Retirement Benefits (Tables)

12 Months Ended

Dec. 31, 2018

Defined Benefit Plan [Abstract]

Schedule of weighted average actuarial assumptions utilized defined benefit plans

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 Benefits				U.S. Postretirement Benefits
	2018		2017		2018		2017
Weighted average assumptions:
Discount rate (for expense)	3.86	%	4.58	%	3.67	%	4.12	%
Expected return on plan assets	7.95	%	7.95	%	—		—
Discount rate (for benefit obligation)	4.45	%	3.86	%	4.24	%	3.67	%

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  Benefits				Postretirement Benefits
	2018		2017		2018		2017
Weighted average assumptions:
Discount rate (for expense)	3.07	%	3.40	%	3.21	%	3.64	%
Expected return on plan assets	5.83	%	6.64	%	—		—
Rate of compensation increase (for expense)	1.73	%	1.77	%	—		—
Discount rate (for benefit obligation)	3.48	%	3.07	%	3.65	%	3.21	%
Rate of compensation increase (for benefit obligation)	1.74	%	1.73	%	—		—

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 Benefits				Non-U.S. Postretirement Benefits
	2018		2017		2018		2017
Weighted average assumptions:
Discount rate (for expense)	2.58	%	2.69	%	2.97	%	3.42	%
Expected return on plan assets	4.94	%	6.07	%	—		—
Rate of compensation increase (for expense)	2.80	%	2.85	%	—		—
Discount rate (for benefit obligation)	2.89	%	2.58	%	3.32	%	2.97	%
Rate of compensation increase (for benefit obligation)	2.82	%	2.80	%	—		—

Schedule of components of net periodic benefit cost for U.S. defined benefit and other postretirement benefit plans

As discussed in Note 1, effective January 1, 2018, the Company adopted the new guidance that changes the presentation of net periodic pension cost and net periodic post-retirement cost (''net periodic benefit costs"). The new guidance requires employers to report the service cost component of net periodic benefit costs in the same line item as other compensation costs in the income statement. The other components of net periodic benefit costs are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations. The new guidance requires retrospective application for the presentation of the service cost component and the other components of net periodic benefit costs. Accordingly, the Company has reclassified prior period information in the following chart to conform with the current year's presentation:

Amounts Recorded in the Consolidated Statement of Income
Combined U.S. and significant non-U.S. plans	Pension Benefits									Post-retirement Benefits
For the Years Ended December 31,
(In millions)	2018			2017			2016			2018		2017		2016
Compensation and benefits expense (Operating income)	$	34		$	76		$	178		$	1	$	1	$	2
Other net benefit (credit) cost	(215		)	(206		)	(240		)	—		5		7
Total (credit) cost	$	(181	)	$	(130	)	$	(62	)	$	1	$	6	$	9

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)	2018			2017			2016			2018			2017		2016
Service cost	$	34		$	76		$	178		$	1		$	1	$	2
Interest cost	463			497			537			3			4		5
Expected return on plan assets	(864		)	(921		)	(940		)	—			—		—
Amortization of prior service (credit) cost	(2		)	(2		)	(1		)	(2		)	1		4
Recognized actuarial loss (gain)	146			167			168			(1		)	—		(2		)
Net periodic benefit (credit) cost	$	(223	)	$	(183	)	$	(58	)	$	1		$	6	$	9
Curtailment (loss) gain	—			(1		)	(4		)	—			—		—
Settlement loss	42			54			—			—			—		—
Total (credit) cost	$	(181	)	$	(130	)	$	(62	)	$	1		$	6	$	9

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 Benefits									Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2016			2018			2017			2016
Service cost	$	34		$	76		$	72		$	1		$	1		$	2
Interest cost	228			233			273			2			2			3
Expected return on plan assets	(507		)	(564		)	(561		)	—			—			—
Amortization of prior service credit	(2		)	(2		)	(1		)	(2		)	(2		)	—
Recognized actuarial loss	91			130			94			—			1			—
Net periodic benefit (credit) cost	(156		)	(127		)	(123		)	1			2			5
Settlement loss	42			54			—			—			—			—
Curtailment (gain) loss	—			(1		)	(4		)	—			—			—
Total (credit) cost	$	(114	)	$	(74	)	$	(127	)	$	1		$	2		$	5

The components of the net periodic benefit cost (credit) for the U.S. defined benefit and other postretirement benefit plans are as follows:

