Retirement Benefits (Tables)

12 Months Ended

Dec. 31, 2020

Retirement Benefits [Abstract]

Schedule of weighted average actuarial assumptions utilized defined benefit plans

The weighted average actuarial assumptions utilized for the U.S. and significant non-U.S. defined benefit plans and post-retirement benefit plans are as follows:

	Pension  Benefits				Post-retirement Benefits
	2020		2019		2020		2019
Weighted average assumptions:
Discount rate (for expense)	2.57	%	3.48	%	2.72	%	3.65	%
Expected return on plan assets	5.31	%	5.74	%	—		—
Rate of compensation increase (for expense)*	1.76	%	1.74	%	—		—
Discount rate (for benefit obligation)	1.92	%	2.57	%	2.42	%	2.72	%
Rate of compensation increase (for benefit obligation)*	1.85	%	1.76	%	—		—

The weighted average actuarial assumptions utilized in determining expense during the year and benefit obligation at the end of the year for the U.S. defined benefit and other U.S. post-retirement plans are as follows:

	U.S. Pension Benefits				U.S. Post-retirement Benefits
	2020		2019		2020		2019
Weighted average assumptions:
Discount rate (for expense)	3.44	%	4.45	%	3.10	%	4.24	%
Expected return on plan assets	7.82	%	7.95	%	—		—
Discount rate (for benefit obligation)	2.73	%	3.44	%	2.18	%	3.10	%

	Non-U.S. Pension Benefits				Non-U.S. Post-retirement Benefits
	2020		2019		2020		2019
Weighted average assumptions:
Discount rate (for expense)	2.09	%	2.89	%	2.53	%	3.32	%
Expected return on plan assets	4.35	%	4.87	%	—		—
Rate of compensation increase (for expense)	2.75	%	2.82	%	—		—
Discount rate (for benefit obligation)	1.49	%	2.09	%	1.96	%	2.53	%
Rate of compensation increase (for benefit obligation)	2.84	%	2.75	%	—		—

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 defined benefit and other post-retirement plans are as follows:

Combined U.S. and significant non-U.S. Plans	Pension						Post-retirement
For the Years Ended December 31,	Benefits						Benefits
(In millions)	2020		2019		2018		2020		2019		2018
Service cost	$	36	$	31	$	34	$	—	$	—	$	1
Interest cost	421		487		463		3		3		3
Expected return on plan assets	(844)		(863)		(864)		—		—		—
Amortization of prior service (credit)	—		—		(2)		(2)		(2)		(2)
Recognized actuarial loss (gain)	161		104		146		—		(1)		(1)
Net periodic benefit (credit) cost	$	(226)	$	(241)	$	(223)	$	1	$	—	$	1
Plan termination	1		—		—		—		—		—
Settlement loss	3		7		42		—		—		—
Total (credit) cost	$	(222)	$	(234)	$	(181)	$	1	$	—	$	1

The following chart provides the amounts reported in the consolidated statements of income:

Combined U.S. and significant non-U.S. Plans	Pension Benefits						Post-retirement Benefits
For the Years Ended December 31,
(In millions)	2020		2019		2018		2020		2019		2018
Compensation and benefits expense (Operating income)	$	36	$	31	$	34	$	—	$	—	$	1
Other net benefit (credit) cost	(258)		(265)		(215)		1		—		—
Total (credit) cost	$	(222)	$	(234)	$	(181)	$	1	$	—	$	1

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

U.S. Plans only	Pension Benefits						Post-retirement Benefits
For the Years Ended December 31,
(In millions)	2020		2019		2018		2020		2019		2018
Interest cost	213		241		235		1		1		1
Expected return on plan assets	(345)		(343)		(357)		—		—		—
Recognized actuarial loss (gain)	72		44		55		—		(1)		(1)
Net periodic benefit (credit) cost	$	(60)	$	(58)	$	(67)	$	1	$	—	$	—

The components of the net periodic benefit cost for the non-U.S. defined benefit and other post-retirement 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. Post-retirement Benefits
(In millions)	2020		2019		2018		2020		2019		2018
Service cost	$	36	$	31	$	34	$	—	$	—	$	1
Interest cost	208		246		228		2		2		2
Expected return on plan assets	(499)		(520)		(507)		—		—		—
Amortization of prior service credit	—		—		(2)		(2)		(2)		(2)
Recognized actuarial loss	89		60		91		—		—		—
Net periodic benefit (credit) cost	(166)		(183)		(156)		—		—		1
Settlement loss	3		7		42		—		—		—
Special termination benefits	1		—		—		—		—		—
Total (credit) cost	$	(162)	$	(176)	$	(114)	$	—	$	—	$	1

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 post-retirement benefit plans:

