Retirement Benefits (Tables)

12 Months Ended

Dec. 31, 2016

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 Benefits				Non-U.S. Postretirement Benefits
	2016		2015		2016		2015
Weighted average assumptions:
Discount rate (for expense)	3.71	%	3.49	%	4.00	%	3.85	%
Expected return on plan assets	6.36	%	6.57	%	—		—
Rate of compensation increase (for expense)	2.72	%	2.67	%	—		—
Discount rate (for benefit obligation)	2.69	%	3.71	%	3.42	%	4.00	%
Rate of compensation increase (for benefit obligation)	2.85	%	2.72	%	—		—

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
	2016		2015		2016		2015
Weighted average assumptions:
Discount rate (for expense)	4.71	%	4.41	%	4.36	%	3.90	%
Expected return on plan assets	8.72	%	8.75	%	—		—
Rate of compensation increase (for expense)	2.00	%	2.00	%	—		—
Discount rate (for benefit obligation)	4.58	%	4.74	%	4.12	%	4.36	%
Rate of compensation increase (for benefit obligation) *	—		2.00	%	—		—

* No assumption is required since the U.S. plans were frozen to future benefits after December 31, 2016.

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
	2016		2015		2016		2015
Weighted average assumptions:
Discount rate (for expense)	4.10	%	3.83	%	4.12	%	3.87	%
Expected return on plan assets	7.06	%	7.23	%	—		—
Rate of compensation increase (for expense)	2.44	%	2.42	%	—		—
Discount rate (for benefit obligation)	3.40	%	4.11	%	3.64	%	4.12	%
Rate of compensation increase (for benefit obligation)*	1.77	%	2.44	%	—		—

*The 2016 assumption does not include a rate of compensation increase for the U.S. defined benefit plans since those plans were frozen to future benefits after December 31, 2016.

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 Benefits									Postretirement Benefits
For the Years Ended December 31,
(In millions of dollars)	2016			2015			2014			2016			2015			2014
Service cost	$	106		$	114		$	91		$	—		$	1		$	2
Interest cost	264			254			253			2			2			7
Expected return on plan assets	(379		)	(373		)	(346		)	—			—			—
Amortization of prior service (credit) cost	—			—			(7		)	4			3			—
Recognized actuarial loss (gain)	74			146			112			(2		)	(2		)	(2		)
Net periodic benefit cost (credit)	$	65		$	141		$	103		$	4		$	4		$	7
Plan termination	—			—			—			—			(128		)	—
Total cost (credit)	$	65		$	141		$	103		$	4		$	(124	)	$	7

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)	2016			2015			2014			2016		2015		2014
Service cost	$	72		$	82		$	122		$	2	$	2	$	2
Interest cost	273			333			388			3		3		4
Expected return on plan assets	(561		)	(604		)	(644		)	—		—		—
Amortization of prior service cost	(1		)	(1		)	(9		)	—		—		—
Recognized actuarial loss	94			125			131			—		1		1
Net periodic benefit (credit) cost	(123		)	(65		)	(12		)	5		6		7
Settlement loss	—			1			—			—		—		—
Curtailment (gain) loss	(4		)	5			(65		)	—		—		—
Total (credit) cost	$	(127	)	$	(59	)	$	(77	)	$	5	$	6	$	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)	2016			2015			2014			2016			2015			2014
Service cost	$	178		$	196		$	213		$	2		$	3		$	4
Interest cost	537			587			641			5			5			11
Expected return on plan assets	(940		)	(977		)	(990		)	—			—			—
Amortization of prior service (credit) cost	(1		)	(1		)	(16		)	4			3			—
Recognized actuarial loss (gain)	168			271			243			(2		)	(1		)	(1		)
Net periodic benefit (credit) cost	$	(58	)	$	76		$	91		$	9		$	10		$	14
Curtailment (loss) gain	(4		)	5			(65		)	—			—			—
Plan termination	—			—			—			—			(128		)	—
Settlement loss	—			1			—			—			—			—
Total (credit) cost	$	(62	)	$	82		$	26		$	9		$	(118	)	$	14

