Employee Benefits (Tables)

12 Months Ended

Dec. 31, 2017

Defined Benefit Plan Disclosure [Line Items]

Change in Plan Assets

	U.S. PLAN						NON-U.S. PLANS
(In Millions)	2017			2016			2017			2016
Fair Value of Assets at Beginning of Period	$	1,393.5		$	1,342.0		$	139.3		$	144.3
Actual Return on Assets	192.5			110.0			12.4			21.9
Employer Contributions	—			—			3.0			4.3
Employee Contributions							0.1
Settlement	—			—			(6.8		)	(7.3		)
Acquisitions/Divestitures	—			—			18.5			—
Benefits Paid	(79.6		)	(58.5		)	(3.0		)	(2.4		)
Foreign Exchange Rate Changes	—			—			15.2			(21.5		)
Fair Value of Assets at End of Period	$	1,506.4		$	1,393.5		$	178.7		$	139.3

Fair Values of U.S. Pension Plan Assets by Major Asset Category, and their Level within Fair Value Hierarchy

The following table presents the fair values of Northern Trust’s U.S. pension plan assets, by major asset category, and their level within the fair value hierarchy defined by GAAP as of December 31, 2017 and 2016.

TABLE 98: FAIR VALUE OF U.S. PENSION PLAN ASSETS

	December 31, 2017
(In Millions)	LEVEL 1		LEVEL 2		LEVEL 3		TOTAL
Domestic Common Stock	$	38.6	$	—	$	—	$	38.6
Foreign Common Stock	—		—		—		—
U.S. Government Obligations	—		1,072.0		—		1,072.0
Northern Trust Mutual Fund	44.5		—		—		44.5
Northern Trust Collective Trust Funds	—		268.6		—		268.6
Northern Trust Private Equity Funds	—		—		29.3		29.3
Northern Trust Hedge Funds	—		—		44.6		44.6
Cash and Other	8.7		—		—		8.7
Total Assets at Fair Value	$	91.9	$	1,340.6	$	73.9	$	1,506.4

	December 31, 2016
(In Millions)	LEVEL 1		LEVEL 2		LEVEL 3		TOTAL
Domestic Common Stock	$	52.1	$	—	$	—	$	52.1
Foreign Common Stock	0.1		—		—		0.1
U.S. Government Obligations	—		134.6		—		134.6
Northern Trust Mutual Fund	70.7		—		—		70.7
Northern Trust Collective Trust Funds	—		1,029.9		—		1,029.9
Northern Trust Private Equity Funds	—		—		35.7		35.7
Northern Trust Hedge Funds	—		—		64.8		64.8
Cash and Other	5.6		—		—		5.6
Total Assets at Fair Value	$	128.5	$	1,164.5	$	100.5	$	1,393.5

Health Care Cost Trend Rate Sensitivity Analysis

For example, increasing or decreasing the assumed health care trend rate by one percentage point in each year would have the following effect.

TABLE 105: HEALTH CARE COST TREND RATE ASSUMPTION

(In Millions)	1–PERCENTAGE POINT INCREASE		1–PERCENTAGE POINT DECREASE
Effect on Postretirement Benefit Obligation	$	0.8	$	(0.7	)
Effect on Total Service and Interest Cost Components	—		—

Pension Plans, Defined Benefit

Defined Benefit Plan Disclosure [Line Items]

Plan Status of U.S. Plan, Non-U.S. Plans and Supplemental Plan

The following tables set forth the status, amounts included in AOCI, and net periodic pension expense of the U.S. plan, non-U.S. plans, and supplemental plan for 2017, 2016, and 2015. Prior service costs are being amortized on a straight-line basis over 11 years for the U.S. plan and 9 years for the supplemental plan.