	U.S. Pension Benefits				U.S. Post-retirement Benefits
(In millions)	2020		2019		2020		2019
Change in benefit obligation:
Benefit obligation at beginning of year	$	6,322	$	5,529	$	31	$	32
Interest cost	213		241		1		1
Employee contributions	—		—		4		4
Plan combination	—		64		—		—
Actuarial (gain) loss	650		753		1		1
Benefits paid	(271)		(265)		(6)		(7)
Benefit obligation, December 31	$	6,914	$	6,322	$	31	$	31
Change in plan assets:
Fair value of plan assets at beginning of year	$	4,715	$	4,062	$	2	$	1
Actual return on plan assets	591		834		—		—
Employer contributions	65		35		3		4
Employee contributions	—		—		4		4
Benefits paid	(271)		(265)		(6)		(7)
Other	—		49		(1)		—
Fair value of plan assets, December 31	$	5,100	$	4,715	$	2	$	2
Net funded status, December 31	$	(1,814)	$	(1,607)	$	(29)	$	(29)
Amounts recognized in the consolidated balance sheets:
Current liabilities	$	(30)	$	(29)	$	(1)	$	(1)
Non-current liabilities	(1,784)		(1,578)		(28)		(28)
Net liability recognized, December 31	$	(1,814)	$	(1,607)	$	(29)	$	(29)
Amounts recognized in other comprehensive income (loss):
Net actuarial (loss) gain	(2,446)		(2,114)		3		4
Total recognized accumulated other comprehensive (loss) income, December 31	$	(2,446)	$	(2,114)	$	3	$	4
Cumulative employer contributions in excess of (less than) net periodic cost	632		507		(32)		(33)
Net amount recognized in consolidated balance sheet	$	(1,814)	$	(1,607)	$	(29)	$	(29)
Accumulated benefit obligation at December 31	$	6,914	$	6,322	$	—	$	—

The following schedules provide information concerning the Company’s non-U.S. defined benefit pension plans and non-U.S. post-retirement benefit plans:

	Non-U.S. Pension Benefits				Non-U.S. Post-retirement Benefits
(In millions)	2020		2019		2020		2019
Change in benefit obligation:
Benefit obligation at beginning of year	$	11,321	$	8,969	$	61	$	57
Service cost	36		31		—		—
Interest cost	208		246		2		2
Employee contributions	2		2		—		—
Plan combination	—		915		—		—
Actuarial loss	1,273		1,339		10		3
Plan amendments	11		(1)		—		—
Effect of settlement	(13)		(25)		—		—
Special termination benefits	1		—		—		—
Benefits paid	(402)		(364)		(2)		(3)
Foreign currency changes	561		209		2		2
Benefit obligation, December 31	$	12,998	$	11,321	$	73	$	61
Change in plan assets:
Fair value of plan assets at beginning of year	$	12,313	$	10,306	$	—	$	—
Plan combination	—		683		—		—
Actual return on plan assets	1,415		1,367		—		—
Effect of settlement	(13)		(25)		—		—
Company contributions	78		87		2		3
Employee contributions	2		2		—		—
Benefits paid	(402)		(364)		(2)		(3)
Foreign currency changes	635		257		—		—
Fair value of plan assets, December 31	$	14,028	$	12,313	$	—	$	—
Net funded status, December 31	$	1,030	$	992	$	(73)	$	(61)
Amounts recognized in the consolidated balance sheets:
Non-current assets	$	1,764	$	1,632	$	—	$	—
Current liabilities	(7)		(6)		(3)		(3)
Non-current liabilities	(727)		(634)		(70)		(58)
Net asset (liability) recognized, December 31	$	1,030	$	992	$	(73)	$	(61)
Amounts recognized in other comprehensive (loss) income:
Prior service credit	$	(13)	$	(2)	$	9	$	11
Net actuarial loss	(3,467)		(3,055)		(16)		(5)
Total recognized accumulated other comprehensive (loss) income, December 31	$	(3,480)	$	(3,057)	$	(7)	$	6
Cumulative employer contributions in excess of (less than) net periodic cost	4,510		4,049		(66)		(67)
Net asset (liability) recognized in consolidated balance sheets, December 31	$	1,030	$	992	$	(73)	$	(61)
Accumulated benefit obligation, December 31	$	12,736	$	11,079	$	—	$	—

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

	U.S. Pension Benefits				U.S. Post-retirement Benefits
(In millions)	2020		2019		2020		2019
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(2,114)	$	(1,896)	$	4	$	6
Recognized as component of net periodic benefit cost (credit)	72		44		—		(1)
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Liability experience	(650)		(753)		(1)		(1)
Asset experience	246		491		—		—
Total loss recognized as change in plan assets and benefit obligations	(404)		(262)		(1)		(1)
Net actuarial (loss) gain, December 31	$	(2,446)	$	(2,114)	$	3	$	4

