 2014, 2013 and 2012, respectively.
[2]	Includes inter-segment revenue of $35 million, $31 million and $34 million in 2014, 2013 and 2012, respectively, interest income on fiduciary funds of $6 million in 2014, $5 million in 2013 and $4 million 2012 and equity method income of $2 million in 2014, and $0 million in both 2013 and 2012.
[3]	Corporate assets primarily include insurance recoverables, pension related assets, the owned portion of the Company headquarters building and intercompany eliminations.

Segment_Information_Selected_I

Segment Information (Selected Information and Details Of Operating Segment Revenue) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$3,246	$3,141	$3,300	$3,264	$3,115	$2,932	$3,088	$3,126	$12,951		$12,261		$11,924
Fixed Assets, Net	809				828				809		828		809
United States [Member]
Segment Reporting Information [Line Items]
Revenue									5,865		5,485		5,300
Fixed Assets, Net	483				494				483		494		494
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenue									2,111		1,979		1,960
Fixed Assets, Net	120				121				120		121		121
Europe [Member]
Segment Reporting Information [Line Items]
Revenue									2,077		1,943		1,879
Fixed Assets, Net	60				64				60		64		63
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenue									1,420		1,396		1,346
Fixed Assets, Net	62				72				62		72		62
Other Geographic Areas [Member]
Segment Reporting Information [Line Items]
Revenue									1,517		1,494		1,478
Fixed Assets, Net	84				77				84		77		69
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue									12,990		12,297		11,963
Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									6,931	[1]	6,596	[1]	6,350	[1]
Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									6,059	[2]	5,701	[2]	5,613	[2]
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue									-39		-36		-39
Intersegment Eliminations [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									4		5		5
Intersegment Eliminations [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									35		31		34
Marsh Insurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									5,774		5,461		5,265
Guy Carpenter Reinsurance Group [Member] | Operating Segments [Member] | Risk and Insurance Services Segment [Member]
Segment Reporting Information [Line Items]
Revenue									1,157		1,135		1,085
Mercer Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									4,350		4,241		4,147
Oliver Wyman Group Consulting Group [Member] | Operating Segments [Member] | Consulting Segment [Member]
Segment Reporting Information [Line Items]
Revenue									$1,709		$1,460		$1,466
[1]	Includes inter-segment revenue of $4 million in 2014 and $5 million in both 2013 and 2012, interest income on fiduciary funds of $24 million, $27 million and $39 million in 2014, 2013 and 2012, respectively, and equity method income of $9 million, $8 million and $11 million in 2014, 2013 and 2012, respectively.
[2]	Includes inter-segment revenue of $35 million, $31 million and $34 million in 2014, 2013 and 2012, respectively, interest income on fiduciary funds of $6 million in 2014, $5 million in 2013 and $4 million 2012 and equity method income of $2 million in 2014, and $0 million in both 2013 and 2012.

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data and Supplemental Information(Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 20, 2015
												stockholder
Subsequent Event [Line Items]
Revenue	$3,246	$3,141	$3,300	$3,264	$3,115	$2,932	$3,088	$3,126	$12,951	$12,261	$11,924
Operating income	536	445	647	673	489	404	577	607	2,301	2,077	1,829
Income from continuing operations	269	305	440	457	307	260	400	412	1,471	1,379	1,204
Discontinued operations, net of tax	30	-1	-2	-1	0	-1	-5	12	26	6	-3
Net income attributable to the Company	$294	$297	$431	$443	$303	$253	$388	$413	$1,465	$1,357	$1,176
Basic Per Share Data:
Continuing operations	$0.49	$0.55	$0.79	$0.81	$0.55	$0.46	$0.71	$0.73	$2.64	$2.46	$2.16
Discontinued operations, net of tax	$0.05	$0	($0.01)	$0	$0	$0	$0	$0.02
Net income attributable to the Company	$0.54	$0.55	$0.78	$0.81	$0.55	$0.46	$0.71	$0.75	$2.69	$2.47	$2.16
Diluted Per Share Data:
Continuing operations	$0.48	$0.54	$0.78	$0.80	$0.54	$0.45	$0.70	$0.72	$2.61	$2.42	$2.13
Discontinued operations, net of tax	$0.06	$0	($0.01)	$0	$0	$0	($0.01)	$0.02
Net income attributable to the Company	$0.54	$0.54	$0.77	$0.80	$0.54	$0.45	$0.69	$0.74	$2.65	$2.43	$2.13
Dividends Paid Per Share	$0.28	$0.28	$0.25	$0.25	$0.25	$0.25	$0.23	$0.23
Subsequent Event [Member]
Diluted Per Share Data:
Stockholders of record												6,197