TABLE 92: EMPLOYEE BENEFIT PLAN STATUS

	U.S. PLAN						NON-U.S. PLANS						SUPPLEMENTAL PLAN
($ In Millions)	2017			2016			2017			2016			2017			2016
Accumulated Benefit Obligation	$	1,088.4		$	953.2		$	192.2		$	158.3		$	129.0		$	108.9
Projected Benefit Obligation	1,209.9			1,062.7			198.3			155.9			144.5			121.1
Plan Assets at Fair Value	1,506.4			1,393.5			178.7			139.3			—			—
Funded Status at December 31	$	296.5		$	330.8		$	(19.6	)	$	(16.6	)	$	(144.5	)	$	(121.1	)
Weighted-Average Assumptions:
Discount Rates	3.79		%	4.46		%	2.08		%	2.39		%	3.79		%	4.46		%
Rate of Increase in Compensation Level	4.39			4.39			N/A			N/A			4.39			4.39
Expected Long-Term Rate of Return on Assets	6.00			6.75			2.61			3.22			N/A			N/A

Amount Included in Accumulated Other Comprehensive Income

	U.S. PLAN						NON-U.S. PLANS				SUPPLEMENTAL PLAN
(In Millions)	2017			2016			2017		2016		2017		2016
Net Actuarial Loss	$	399.0		$	378.1		$	44.2	$	55.8	$	83.2	$	67.4
Prior Service Cost	(1.8		)	(2.3		)	2.5		—		0.6		0.8
Gross Amount in Accumulated Other Comprehensive Income	397.2			375.8			46.7		55.8		83.8		68.2
Income Tax Effect	151.6			142.3			5.3		6.7		31.9		25.8
Net Amount in Accumulated Other Comprehensive Income	$	245.6		$	233.5		$	41.4	$	49.1	$	51.9	$	42.4

Components of Company's Net Periodic Benefit Cost

	U.S. PLAN									NON-U.S. PLANS									SUPPLEMENTAL PLAN
($ In Millions)	2017			2016			2015			2017			2016			2015			2017			2016			2015
Service Cost	$	38.3		$	37.4		$	37.8		$	0.4		$	—		$	—		$	3.7		$	3.5		$	3.6
Interest Cost	45.9			45.8			44.7			4.0			4.7			5.7			5.2			5.1			5.0
Expected Return on Plan Assets	(93.8		)	(94.4		)	(96.5		)	(4.5		)	(4.6		)	(5.9		)	—			N/A			N/A
Settlement Expense	—			—			—			1.1			3.7			—			—			—			—
Amortization:
Net Loss	19.0			18.8			29.7			1.3			1.0			1.5			5.7			5.8			7.3
Prior Service Cost	(0.4		)	(0.4		)	(0.4		)	0.1			—			—			0.2			0.2			0.2
Net Periodic Pension Expense	$	9.0		$	7.2		$	15.3		$	2.4		$	4.8		$	1.3		$	14.8		$	14.6		$	16.1
Weighted-Average Assumptions:
Discount Rates	4.46		%	4.71		%	4.25		%	2.33		%	3.39		%	3.20		%	4.46		%	4.71		%	4.25		%
Rate of Increase in Compensation Level	4.39			4.25			4.25			1.75			N/A			N/A			4.39			4.25			4.25
Expected Long-Term Rate of Return on Assets	6.75			7.00			7.25			3.13			3.73			4.00			N/A			N/A			N/A

Change in Benefit Obligation, Postretirement Health Care, and in Accumulated Postretirement Benefit Obligation

	U.S. PLAN						NON-U.S. PLANS						SUPPLEMENTAL PLAN
(In Millions)	2017			2016			2017			2016			2017			2016
Beginning Balance	$	1,062.7		$	1,006.5		$	155.9		$	156.5		$	121.1		$	113.9
Service Cost	38.3			37.4			0.4			—			3.7			3.5
Interest Cost	45.9			45.8			4.0			4.7			5.2			5.1
Employee Contributions	—			—			0.1			—			—			—
Plan Amendment	—			—			2.5			—			—			—
Actuarial (Gain) Loss	142.6			31.5			0.4			26.0			21.5			7.4
Settlement	—			—			(6.8		)	(7.3		)	—			—
Acquisitions/Divestitures	—			—			27.0			—			—			—
Benefits Paid	(79.6		)	(58.5		)	(3.0		)	(2.4		)	(7.0		)	(8.8		)
Foreign Exchange Rate Changes	—			—			17.8			(21.6		)	—			—
Ending Balance	$	1,209.9		$	1,062.7		$	198.3		$	155.9		$	144.5		$	121.1