U.S. Plans only	Pension Benefits									Postretirement Benefits
For the Years Ended December 31,
(In millions of dollars)	2018			2017			2016			2018			2017			2016
Service cost	$	—		$	—		$	106		$	—		$	—		$	—
Interest cost	235			264			264			1			2			2
Expected return on plan assets	(357		)	(357		)	(379		)	—			—			—
Amortization of prior service cost	—			—			—			—			3			4
Recognized actuarial loss (gain)	55			37			74			(1		)	(1		)	(2		)
Net periodic benefit (credit) cost	$	(67	)	$	(56	)	$	65		$	—		$	4		$	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 Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Change in benefit obligation:
Benefit obligation at beginning of year	$	10,053		$	9,670		$	68		$	81
Service cost	34			76			1			1
Interest cost	228			233			2			2
Employee contributions	2			7			—			—
Actuarial (gain) loss	(450		)	(149		)	(8		)	—
Plan amendments	44			—			—			(17		)
Effect of settlement	(162		)	(211		)	—			—
Effect of curtailment	—			(1		)	—			—
Benefits paid	(290		)	(291		)	(3		)	(3		)
Foreign currency changes	(491		)	703			(3		)	4
Other	1			16			—			—
Benefit obligation, December 31	$	8,969		$	10,053		$	57		$	68
Change in plan assets:
Fair value of plan assets at beginning of year	$	11,388		$	10,017		$	—		$	—
Actual return on plan assets	(141		)	875			—			—
Effect of settlement	(162		)	(211		)	—			—
Company contributions	82			229			3			3
Employee contributions	2			7			—			—
Benefits paid	(290		)	(291		)	(3		)	(3		)
Foreign currency changes	(573		)	749			—			—
Other	—			13			—			—
Fair value of plan assets, December 31	$	10,306		$	11,388		$	—		$	—
Net funded status, December 31	$	1,337		$	1,335		$	(57	)	$	(68	)
Amounts recognized in the consolidated balance sheets:
Non-current assets	$	1,687		$	1,684		$	—		$	—
Current liabilities	(5		)	(6		)	(3		)	(4		)
Non-current liabilities	(345		)	(343		)	(54		)	(64		)
Net asset (liability) recognized, December 31	$	1,337		$	1,335		$	(57	)	$	(68	)
Amounts recognized in other comprehensive (loss) income:
Prior service credit	$	(2	)	$	43		$	12		$	15
Net actuarial loss	(2,568		)	(2,646		)	(1		)	(10		)
Total recognized accumulated other comprehensive (loss) income, December 31	$	(2,570	)	$	(2,603	)	$	11		$	5
Cumulative employer contributions in excess of (less than) net periodic cost	3,907			3,938			(68		)	(73		)
Net asset (liability) recognized in consolidated balance sheets, December 31	$	1,337		$	1,335		$	(57	)	$	(68	)
Accumulated benefit obligation, December 31	$	8,752		$	9,783		$	—		$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	43		$	43		$	15		$	—
Recognized as component of net periodic benefit credit:
Amortization of prior service credit	(2		)	(2		)	(2		)	(2		)
Effect of curtailment	—			(1		)	—			—
Total recognized as component of net periodic benefit credit	(2		)	(3		)	(2		)	(2		)
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	(44		)	—			—			17
Exchange rate adjustments	1			3			(1		)	—
Prior service credit, December 31	$	(2	)	$	43		$	12		$	15

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive (loss) income:
Beginning balance	$	(2,646	)	$	(3,081	)	$	(10	)	$	(11	)
Recognized as component of net periodic benefit cost:
Amortization of net loss	91			130			—			1
Effect of settlement	42			54			—			—
Total recognized as component of net periodic benefit credit	133			184			—			1
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Liability experience	450			149			8			—
Asset experience	(648		)	311			—			—
Other	3			(5		)	—			—
Effect of curtailment	—			1			—			—
Total amount recognized as change in plan assets and benefit obligations	(195		)	456			8			—
Exchange rate adjustments	140			(205		)	1			—
Net actuarial loss, December 31	$	(2,568	)	$	(2,646	)	$	(1	)	$	(10	)

For the Years Ended December 31,	Non-U.S. Pension Benefits								Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2016		2018			2017			2016
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$	(147	)	$	(513	)	$	21	$	(5	)	$	(14	)	$	10

The following schedules provide information concerning the Company’s U.S. defined benefit pension plans and postretirement benefit plans:

