Retirement Benefits (Tables)

12 Months Ended

Dec. 31, 2015

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
	2015		2014		2015		2014
Weighted average assumptions:
Discount rate (for expense)	3.49	%	4.55	%	3.85	%	4.80	%
Expected return on plan assets	6.57	%	6.95	%	—		—
Rate of compensation increase (for expense)	2.67	%	2.99	%	—		—
Discount rate (for benefit obligation)	3.71	%	3.49	%	4.00	%	3.85	%
Rate of compensation increase (for benefit obligation)	2.72	%	2.67	%	—		—

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
	2015		2014		2015		2014
Weighted average assumptions:
Discount rate (for expense)	4.41	%	5.30	%	3.90	%	4.99	%
Expected return on plan assets	8.75	%	8.75	%	—		—
Rate of compensation increase (for expense)	2.00	%	2.00	%	—		—
Discount rate (for benefit obligation)	4.74	%	4.30	%	4.36	%	4.19	%
Rate of compensation increase (for benefit obligation)	2.00	%	2.00	%	—		—

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
	2015		2014		2015		2014
Weighted average assumptions:
Discount rate (for expense)	3.83	%	4.82	%	3.87	%	4.92	%
Expected return on plan assets	7.23	%	7.52	%	—		—
Rate of compensation increase (for expense)	2.42	%	2.64	%	—		—
Discount rate (for benefit obligation)	4.11	%	3.79	%	4.12	%	4.08	%
Rate of compensation increase (for benefit obligation)	2.44	%	2.42	%	—		—

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)	2015			2014			2013			2015			2014			2013
Service cost	$	114		$	91		$	104		$	1		$	2		$	3
Interest cost	254			253			229			2			7			7
Expected return on plan assets	(373		)	(346		)	(324		)	—			—			—
Amortization of prior service (credit) cost	—			(7		)	(16		)	3			—			—
Recognized actuarial loss (gain)	146			112			207			(2		)	(2		)	—
Net periodic benefit cost (credit)	$	141		$	103		$	200		$	4		$	7		$	10
Plan termination	—			—			—			(128		)	—			—
Total cost (credit)	$	141		$	103		$	200		$	(124	)	$	7		$	10

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)	2015			2014			2013			2015		2014		2013
Service cost	$	82		$	122		$	148		$	2	$	2	$	2
Interest cost	333			388			352			3		4		4
Expected return on plan assets	(604		)	(644		)	(587		)	—		—		—
Amortization of prior service cost	(1		)	(9		)	(6		)	—		—		—
Recognized actuarial loss	125			131			108			1		1		2
Net periodic benefit (credit) cost	(65		)	(12		)	15			6		7		8
Settlement loss	1			—			—			—		—		—
Curtailment loss (gain)	5			(65		)	—			—		—		—
Total (credit) cost	$	(59	)	$	(77	)	$	15		$	6	$	7	$	8

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)	2015			2014			2013			2015			2014			2013
Service cost	$	196		$	213		$	252		$	3		$	4		$	5
Interest cost	587			641			581			5			11			11
Expected return on plan assets	(977		)	(990		)	(911		)	—			—			—
Amortization of prior service (credit) cost	(1		)	(16		)	(22		)	3			—			—
Recognized actuarial loss (gain)	271			243			315			(1		)	(1		)	2
Net periodic benefit cost	$	76		$	91		$	215		$	10		$	14		$	18
Curtailment (loss) gain	5			(65		)	—			—			—			—
Plan termination	—			—			—			(128		)	—			—
Settlement loss	1			—			—			—			—			—
Total cost	$	82		$	26		$	215		$	(118	)	$	14		$	18

Schedule of MMC's defined benefit plans and postretirement plans

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)	2015			2014			2015			2014
Change in benefit obligation:
Benefit obligation at beginning of year	$	5,924		$	4,827		$	177		$	158
Service cost	114			91			1			2
Interest cost	254			253			2			7
Employee contributions	—			—			3			13
Plan amendments	—			—			—			(4		)
Plan termination	—			—			(128		)	—
Actuarial (gain) loss	(392		)	955			(5		)	21
Medicare part D subsidy	—			—			—			1
Benefits paid	(215		)	(202		)	(10		)	(21		)
Benefit obligation, December 31	$	5,685		$	5,924		$	40		$	177
Change in plan assets:
Fair value of plan assets at beginning of year	$	4,516		$	4,279		$	18		$	—
Plan combination	—			—			—			—
Actual return on plan assets	(170		)	414			—			—
Employer contributions	29			25			4			13
Employee contributions	—			—			3			13
Medicare part D subsidy	—			—			—			1
Benefits paid	(215		)	(202		)	(10		)	(21		)
Other	—			—			(12		)	12
Fair value of plan assets, December 31	$	4,160		$	4,516		$	3		$	18
Net funded status, December 31	$	(1,525	)	$	(1,408	)	$	(37	)	$	(159	)
Amounts recognized in the consolidated balance sheets:
Current liabilities	$	(26	)	$	(25	)	$	(2	)	$	(2	)
Non-current liabilities	(1,499		)	(1,383		)	(35		)	(157		)
Net liability recognized, December 31	$	(1,525	)	$	(1,408	)	$	(37	)	$	(159	)
Amounts recognized in other comprehensive income (loss):
Prior service (cost) credit	$	—		$	—		$	(7	)	$	4
Net actuarial (loss) gain	(1,754		)	(1,749		)	13			2
Total recognized accumulated other comprehensive (loss) income, December 31	$	(1,754	)	$	(1,749	)	$	6		$	6
Cumulative employer contributions in excess (less than) net periodic cost	229			341			(43		)	(165		)
Net amount recognized in consolidated balance sheet	$	(1,525	)	$	(1,408	)	$	(37	)	$	(159	)
Accumulated benefit obligation at December 31	$	5,600		$	5,825		$	—		$	—