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)	2016			2015			2016			2015
Change in benefit obligation:
Benefit obligation at beginning of year	$	9,076		$	10,018		$	79		$	93
Service cost	72			82			2			2
Interest cost	273			333			3			3
Employee contributions	7			8			—			—
Actuarial loss (gain)	1,966			(432		)	5			(6		)
Plan amendments	(49		)	(5		)	—			—
Effect of settlement	(27		)	(12		)	—			—
Effect of curtailment	(7		)	8			—			—
Benefits paid	(352		)	(337		)	(3		)	(3		)
Foreign currency changes	(1,290		)	(632		)	(5		)	(10		)
Other	1			45			—			—
Benefit obligation December 31	$	9,670		$	9,076		$	81		$	79
Change in plan assets:
Fair value of plan assets at beginning of year	$	9,826		$	10,410		$	—		$	—
Actual return on plan assets	1,815			187			—			—
Effect of settlement	(27		)	(12		)	—			—
Company contributions	187			166			3			3
Employee contributions	7			8			—			—
Benefits paid	(352		)	(337		)	(3		)	(3		)
Foreign currency changes	(1,439		)	(620		)	—			—
Other	—			24			—			—
Fair value of plan assets, December 31	$	10,017		$	9,826		$	—		$	—
Net funded status, December 31	$	347		$	750		$	(81	)	$	(79	)
Amounts recognized in the consolidated balance sheets:
Non-current assets	$	766		$	1,144		$	—		$	—
Current liabilities	(5		)	(5		)	(3		)	(3		)
Non-current liabilities	(414		)	(389		)	(78		)	(76		)
Net asset (liability) recognized, December 31	$	347		$	750		$	(81	)	$	(79	)
Amounts recognized in other comprehensive (loss) income:
Prior service (cost) credit	$	43		$	(3	)	$	—		$	—
Net actuarial loss	(3,081		)	(2,887		)	(11		)	(6		)
Total recognized accumulated other comprehensive (loss) income, December 31	$	(3,038	)	$	(2,890	)	$	(11	)	$	(6	)
Cumulative employer contributions in excess of (less than) net periodic cost	3,385			3,640			(70		)	(73		)
Net asset (liability) recognized in consolidated balance sheet, December 31	$	347		$	750		$	(81	)	$	(79	)
Accumulated benefit obligation, December 31	$	9,397		$	8,830		$	—		$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2016			2015			2016		2015
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(3	)	$	(2	)	$	—	$	—
Recognized as component of net periodic benefit credit	(1		)	(1		)	—		—
Effect of curtailment	—			(5		)	—		—
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	49			5			—		—
Exchange rate adjustments	(2		)	—			—		—
Prior service (cost) credit, December 31	$	43		$	(3	)	$	—	$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2016			2015			2016			2015
Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(2,887	)	$	(3,215	)	$	(6	)	$	(14	)
Recognized as component of net periodic benefit cost	94			125			—			1
Effect of settlement	—			2			—			—
Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
Liability experience	(1,966		)	432			(5		)	6
Asset experience	1,254			(417		)	—			—
Other	—			(20		)	—			—
Effect of curtailment	3			—			—			—
Total amount recognized as change in plan assets and benefit obligations	(709		)	(5		)	(5		)	6
Exchange rate adjustments	421			206			—			1
Net actuarial loss, December 31	$	(3,081	)	$	(2,887	)	$	(11	)	$	(6	)

For the Years Ended December 31,	Non-U.S. Pension Benefits							Non-U.S. Postretirement Benefits
(In millions of dollars)	2016		2015			2014		2016		2015			2014
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	21	$	(407	)	$	201	$	10	$	(2	)	$	5

