Northern Trust (NTRS) 10-Q 28 Apr 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	NTRS
Entity Registrant Name	NORTHERN TRUST CORP
Entity Central Index Key	73124
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding	233,368,512

Consolidated_Balance_Sheet

Consolidated Balance Sheet (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Assets
Cash and Due from Banks	$4,074.80	$3,050.60
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,113.90	1,062.70
Interest-Bearing Deposits with Banks	15,432.30	14,928.30
Federal Reserve Deposits	8,804.20	17,386.30
Securities
Available for Sale	30,945.70	29,558.50
Held to Maturity (Fair value of $5,665.0 and $4,176.1)	5,655.80	4,170.80
Trading Account	1.4	4.7
Total Securities	36,602.90	33,734
Loans and Leases
Commercial	15,491.30	14,353.60
Personal	17,138.90	17,286.60
Total Loans and Leases	32,630.20	31,640.20
Allowance for Credit Losses Assigned to Loans and Leases	-259	-267
Buildings and Equipment	442.4	444.3
Client Security Settlement Receivables	2,219.30	1,568.80
Goodwill	527.3	533.2
Other Assets	5,363.70	5,865.10
Total Assets	106,952	109,946.50
Deposits
Demand and Other Noninterest-Bearing	20,816.60	22,815
Savings and Money Market	15,892.20	15,916.40
Savings Certificates and Other Time	1,777.90	1,757.40
Non U.S. Offices - Noninterest-Bearing	2,307.60	2,723.20
Non U.S. Offices - Interest-Bearing	45,942.20	47,545
Total Deposits	86,736.50	90,757
Federal Funds Purchased	410.7	932.9
Securities Sold under Agreements to Repurchase	613.2	885.1
Other Borrowings	2,874.10	1,685.20
Senior Notes	1,497.10	1,497
Long-Term Debt	1,399.10	1,615.10
Floating Rate Capital Debt	277.2	277.2
Other Liabilities	4,536.50	3,848.10
Total Liabilities	98,344.40	101,497.60
Stockholders' Equity
Preferred Stock, No Par Value; Authorized 10,000,000 shares: Series C, outstanding shares of 16,000	388.5	388.5
Common Stock, $1.66 2/3 Par Value; Authorized 560,000,000 shares; Outstanding shares of 233,368,512 and 233,390,705	408.6	408.6
Additional Paid-In Capital	1,024.90	1,050.90
Retained Earnings	7,771.70	7,625.40
Accumulated Other Comprehensive Loss	-266.4	-319.7
Treasury Stock (11,803,012 and 11,780,819 shares, at cost)	-719.7	-704.8
Total Stockholders' Equity	8,607.60	8,448.90
Total Liabilities and Stockholders' Equity	$106,952	$109,946.50

Consolidated_Balance_Sheet_Par

Consolidated Balance Sheet (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, except Share data, unless otherwise specified
Held to Maturity, Fair value	$5,665	$4,176.10
Total Loans and Leases, unearned income	$277	$287.70
Preferred Stock, Par Value	$0	$0
Preferred Stock, Authorized	10,000,000	10,000,000
Preferred Stock, Outstanding	16,000	16,000
Common Stock, Par Value	$1.67	$1.67
Common Stock, Authorized	560,000,000	560,000,000
Common Stock, Outstanding	233,368,512	233,390,705
Treasury Stock, shares	11,803,012	11,780,819