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Change in benefit obligation:
Benefit obligation at beginning of year	$	6,221		$	5,894		$	36		$	37
Interest cost	235			264			1			2
Employee contributions	—			—			4			3
Actuarial (gain) loss	(502		)	538			(1		)	3
Benefits paid	(425		)	(475		)	(8		)	(9		)
Benefit obligation, December 31	$	5,529		$	6,221		$	32		$	36
Change in plan assets:
Fair value of plan assets at beginning of year	$	4,787		$	4,365		$	2		$	2
Actual return on plan assets	(330		)	812			—			—
Employer contributions	30			85			3			6
Employee contributions	—			—			4			3
Benefits paid	(425		)	(475		)	(8		)	(9		)
Fair value of plan assets, December 31	$	4,062		$	4,787		$	1		$	2
Net funded status, December 31	$	(1,467	)	$	(1,434	)	$	(31	)	$	(34	)
Amounts recognized in the consolidated balance sheets:
Current liabilities	$	(28	)	$	(27	)	$	(2	)	$	(2	)
Non-current liabilities	(1,439		)	(1,407		)	(29		)	(32		)
Net liability recognized, December 31	$	(1,467	)	$	(1,434	)	$	(31	)	$	(34	)
Amounts recognized in other comprehensive income (loss):
Net actuarial (loss) gain	(1,896		)	(1,766		)	6			6
Total recognized accumulated other comprehensive (loss) income, December 31	$	(1,896	)	$	(1,766	)	$	6		$	6
Cumulative employer contributions in excess of (less than) net periodic cost	429			332			(37		)	(40		)
Net amount recognized in consolidated balance sheet	$	(1,467	)	$	(1,434	)	$	(31	)	$	(34	)
Accumulated benefit obligation at December 31	$	5,529		$	6,221		$	—		$	—

	U.S. Pension Benefits				U.S. Postretirement Benefits
(In millions of dollars)	2018		2017		2018		2017
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	—	$	—	$	—	$	(3	)
Recognized as component of net periodic benefit cost	—		—		—		3
Prior service cost, December 31	$	—	$	—	$	—	$	—

Schedule of total recognized in net periodic benefit cost and other comprehensive income (loss)

	U.S. Pension Benefits				U.S. Postretirement Benefits
(In millions of dollars)	2018		2017		2018		2017
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	—	$	—	$	—	$	(3	)
Recognized as component of net periodic benefit cost	—		—		—		3
Prior service cost, December 31	$	—	$	—	$	—	$	—

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(1,766	)	$	(1,720	)	$	6		$	11
Recognized as component of net periodic benefit cost (credit)	55			37			(1		)	(1		)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Other	—			—			—			(1		)
Liability experience	502			(538		)	1			(3		)
Asset experience	(687		)	455			—			—
Total (loss) gain recognized as change in plan assets and benefit obligations	(185		)	(83		)	1			(4		)
Net actuarial (loss) gain, December 31	$	(1,896	)	$	(1,766	)	$	6		$	6

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(1,766	)	$	(1,720	)	$	6		$	11
Recognized as component of net periodic benefit cost (credit)	55			37			(1		)	(1		)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Other	—			—			—			(1		)
Liability experience	502			(538		)	1			(3		)
Asset experience	(687		)	455			—			—
Total (loss) gain recognized as change in plan assets and benefit obligations	(185		)	(83		)	1			(4		)
Net actuarial (loss) gain, December 31	$	(1,896	)	$	(1,766	)	$	6		$	6

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	43		$	43		$	15		$	—
Recognized as component of net periodic benefit credit:
Amortization of prior service credit	(2		)	(2		)	(2		)	(2		)
Effect of curtailment	—			(1		)	—			—
Total recognized as component of net periodic benefit credit	(2		)	(3		)	(2		)	(2		)
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	(44		)	—			—			17
Exchange rate adjustments	1			3			(1		)	—
Prior service credit, December 31	$	(2	)	$	43		$	12		$	15

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2018			2017
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive (loss) income:
Beginning balance	$	(2,646	)	$	(3,081	)	$	(10	)	$	(11	)
Recognized as component of net periodic benefit cost:
Amortization of net loss	91			130			—			1
Effect of settlement	42			54			—			—
Total recognized as component of net periodic benefit credit	133			184			—			1
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Liability experience	450			149			8			—
Asset experience	(648		)	311			—			—
Other	3			(5		)	—			—
Effect of curtailment	—			1			—			—
Total amount recognized as change in plan assets and benefit obligations	(195		)	456			8			—
Exchange rate adjustments	140			(205		)	1			—
Net actuarial loss, December 31	$	(2,568	)	$	(2,646	)	$	(1	)	$	(10	)

Schedule of amounts recognized in other comprehensive income (loss)