Estimated Future Benefit Payments

(In Millions)	U.S. PLAN		NON-U.S. PLANS		SUPPLEMENTAL PLAN
2018	$	77.2	$	3.7	$	8.0
2019	75.7		3.5		12.5
2020	78.0		3.9		12.7
2021	75.9		4.2		15.5
2022	74.8		4.0		14.9
2023-2027	375.1		24.9		66.8

Changes in Level 3 U.S. Pension Plan Assets

The following table presents the changes in Level 3 assets for the years ended December 31, 2017 and 2016.

TABLE 99: CHANGE IN LEVEL 3 ASSETS

	PRIVATE EQUITY                       FUNDS						HEDGE FUNDS
(In Millions)	2017			2016			2017			2016
Fair Value at January 1	$	35.7		$	47.5		$	64.8		$	63.4
Actual Return on Plan Assets	(5.4		)	(5.6		)	(3.1		)	1.5
Realized Gain	—			—			5.0			—
Purchases	0.8			2.0			—			—
Sales	(1.8		)	(8.2		)	(22.1		)	(0.1		)
Fair Value at December 31	$	29.3		$	35.7		$	44.6		$	64.8

Note: The return on plan assets represents the change in the unrealized gain (loss) on assets still held at December 31.

Other Postretirement Plan

Defined Benefit Plan Disclosure [Line Items]

Plan Status of U.S. Plan, Non-U.S. Plans and Supplemental Plan

The following tables set forth the postretirement health care plan status and amounts included in AOCI at December 31, the net periodic postretirement benefit cost of the plan for 2017 and 2016, and the change in the accumulated postretirement benefit obligation during 2017 and 2016.

TABLE 100: POSTRETIREMENT HEALTH CARE PLAN STATUS

	DECEMBER 31,
(In Millions)	2017		2016
Accumulated Postretirement Benefit Obligation at Measurement Date:
Retirees and Dependents	$	27.7	$	26.4
Actives Eligible for Benefits	6.7		7.7
Net Postretirement Benefit Obligation	$	34.4	$	34.1

Amount Included in Accumulated Other Comprehensive Income

	DECEMBER 31,
(In Millions)	2017		2016
Net Actuarial Loss / (Gain)	$	3.9	$	0.3
Prior Service Cost	—		—
Gross Amount in Accumulated Other Comprehensive Income	3.9		0.3
Income Tax Effect	1.5		0.1
Net Amount in Accumulated Other Comprehensive Income	$	2.4	$	0.2

Components of Company's Net Periodic Benefit Cost

	FOR THE YEAR ENDED DECEMBER 31,
(In Millions)	2017		2016		2015
Service Cost	$	0.1	$	0.1	$	0.1
Interest Cost	1.4		1.5		1.4
Expected Return on Plan Assets	—		—		—
Amortization
Net Gain	—		—		—
Prior Service Benefit	—		—		—
Net Periodic Postretirement Expense	$	1.5	$	1.6	$	1.5

Change in Benefit Obligation, Postretirement Health Care, and in Accumulated Postretirement Benefit Obligation

	FOR THE YEAR ENDED                       DECEMBER 31,
(In Millions)	2017			2016
Beginning Balance	$	34.1		$	32.2
Service Cost	0.1			0.1
Interest Cost	1.4			1.5
Actuarial Loss / (Gain)	(0.2		)	2.7
Net Claims Paid	(1.0		)	(2.4		)
Medicare Subsidy	—			—
Ending Balance	$	34.4		$	34.1

Estimated Future Benefit Payments

(In Millions)	TOTAL POSTRETIREMENT MEDICAL BENEFITS
2018	$	2.7
2019	2.7
2020	2.7
2021	2.6
2022	2.5
2023-2027	11.4