Consolidated_Statement_of_Inco

Consolidated Statement of Income (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Noninterest Income
Trust, Investment and Other Servicing Fees	$727.50		$679.50
Foreign Exchange Trading Income	71.6		50.1
Treasury Management Fees	16.3		16.8
Security Commissions and Trading Income	19.8		14.7
Other Operating Income	38.6		37.7
Investment Security Gains (Losses), net	0.1	[1]	-4	[1]
Total Noninterest Income	873.9		794.8
Net Interest Income
Interest Income	298.8		295.4
Interest Expense	38.2		49.7
Net Interest Income	260.6		245.7
Provision for Credit Losses	-4.5		3
Net Interest Income after Provision for Credit Losses	265.1		242.7
Noninterest Expense
Compensation	354.3		341.8
Employee Benefits	72.9		66.9
Outside Services	135.1		144.4
Equipment and Software	110.3		101.3
Occupancy	43		44.2
Other Operating Expense	73.4		69.4
Total Noninterest Expense	789		768
Income before Income Taxes	350		269.5
Provision for Income Taxes	119.3		88.1
Net Income	230.7		181.4
Preferred Stock Dividends	5.9
Net Income Applicable to Common Stock	$224.80		$181.40
Per Common Share
Net Income - Basic	$0.95		$0.75
Net Income - Diluted	$0.94		$0.75
Average Number of Common Shares Outstanding - Basic	233,381,168		237,208,151
Average Number of Common Shares Outstanding - Diluted	235,288,695		239,050,714
[1]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ (4.6 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - 0.7 Other Security Gains (Losses), net 0.1 (0.1 ) Investment Security Gains (Losses), net $ 0.1 $ (4.0 )

Consolidated_Statement_of_Comp

Consolidated Statement of Comprehensive Income (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Net Income	$230.70	$181.40
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)
Net Unrealized Gains on Securities Available for Sale	54.1	4.5
Net Unrealized Gains (Losses) on Cash Flow Hedges	-1.1	1.6
Foreign Currency Translation Adjustments	-5.9	-1.3
Pension and Other Postretirement Benefit Adjustments	6.2	3.4
Other Comprehensive Income (Loss)	53.3	8.2
Comprehensive Income	$284	$189.60

Consolidated_Statement_of_Inco1

Consolidated Statement of Income (Parenthetical) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Changes in Other-Than-Temporary-Impairment (OTTI) Losses			($4.60)	[1]
Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI			0.7	[2]
Other Security Gains (Losses), net	0.1		-0.1
Investment Security Gains (Losses), net	$0.10	[3]	($4)	[3]
[1]	For initial OTTI in the applicable period, the balance includes the excess of the amortized cost over the fair value of the impaired securities. For subsequent impairments of the same security, the balance includes any additional changes in fair value of the security subsequent to its most recently recorded OTTI.
[2]	For initial OTTI in the applicable period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in other comprehensive income (OCI). For subsequent impairments of the same security, the balance includes additional changes in OCI for that security subsequent to its most recently recorded OTTI.
[3]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ (4.6 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - 0.7 Other Security Gains (Losses), net 0.1 (0.1 ) Investment Security Gains (Losses), net $ 0.1 $ (4.0 )

Consolidated_Statement_of_Chan

Consolidated Statement of Changes in Stockholders' Equity (USD $)	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
In Millions
Balance at Dec. 31, 2013			$408.60	$1,035.70	$7,134.80	($244.30)	($422.80)
Net Unrealized Gains (Losses) on Securities Available for Sale	4.5					4.5
Stock Options and Awards				-43.2			97.3
Net Income	181.4				181.4
Net Unrealized Gains (Losses) on Cash Flow Hedges	1.6					1.6
Stock Options and Awards - Amortization				23.3
Dividends Declared - Common Stock					-74.8
Stock Purchased							-163
Foreign Currency Translation Adjustments	-1.3					-1.3
Stock Options and Awards - Tax Benefits				7.1
Pension and Other Postretirement Benefit Adjustments	3.4					3.4
Balance at Mar. 31, 2014	7,948.30		408.6	1,022.90	7,241.40	-236.1	-488.5
Balance at Dec. 31, 2014	8,448.90	388.5	408.6	1,050.90	7,625.40	-319.7	-704.8
Net Unrealized Gains (Losses) on Securities Available for Sale	54.1					54.1
Stock Options and Awards				-60.1			92.3
Net Income	230.7				230.7
Net Unrealized Gains (Losses) on Cash Flow Hedges	-1.1					-1.1
Stock Options and Awards - Amortization				23.4
Dividends Declared - Common Stock					-78.5
Stock Purchased	-107.2						-107.2
Foreign Currency Translation Adjustments	-5.9					-5.9
Stock Options and Awards - Tax Benefits				10.7
Dividends Declared - Preferred Stock					-5.9
Pension and Other Postretirement Benefit Adjustments	6.2					6.2
Balance at Mar. 31, 2015	$8,607.60		$408.60	$1,024.90	$7,771.70	($266.40)	($719.70)