For the Years Ended December 31,	U.S. Pension Benefits							U.S. Postretirement Benefits
(In millions of dollars)	2018		2017			2016		2018		2017		2016
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	63	$	(10	)	$	31	$	—	$	5	$	2

For the Years Ended December 31,	Non-U.S. Pension Benefits								Non-U.S. Postretirement Benefits
(In millions of dollars)	2018			2017			2016		2018			2017			2016
Total recognized in net periodic benefit cost and other comprehensive (income) loss	$	(147	)	$	(513	)	$	21	$	(5	)	$	(14	)	$	10

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 loss to net periodic pension cost in the next fiscal year:

	Non-U.S. Pension Benefits		Non-U.S. Postretirement Benefits
(In millions of dollars)	2019		2019
Prior service credit	$	—	$	(2	)
Net actuarial loss	59		—
Projected cost	$	59	$	(2	)

Estimated amounts that will be amortized from accumulated other comprehensive loss to net periodic pension cost in the next fiscal year:

	U.S. Pension Benefits		U.S. Postretirement Benefits
(In millions of dollars)	2019		2019
Net actuarial loss	$	44	$	1

Schedule of change in assumed health care cost trend rates

A one percentage point change in assumed health care cost trend rates would have the following effects:

(In millions of dollars)	1 Percentage Point Increase		1 Percentage Point Decrease
Effect on total of service and interest cost components	$	—	$	—
Effect on postretirement benefit obligation	$	6	$	(5	)

Schedule of estimated future benefit payments for its pension and postretirement benefits

The estimated future benefit payments for the Company's pension and postretirement benefit plans are as follows:

For the Years Ended December 31,	Pension Benefits				Postretirement Benefits
(In millions of dollars)	U.S.		Non-U.S.		U.S.		Non-U.S.
2019	$	264	$	272	$	4	$	3
2020	$	277	$	282	$	4	$	3
2021	$	291	$	291	$	4	$	3
2022	$	299	$	302	$	3	$	3
2023	$	306	$	320	$	3	$	3
2024-2028	$	1,626	$	1,768	$	12	$	15

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, 2018 and 2017:

	Fair Value Measurements at December 31, 2018
Assets (In millions of dollars)	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		NAV		Total
Common/collective trusts	$	291	$	—	$	—	$	5,329	$	5,620
Corporate obligations	—		3,673		—		—		3,673
Corporate stocks	2,046		33		1		—		2,080
Private equity/partnerships	—		—		—		921		921
Government securities	15		535		—		—		550
Real estate	—		—		—		563		563
Short-term investment funds	286		—		—		—		286
Company common stock	319		—		—		—		319
Other investments	13		20		333		—		366
Total investments	$	2,970	$	4,261	$	334	$	6,813	$	14,378

	Fair Value Measurements at December 31, 2017
Assets (In millions of dollars)	Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		NAV		Total
Common/collective trusts	$	375	$	—	$	—	$	7,611	$	7,986
Corporate obligations	—		3,620		20		—		3,640
Corporate stocks	1,467		34		2		—		1,503
Private equity/partnerships	—		—		—		803		803
Government securities	15		556		—		—		571
Real estate	—		—		—		566		566
Short-term investment funds	391		16		—		—		407
Company common stock	326		—		—		—		326
Other investments	12		12		350		—		374
Total investments	$	2,586	$	4,238	$	372	$	8,980	$	16,176

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, 2018 and December 31, 2017:

Assets (In millions)	Fair Value, January 1, 2018		Purchases		Sales			Unrealized Gain/ (Loss)			Realized Gain/ (Loss)		Exchange Rate Impact			Transfers in/(out) and Other			Fair Value, December 31, 2018
Other investments	$	350	$	20	$	(19	)	$	(5	)	$	1	$	(14	)	$	—		$	333
Corporate stocks	2		—		—			—			—		—			(1		)	1
Corporate obligations	20		—		—			—			—		—			(20		)	—
Total assets	$	372	$	20	$	(19	)	$	(5	)	$	1	$	(14	)	$	(21	)	$	334

Assets (In millions)	Fair Value, January 1, 2017		Purchases		Sales			Unrealized Gain/ (Loss)			Realized Gain/ (Loss)		Exchange Rate Impact		Transfers in/(out) and Other			Fair Value, December 31, 2017
Other investments	$	312	$	20	$	(15	)	$	(7	)	$	—	$	40	$	—		$	350
Corporate stocks	2		—		—			—			—		—		—			2
Corporate obligations	9		9		(1		)	9			—		1		(7		)	20
Total assets	$	323	$	29	$	(16	)	$	2		$	—	$	41	$	(7	)	$	372