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)	2016			2015			2016			2015
Change in benefit obligation:
Benefit obligation at beginning of year	$	5,685		$	5,924		$	40		$	177
Service cost	106			114			—			1
Interest cost	264			254			2			2
Employee contributions	—			—			3			3
Effect of curtailment	(98		)	—			—			—
Plan termination	—			—			—			(128		)
Actuarial loss (gain)	160			(392		)	—			(5		)
Benefits paid	(223		)	(215		)	(8		)	(10		)
Benefit obligation, December 31	$	5,894		$	5,685		$	37		$	40
Change in plan assets:
Fair value of plan assets at beginning of year	$	4,160		$	4,516		$	3		$	18
Actual return on plan assets	401			(170		)	—			—
Employer contributions	27			29			5			4
Employee contributions	—			—			3			3
Benefits paid	(223		)	(215		)	(8		)	(10		)
Other	—			—			(1		)	(12		)
Fair value of plan assets, December 31	$	4,365		$	4,160		$	2		$	3
Net funded status, December 31	$	(1,529	)	$	(1,525	)	$	(35	)	$	(37	)
Amounts recognized in the consolidated balance sheets:
Current liabilities	$	(27	)	$	(26	)	$	(2	)	$	(2	)
Non-current liabilities	(1,502		)	(1,499		)	(33		)	(35		)
Net liability recognized, December 31	$	(1,529	)	$	(1,525	)	$	(35	)	$	(37	)
Amounts recognized in other comprehensive income (loss):
Prior service (cost) credit	$	—		$	—		$	(3	)	$	(7	)
Net actuarial (loss) gain	(1,720		)	(1,754		)	11			13
Total recognized accumulated other comprehensive (loss) income, December 31	$	(1,720	)	$	(1,754	)	$	8		$	6
Cumulative employer contributions in excess of (less than) net periodic cost	191			229			(43		)	(43		)
Net amount recognized in consolidated balance sheet	$	(1,529	)	$	(1,525	)	$	(35	)	$	(37	)
Accumulated benefit obligation at December 31	$	5,894		$	5,600		$	—		$	—

	U.S. Pension Benefits				U.S. Postretirement Benefits
(In millions of dollars)	2016		2015		2016			2015
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	—	$	—	$	(7	)	$	4
Recognized as component of net periodic benefit cost	—		—		4			3
Plan termination	—		—		—			(14		)
Prior service cost, December 31	$	—	$	—	$	(3	)	$	(7	)

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)	2016		2015		2016			2015
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	—	$	—	$	(7	)	$	4
Recognized as component of net periodic benefit cost	—		—		4			3
Plan termination	—		—		—			(14		)
Prior service cost, December 31	$	—	$	—	$	(3	)	$	(7	)

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2016			2015			2016			2015
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(1,754	)	$	(1,749	)	$	13		$	2
Recognized as component of net periodic benefit cost (credit)	74			146			(2		)	(2		)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Effect of curtailment	98			—			—			—
Other	—			—			—			8
Liability experience	(160		)	392			—			5
Asset experience	22			(543		)	—			—
Total (loss) gain recognized as change in plan assets and benefit obligations	(40		)	(151		)	—			5
Net actuarial (loss) gain, December 31	$	(1,720	)	$	(1,754	)	$	11		$	13

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2016			2015			2016			2015
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(1,754	)	$	(1,749	)	$	13		$	2
Recognized as component of net periodic benefit cost (credit)	74			146			(2		)	(2		)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Effect of curtailment	98			—			—			—
Other	—			—			—			8
Liability experience	(160		)	392			—			5
Asset experience	22			(543		)	—			—
Total (loss) gain recognized as change in plan assets and benefit obligations	(40		)	(151		)	—			5
Net actuarial (loss) gain, December 31	$	(1,720	)	$	(1,754	)	$	11		$	13

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2016			2015			2016		2015
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(3	)	$	(2	)	$	—	$	—
Recognized as component of net periodic benefit credit	(1		)	(1		)	—		—
Effect of curtailment	—			(5		)	—		—
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	49			5			—		—
Exchange rate adjustments	(2		)	—			—		—
Prior service (cost) credit, December 31	$	43		$	(3	)	$	—	$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2016			2015			2016			2015
Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(2,887	)	$	(3,215	)	$	(6	)	$	(14	)
Recognized as component of net periodic benefit cost	94			125			—			1
Effect of settlement	—			2			—			—
Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
Liability experience	(1,966		)	432			(5		)	6
Asset experience	1,254			(417		)	—			—
Other	—			(20		)	—			—
Effect of curtailment	3			—			—			—
Total amount recognized as change in plan assets and benefit obligations	(709		)	(5		)	(5		)	6
Exchange rate adjustments	421			206			—			1
Net actuarial loss, December 31	$	(3,081	)	$	(2,887	)	$	(11	)	$	(6	)