Consolidated_Statement_of_Cash

Consolidated Statement of Cash Flows (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Cash Flows from Operating Activities:
Net Income	$230.70		$181.40
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Investment Security Losses (Gains), net	-0.1	[1]	4	[1]
Amortization and Accretion of Securities and Unearned Income, net	10.7		3
Provision for Credit Losses	-4.5		3
Depreciation on Buildings and Equipment	23.1		23
Pension Plan Contributions	-16.5		-13.9
Change in Receivables	50.3		-254.9
Change in Interest Payable	-6.2		-0.1
Change in Collateral With Derivative Counterparties, net	289.5		505.7
Other Operating Activities, net	-151.5		26.8
Net Cash Provided by Operating Activities	490.8		538.5
Cash Flows from Investing Activities:
Net Change in Federal Funds Sold and Securities Purchased under Agreements to Resell	-51.2		19.6
Change in Interest-Bearing Deposits with Banks	-1,242.60		2,329.10
Net Change in Federal Reserve Deposits	8,582.10		700.2
Purchases of Securities - Held to Maturity	-2,101.10		-2,467.40
Proceeds from Maturity and Redemption of Securities - Held to Maturity	278.4		597.6
Purchases of Securities - Available for Sale	-2,992.50		-3,939.10
Proceeds from Sale, Maturity and Redemption of Securities - Available for Sale	1,686.80		2,803.70
Change in Loans and Leases	-1,004.20		-274.9
Purchases of Buildings and Equipment	-25.5		-13.5
Purchases and Development of Computer Software	-68.4		-74.9
Change in Client Security Settlement Receivables	-660.7		-485.6
Other Investing Activities, net	404.8		-16.9
Net Cash Provided by (Used in) Investing Activities	2,805.90		-822.1
Cash Flows from Financing Activities:
Change in Deposits	-2,817.70		2,082.40
Change in Federal Funds Purchased	-522.2		48.8
Change in Securities Sold under Agreements to Repurchase	-271.9		-180.7
Change in Short-Term Other Borrowings	1,064.20		-1,004.40
Repayments of Senior Notes and Long-Term Debt	-230.1		-1.1
Treasury Stock Purchased	-107.2		-163
Net Proceeds from Stock Options	32.2		54
Cash Dividends Paid on Common Stock	-77		-75
Cash Dividends Paid on Preferred Stock	-9.5
Other Financing Activities, net	747.9		27.1
Net Cash (Used in) Provided by Financing Activities	-2,191.30		788.1
Effect of Foreign Currency Exchange Rates on Cash	-81.2		9.4
Increase in Cash and Due from Banks	1,024.20		513.9
Cash and Due from Banks at Beginning of Year	3,050.60		3,162.40
Cash and Due from Banks at End of Period	4,074.80		3,676.30
Supplemental Disclosures of Cash Flow Information:
Interest Paid	44.9		50.4
Income Taxes Paid	40.5		15.1
Transfers from Loans to OREO	4.6		1.9
Computer Software
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Amortization of Intangibles	60.7		55.6
Other Intangible Assets
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Amortization of Intangibles	$4.60		$4.90
[1]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ (4.6 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - 0.7 Other Security Gains (Losses), net 0.1 (0.1 ) Investment Security Gains (Losses), net $ 0.1 $ (4.0 )