	Non-U.S. Pension Benefits				Non-U.S. Post-retirement Benefits
(In millions)	2020		2019		2020		2019
Reconciliation of prior service (cost) credit recognized in accumulated other comprehensive income (loss):
Beginning balance	$	(2)	$	(2)	$	11	$	12
Recognized as component of net periodic benefit credit:
Amortization of prior service credit	—		—		(2)		(2)
Total recognized as component of net periodic benefit credit	—		—		(2)		(2)
Changes in plan assets and benefit obligations recognized in other comprehensive income:
Plan amendments	(11)		1		—		—
Exchange rate adjustments	—		(1)		—		1
Prior service (cost) credit, December 31	$	(13)	$	(2)	$	9	$	11

	Non-U.S. Pension Benefits				Non-U.S. Post-retirement Benefits
(In millions)	2020		2019		2020		2019
Reconciliation of net actuarial (loss) gain recognized in accumulated other comprehensive (loss) income:
Beginning balance	$	(3,055)	$	(2,568)	$	(5)	$	(1)
Recognized as component of net periodic benefit cost:
Amortization of net loss	89		60		—		—
Effect of settlement	3		7		—		—
Total recognized as component of net periodic benefit credit	92		67		—		—
Changes in plan assets and benefit obligations recognized in other comprehensive income (loss):
Liability experience	(1,273)		(1,339)		(10)		(3)
Asset experience	916		847		—		—
Total amount recognized as change in plan assets and benefit obligations	(357)		(492)		(10)		(3)
Exchange rate adjustments	(147)		(62)		(1)		(1)
Net actuarial loss, December 31	$	(3,467)	$	(3,055)	$	(16)	$	(5)

Schedule of amounts recognized in other comprehensive income (loss)

For the Years Ended December 31,	U.S. Pension Benefits						U.S. Post-retirement Benefits
(In millions)	2020		2019		2018		2020		2019		2018
Total recognized in net periodic benefit cost and other comprehensive loss	$	272	$	160	$	63	$	2	$	2	$	—

For the Years Ended December 31,	Non-U.S. Pension Benefits						Non-U.S. Post-retirement Benefits
(In millions)	2020		2019		2018		2020		2019		2018
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$	261	$	311	$	(147)	$	13	$	5	$	(5)

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

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

For the Years Ended December 31,	Pension Benefits				Post-retirement Benefits
(In millions)	U.S.		Non-U.S.		U.S.		Non-U.S.
2021	$	291	$	344	$	4	$	3
2022	$	302	$	353	$	3	$	3
2023	$	315	$	373	$	3	$	3
2024	$	321	$	382	$	3	$	3
2025	$	327	$	394	$	3	$	3
2026-2030	$	1,701	$	2,160	$	10	$	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, 2020 and 2019:

	Fair Value Measurements at December 31, 2020
Assets (In millions)	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	$	561	$	—	$	—	$	4,298	$	4,859
Corporate obligations	—		4,707		2		—		4,709
Corporate stocks	2,737		39		1		—		2,777
Private equity/partnerships	—		—		—		1,353		1,353
Government securities	15		4,331		—		—		4,346
Real estate	—		—		—		487		487
Short-term investment funds	1,040		—		—		—		1,040
Company common stock	234		—		—		—		234
Other investments	13		7		771		—		791
Total investments	$	4,600	$	9,084	$	774	$	6,138	$	20,596
Net derivative liabilities	—		(1,522)		—		—		(1,522)
Net Investments	$	4,600	$	7,562	$	774	$	6,138	$	19,074

	Fair Value Measurements at December 31, 2019
Assets (In millions)	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	$	492	$	—	$	—	$	5,959	$	6,451
Corporate obligations	—		4,063		—		—		4,063
Corporate stocks	2,871		34		1		—		2,906
Private equity/partnerships	—		—		—		1,055		1,055
Government securities	20		679		—		—		699
Real estate	—		—		—		660		660
Short-term investment funds	309		3		—		—		312
Company common stock	223		—		—		—		223
Other investments	15		17		682		2		716
Total investments	$	3,930	$	4,796	$	683	$	7,676	$	17,085

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, 2020 and December 31, 2019:

Assets (In millions)	Fair Value, January 1, 2020		Purchases		Sales		Unrealized Gain/ (Loss)		Realized Gain/ (Loss)		Exchange Rate Impact		Transfers in/(out) and Other		Fair Value, December 31, 2020
Other investments	$	682	$	20	$	(12)	$	25	$	1	$	55	$	2	$	773
Corporate stocks	1		—		—		—		—		—		—		1
Total assets	$	683	$	20	$	(12)	$	25	$	1	$	55	$	2	$	774

Assets (In millions)	Fair Value, January 1, 2019		Purchases		Sales		Unrealized Gain/ (Loss)		Realized Gain/ (Loss)		Exchange Rate Impact		Transfers in/(out) and Other (a)		Fair Value, December 31, 2019
Other investments	$	333	$	17	$	(14)	$	72	$	1	$	(9)	$	282	$	682
Corporate stocks	1		—		—		—		—		—		—		1
Total assets	$	334	$	17	$	(14)	$	72	$	1	$	(9)	$	282	$	683

(a) Transfers in during 2019 are primarily related to the inclusion of JLT plan assets.