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)	2016		2015		2014		2016		2015			2014
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	31	$	146	$	885	$	2	$	(138	)	$	14

For the Years Ended December 31,	Non-U.S. Pension Benefits							Non-U.S. Postretirement Benefits
(In millions of dollars)	2016		2015			2014		2016		2015			2014
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	21	$	(407	)	$	201	$	10	$	(2	)	$	5

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 in the next fiscal year:

	U.S. Pension Benefits		U.S. Postretirement Benefits
(In millions of dollars)	2017		2017
Prior service credit	$	—	$	(3	)
Net actuarial loss	37		1
Projected cost (credit)	$	37	$	(2	)

Estimated amounts that will be amortized from accumulated other comprehensive income in the next fiscal year:

	Non-U.S. Pension Benefits			Non-U.S. Postretirement Benefits
(In millions of dollars)	2017			2017
Prior service credit	$	(2	)	$	—
Net actuarial loss	126			1
Projected cost	$	124		$	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	$	1	$	—
Effect on postretirement benefit obligation	$	8	$	(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 Benefits				Postretirement Benefits
(In millions of dollars)	U.S.		Non-U.S.		U.S.		Non-U.S.
2017	$	244	$	240	$	4	$	3
2018	$	259	$	272	$	4	$	3
2019	$	276	$	281	$	4	$	3
2020	$	287	$	293	$	4	$	3
2021	$	297	$	303	$	3	$	4
2022-2026	$	1,641	$	1,719	$	14	$	19

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, 2016 and 2015:

	Fair Value Measurements at December 31, 2016
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	$	16	$	—	$	—	$	6,805	$	6,821
Corporate obligations	—		3,024		9		—		3,033
Corporate stocks	2,009		3		2		—		2,014
Private equity/partnerships	—		—		—		722		722
Government securities	11		380		—		—		391
Real estate	—		—		—		412		412
Short-term investment funds	297		22		—		—		319
Company common stock	270		—		—		—		270
Other investments	15		23		312		—		350
Total investments	$	2,618	$	3,452	$	323	$	7,939	$	14,332

	Fair Value Measurements at December 31, 2015
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	$	175	$	—	$	—	$	6,861	$	7,036
Corporate obligations	—		2,651		1		—		2,652
Corporate stocks	1,844		6		2		—		1,852
Private equity/partnerships	—		—		—		710		710
Government securities	10		415		—		—		425
Real estate	—		—		—		442		442
Short-term investment funds	312		4		—		—		316
Company common stock	222		—		—		—		222
Other investments	13		47		257		—		317
Total investments	$	2,576	$	3,123	$	260	$	8,013	$	13,972

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, 2016 and December 31, 2015:

Assets (In millions)	Fair Value, January 1, 2016		Purchases		Sales			Unrealized Gain/ (Loss)		Realized Gain/ (Loss)		Exchange Rate Impact			Transfers in/(out) and Other		Fair Value, December 31, 2016
Other investments	257		27		(28		)	67		1		(12		)	—		312
Corporate stocks	2		—		—			—		—		—			—		2
Corporate obligations	1		8		—			1		—		(1		)	—		9
Total assets	$	260	$	35	$	(28	)	$	68	$	1	$	(13	)	$	—	$	323

Assets (In millions)	Fair Value, January 1, 2015		Purchases		Sales			Unrealized Gain/ (Loss)		Realized Gain/ (Loss)		Exchange Rate Impact			Transfers in/(out) and Other			Fair Value, December 31, 2015
Other investments	239		47		(13		)	14		—		(30		)	—			257
Corporate stocks	1		—		—			1		—		—			—			2
Corporate obligations	3		—		—			—		—		—			(2		)	1
Total assets	$	243	$	47	$	(13	)	$	15	$	—	$	(30	)	$	(2	)	$	260