Basis_of_Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2015
Basis of Presentation	1. Basis of Presentation — The consolidated financial statements include the accounts of Northern Trust Corporation (Corporation) and its subsidiaries (collectively, Northern Trust). Significant intercompany balances and transactions have been eliminated. The consolidated financial statements, as of and for the three months ended March 31, 2015, and 2014, have not been audited by the Corporation’s independent registered public accounting firm. In the opinion of management, all accounting entries and adjustments, including normal recurring accruals, necessary for a fair presentation of the financial position and the results of operations for the interim periods have been made. The accounting and financial reporting policies of Northern Trust conform with U.S. generally accepted accounting principles (GAAP) and reporting practices prescribed by the banking industry. For a description of Northern Trust’s significant accounting policies, refer to Note 1 of the Notes to Consolidated Financial Statements in the Annual Report on Form 10-K for the year ended December 31, 2014.

Recent_Accounting_Pronouncemen

Recent Accounting Pronouncements	3 Months Ended
	Mar. 31, 2015
Recent Accounting Pronouncements	2. Recent Accounting Pronouncements — In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2014-09, “Revenue from Contracts with Customers (Topic 606).” The ASU is a converged standard between the FASB and the International Accounting Standards Board (IASB) that provides a single comprehensive revenue recognition model for all contracts with customers across transactions and industries. The primary objective of the ASU is revenue recognition that represents the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The ASU is effective for interim and annual reporting periods beginning after December 15, 2016. Northern Trust is currently assessing the impact of adoption of ASU 2014-09.
	In February 2015, the FASB issued ASU No. 2015-02, “Consolidation (Topic 810): Amendments to the Consolidation Analysis” which changes the guidance with respect to the analysis that a reporting entity must perform to determine whether it should consolidate certain types of legal entities. The amendments include: (1) modifying the evaluation of limited partnerships and similar legal entities, (2) amending when fees paid to a decision maker should be included in the variable interest entity analysis, (3) amending the related party relationship guidance, and (4) providing a scope exception from the consolidation guidance for reporting entities with interests in certain investment funds. The ASU is effective for interim and annual reporting periods beginning after December 15, 2015, although early adoption is permitted. Northern Trust is currently assessing the impact of adoption of ASU 2015-02.
	In April 2015, the FASB issued ASU No. 2015-03, “Interest — Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs” which requires that debt issuance costs be presented in the balance sheet as a direct deduction to the carrying amount of the associated debt liability. The ASU is effective for interim and annual periods beginning after December 15, 2015, although early adoption is permitted. Northern Trust is currently assessing the impact of adoption of ASU 2015-03.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
Fair Value Measurements	3. Fair Value Measurements — Fair Value Hierarchy. The following describes the hierarchy of valuation inputs (Levels 1, 2, and 3) used to measure fair value and the primary valuation methodologies used by Northern Trust for financial instruments measured at fair value on a recurring basis. Observable inputs reflect market data obtained from sources independent of the reporting entity; unobservable inputs reflect the entity’s own assumptions about how market participants would value an asset or liability based on the best information available. GAAP requires an entity measuring fair value to maximize the use of observable inputs and minimize the use of unobservable inputs and establishes a fair value hierarchy of inputs. Financial instruments are categorized within the hierarchy based on the lowest level input that is significant to their valuation. Northern Trust’s policy is to recognize transfers into and transfers out of fair value levels as of the end of the reporting period in which the transfer occurred. No transfers between fair value levels occurred during the three months ended March 31, 2015, or the year ended December 31, 2014.
	Level 1 — Quoted, active market prices for identical assets or liabilities.
	Northern Trust’s Level 1 assets are comprised of available for sale investments in U.S. treasury securities.
	Level 2 — Observable inputs other than Level 1 prices, such as quoted active market prices for similar assets or liabilities, quoted prices for identical or similar assets in inactive markets, and model-derived valuations in which all significant inputs are observable in active markets.