	U.S. Pension Benefits					U.S. Postretirement Benefits
(In millions of dollars)	2015		2014			2015			2014
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	—		(7		)	3			—
Plan termination	—		—			(14		)	—
Plan amendment	—		—			—			4
Prior service (cost) credit, December 31	$	—	$	—		$	(7	)	$	4

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)	2015			2014			2015			2014
Change in benefit obligation:
Benefit obligation at beginning of year	$	10,018		$	8,711		$	93		$	97
Service cost	82			122			2			2
Interest cost	333			388			3			4
Employee contributions	8			10			—			—
Actuarial loss (gain)	(432		)	1,619			(6		)	(1		)
Plan amendments	(5		)	13			—			—
Effect of settlement	(12		)	(11		)	—			—
Effect of curtailment	8			—			—			—
Benefits paid	(337		)	(311		)	(3		)	(3		)
Foreign currency changes	(632		)	(585		)	(10		)	(6		)
Other	45			62			—			—
Benefit obligation December 31	$	9,076		$	10,018		$	79		$	93
Change in plan assets:
Fair value of plan assets at beginning of year	$	10,410		$	9,351		$	—		$	—
Actual return on plan assets	187			1,756			—			—
Effect of settlement	(12		)	(11		)	—			—
Company contributions	166			156			3			3
Employee contributions	8			10			—			—
Benefits paid	(337		)	(311		)	(3		)	(3		)
Foreign currency changes	(620		)	(578		)	—			—
Other	24			37			—			—
Fair value of plan assets, December 31	$	9,826		$	10,410		$	—		$	—
Net funded status, December 31	$	750		$	392		$	(79	)	$	(93	)
Amounts recognized in the consolidated balance sheets:
Non-current assets	$	1,144		$	967		$	—		$	—
Current liabilities	(5		)	(6		)	(3		)	(4		)
Non-current liabilities	(389		)	(569		)	(76		)	(89		)
Net asset (liability) recognized, December 31	$	750		$	392		$	(79	)	$	(93	)
Amounts recognized in other comprehensive (loss) income:
Prior service (cost) credit	$	(3	)	$	(2	)	$	—		$	—
Net actuarial loss	(2,887		)	(3,215		)	(6		)	(14		)
Total recognized accumulated other comprehensive (loss) income, December 31	$	(2,890	)	$	(3,217	)	$	(6	)	$	(14	)
Cumulative employer contributions in excess (deficient) of net periodic cost	3,640			3,609			(73		)	(79		)
Net asset (liability) recognized in consolidated balance sheet, December 31	$	750		$	392		$	(79	)	$	(93	)
Accumulated benefit obligation, December 31	$	8,830		$	9,731		$	—		$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2015			2014			2015		2014
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(2	)	$	85		$	—	$	—
Recognized as component of net periodic benefit credit	(1		)	(9		)	—		—
Effect of curtailment	(5		)	(65		)	—		—
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	5			(13		)	—		—
Exchange rate adjustments	—			—			—		—
Prior service (cost) credit, December 31	$	(3	)	$	(2	)	$	—	$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2015			2014			2015			2014
Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(3,215	)	$	(3,010	)	$	(14	)	$	(16	)
Recognized as component of net periodic benefit cost	125			131			1			1
Effect of settlement	2			—			—			—
Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
Liability experience	432			(1,619		)	6			1
Asset experience	(417		)	1,112			—			—
Other	(20		)	(14		)	—			—
Total amount recognized as change in plan assets and benefit obligations	(5		)	(521		)	6			1
Exchange rate adjustments	206			185			1			—
Net actuarial loss, December 31	$	(2,887	)	$	(3,215	)	$	(6	)	$	(14	)

For the Years Ended December 31,	Non-U.S. Pension Benefits								Non-U.S. Postretirement Benefits
(In millions of dollars)	2015			2014		2013			2015			2014		2013
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	(407	)	$	201	$	(276	)	$	(2	)	$	5	$	(2	)

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

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2015			2014			2015		2014
Reconciliation of prior service credit (cost) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(2	)	$	85		$	—	$	—
Recognized as component of net periodic benefit credit	(1		)	(9		)	—		—
Effect of curtailment	(5		)	(65		)	—		—
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	5			(13		)	—		—
Exchange rate adjustments	—			—			—		—
Prior service (cost) credit, December 31	$	(3	)	$	(2	)	$	—	$	—