	Northern Trust’s Level 2 assets include available for sale and trading account securities, the fair values of which are determined predominantly by external pricing vendors. Prices received from vendors are compared to other vendor and third-party prices. If a security price obtained from a pricing vendor is determined to exceed pre-determined tolerance levels that are assigned based on an asset type’s characteristics, the exception is researched and, if the price is not able to be validated, an alternate pricing vendor is utilized, consistent with Northern Trust’s pricing source hierarchy. As of March 31, 2015, Northern Trust’s available for sale securities portfolio included 924 Level 2 securities with an aggregate market value of $26.3 billion. All 924 securities were valued by external pricing vendors. As of December 31, 2014, Northern Trust’s available for sale securities portfolio included 881 Level 2 securities with an aggregate market value of $25.0 billion. All 881 securities were valued by external pricing vendors. Trading account securities, which totaled $1.4 million and $4.7 million as of March 31, 2015, and December 31, 2014, respectively, were all valued using external pricing vendors.
	Northern Trust has established processes and procedures to assess the suitability of valuation methodologies used by external pricing vendors, including reviews of valuation techniques and assumptions used for selected securities. On a daily basis, periodic quality control reviews of prices received from vendors are conducted which include comparisons to prices on similar security types received from multiple pricing vendors and to the previous day’s reported prices for each security. Predetermined tolerance level exceptions are researched and may result in additional validation through available market information or the use of an alternate pricing vendor. Quarterly, Northern Trust reviews documentation from third-party pricing vendors regarding the valuation processes and assumptions used in their valuations and assesses whether the fair value levels assigned by Northern Trust to each security classification are appropriate. Annually, valuation inputs used within third-party pricing vendor valuations are reviewed for propriety on a sample basis through a comparison of inputs used to comparable market data, including security classifications that are less actively traded and security classifications comprising significant portions of the portfolio.
	Level 2 assets and liabilities also include derivative contracts which are valued internally using widely accepted income-based models that incorporate inputs readily observable in actively quoted markets and reflect the contractual terms of the contracts. Observable inputs include foreign exchange rates and interest rates for foreign exchange contracts; credit spreads, default probabilities, and recovery rates for credit default swap contracts; interest rates for interest rate swap contracts and forward contracts; and interest rates and volatility inputs for interest rate option contracts. Northern Trust evaluates the impact of counterparty credit risk and its own credit risk on the valuation of its derivative instruments. Factors considered include the likelihood of default by Northern Trust and its counterparties, the remaining maturities of the instruments, net exposures after giving effect to master netting arrangements or similar agreements, available collateral, and other credit enhancements in determining the appropriate fair value of derivative instruments. The resulting valuation adjustments have not been considered material.
	Level 3 — Valuation techniques in which one or more significant inputs are unobservable in the marketplace.
	Northern Trust’s Level 3 assets consist of auction rate securities purchased in 2008 from Northern Trust clients. To estimate the fair value of auction rate securities, for which trading is limited and market prices are generally unavailable, Northern Trust developed and maintains a pricing model that discounts estimated cash flows over their estimated remaining lives. Significant inputs to the model include the contractual terms of the securities, credit risk ratings, discount rates, forward interest rates, credit/liquidity spreads, and Northern Trust’s own assumptions about the estimated remaining lives of the securities. The significant unobservable inputs used in the fair value measurement are Northern Trust’s own assumptions about the estimated remaining lives of the securities and the applicable discount rates. Significant increases (decreases) in the estimated remaining lives or the discount rates in isolation would result in a significantly lower (higher) fair value measurement. Level 3 liabilities at January 1, 2014, consisted of an acquisition-related contingent consideration liability, the fair value of which was determined using an income-based (discounted cash flow) model that incorporated Northern Trust’s own assumptions about business growth rates and applicable discount rates, which represented unobservable inputs to the model. In April 2014, Northern Trust made a payment of $55.3 million to extinguish the contingent consideration liability at the value agreed by the parties.