	Non-U.S. Pension Benefits						Non-U.S. Postretirement Benefits
(In millions of dollars)	2015			2014			2015			2014
Reconciliation of net actuarial gain (loss) recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(3,215	)	$	(3,010	)	$	(14	)	$	(16	)
Recognized as component of net periodic benefit cost	125			131			1			1
Effect of settlement	2			—			—			—
Changes in plan assets and benefit obligations recognized in other comprehensive (loss) income:
Liability experience	432			(1,619		)	6			1
Asset experience	(417		)	1,112			—			—
Other	(20		)	(14		)	—			—
Total amount recognized as change in plan assets and benefit obligations	(5		)	(521		)	6			1
Exchange rate adjustments	206			185			1			—
Net actuarial loss, December 31	$	(2,887	)	$	(3,215	)	$	(6	)	$	(14	)

	U.S. Pension Benefits					U.S. Postretirement Benefits
(In millions of dollars)	2015		2014			2015			2014
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	—		(7		)	3			—
Plan termination	—		—			(14		)	—
Plan amendment	—		—			—			4
Prior service (cost) credit, December 31	$	—	$	—		$	(7	)	$	4

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2015			2014			2015			2014
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(1,749	)	$	(974	)	$	2		$	13
Recognized as component of net periodic benefit cost (credit)	146			112			(2		)	(2		)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Other	—			—			8			—
Liability experience	392			(955		)	5			(21		)
Asset experience	(543		)	68			—			12
Total (loss) gain recognized as change in plan assets and benefit obligations	(151		)	(887		)	5			(9		)
Net actuarial (loss) gain, December 31	$	(1,754	)	$	(1,749	)	$	13		$	2

For the Years Ended December 31,	U.S. Pension Benefits							U.S. Postretirement Benefits
(In millions of dollars)	2015		2014		2013			2015			2014		2013
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	146	$	885	$	(696	)	$	(138	)	$	14	$	(5	)

	U.S. Pension Benefits						U.S. Postretirement Benefits
(In millions of dollars)	2015			2014			2015			2014
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(1,749	)	$	(974	)	$	2		$	13
Recognized as component of net periodic benefit cost (credit)	146			112			(2		)	(2		)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Other	—			—			8			—
Liability experience	392			(955		)	5			(21		)
Asset experience	(543		)	68			—			12
Total (loss) gain recognized as change in plan assets and benefit obligations	(151		)	(887		)	5			(9		)
Net actuarial (loss) gain, December 31	$	(1,754	)	$	(1,749	)	$	13		$	2

Schedule of amounts recognized in other comprehensive income (loss)

For the Years Ended December 31,	Non-U.S. Pension Benefits								Non-U.S. Postretirement Benefits
(In millions of dollars)	2015			2014		2013			2015			2014		2013
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	(407	)	$	201	$	(276	)	$	(2	)	$	5	$	(2	)

For the Years Ended December 31,	U.S. Pension Benefits							U.S. Postretirement Benefits
(In millions of dollars)	2015		2014		2013			2015			2014		2013
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	146	$	885	$	(696	)	$	(138	)	$	14	$	(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)	2016		2016
Prior service credit	$	—	$	(4	)
Net actuarial loss	71		2
Projected cost (credit)	$	71	$	(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)	2016			2016
Prior service credit	$	1		$	—
Net actuarial loss	(100		)	—
Projected cost	$	(99	)	$	—

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	$	(1	)
Effect on postretirement benefit obligation	$	7	$	(6	)

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.

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.
2016	$	231	$	260	$	4	$	3
2017	$	247	$	274	$	4	$	4
2018	$	264	$	287	$	4	$	4
2019	$	274	$	299	$	4	$	4
2020	$	286	$	314	$	4	$	4
2021-2025	$	1,586	$	1,837	$	16	$	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, 2015 and 2014:

	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)		Total
Common/collective trusts	$	175	$	6,591	$	270	$	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	—		8		434		442
Short-term investment funds	312		4		—		316
Company common stock	222		—		—		222
Other investments	13		47		257		317
Total investments	$	2,576	$	9,722	$	1,674	$	13,972

	Fair Value Measurements at December 31, 2014
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)		Total
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

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, 2015 and December 31, 2014:

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
Private equity/Partnerships	$	727	$	223	$	(121	)	$	(263	)	$	159	$	(15	)	$	—		$	710
Real estate	375		50		(1		)	32			1		(23		)	—			434
Other investments	239		47		(13		)	14			—		(30		)	—			257
Common/Collective trusts	184		149		(27		)	(6		)	—		(30		)	—			270
Corporate stocks	1		—		—			1			—		—			—			2
Corporate obligations	3		—		—			—			—		—			(2		)	1
Total assets	$	1,529	$	469	$	(162	)	$	(222	)	$	160	$	(98	)	$	(2	)	$	1,674

Assets (In millions)	Fair Value, January 1, 2014		Purchases		Sales			Unrealized Gain/ (Loss)			Realized Gain/ (Loss)		Exchange Rate Impact			Transfers in/(out) and Other			Fair Value, December 31, 2014
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