	Northern Trust believes its valuation methods for its assets and liabilities carried at fair value are appropriate; however, the use of different methodologies or assumptions, particularly as applied to Level 3 assets and liabilities, could have a material effect on the computation of their estimated fair values.
	Management of various businesses and departments of Northern Trust (including Corporate Market Risk, Credit Risk Management, Corporate Financial Management, C&IS and Wealth Management) determine the valuation policies and procedures for Level 3 assets and liabilities. Generally, valuation policies are reviewed by management of each business or department. Fair value measurements are performed upon acquisitions of an asset or liability. As necessary, the valuation models are reviewed by management of the appropriate business or department, and adjusted for changes in inputs. Management of each business or department reviews the inputs in order to substantiate the unobservable inputs used in each fair value measurement. When appropriate, management reviews forecasts used in the valuation process in light of other relevant financial projections to understand any variances between current and previous fair value measurements. In certain circumstances, third-party information is used to support the fair value measurements. If certain third-party information seems inconsistent with consensus views, a review of the information is performed by management of the respective business or department to conclude as to the appropriate fair value of the asset or liability.
	The following presents the fair values of, and the valuation techniques, significant unobservable inputs and quantitative information used to develop significant unobservable inputs for, Northern Trust’s Level 3 assets as of March 31, 2015.
	Table 21: Level 3 Significant Unobservable Inputs
	Financial Instrument		Fair Value			Valuation		Unobservable Inputs	Range of Lives
						Technique			and Rates
	Auction Rate Securities		$	17.5 million		Discounted Cash		Remaining lives	1.2 — 8.6 years
						Flow		Discount rates	0.1% — 8.4%
	The following tables present assets and liabilities measured at fair value on a recurring basis as of March 31, 2015, and December 31, 2014, segregated by fair value hierarchy level.
	Table 22: Recurring Basis Hierarchy Leveling
	(In Millions)		Level 1			Level 2			Level 3		Netting			Assets/Liabilities
														at Fair Value
	March 31, 2015
	Securities
	Available for Sale
	U.S. Government		$	4,627.40		$	—		$	—	$	—		$	4,627.40
	Obligations of States and Political Subdivisions			—			4.5			—		—			4.5
	Government Sponsored Agency			—			16,634.20			—		—			16,634.20
	Non-U.S. Government			—			311.4			—		—			311.4
	Corporate Debt			—			3,651.30			—		—			3,651.30
	Covered Bonds			—			2,049.30			—		—			2,049.30
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			—			561			—		—			561
	Residential Mortgage-Backed			—			4.4			—		—			4.4
	Other Asset-Backed			—			2,949.30			—		—			2,949.30
	Auction Rate			—			—			17.5		—			17.5
	Other			—			135.4			—		—			135.4
	Total Available for Sale			4,627.40			26,300.80			17.5		—			30,945.70
	Trading Account			—			1.4			—		—			1.4
	Total Available for Sale and Trading Securities			4,627.40			26,302.20			17.5		—			30,947.10
	Other Assets
	Derivative Assets
	Foreign Exchange Contracts			—			4,798.60			—		—			4,798.60
	Interest Rate Contracts			—			272			—		—			272
	Total Derivative Assets			—			5,070.60			—		(2,884.2	)		2,186.40
	Other Liabilities
	Derivative Liabilities
	Foreign Exchange Contracts			—			4,714.10			—		—			4,714.10
	Interest Rate Swaps			—			148.2			—		—			148.2
	Total Derivative Liabilities		$	—		$	4,862.30		$	—	$	(3,510.9	)	$	1,351.40
	Note: Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of March 31, 2015, derivative assets and liabilities shown above also include reductions of $440.8 million and $1.1 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.
	(In Millions)		Level 1			Level 2			Level 3		Netting			Assets/Liabilities
														at Fair Value
	December 31, 2014
	Securities
	Available for Sale
	U.S. Government		$	4,506.90		$	—		$	—	$	—		$	4,506.90
	Obligations of States and Political Subdivisions			—			4.6			—		—			4.6
	Government Sponsored Agency			—			16,389.20			—		—			16,389.20
	Non-U.S. Government			—			310.4			—		—			310.4
	Corporate Debt			—			3,577.70			—		—			3,577.70
	Covered Bonds			—			1,907.50			—		—			1,907.50
	Supranational and Non-U.S. Agency Bonds			—			360.6			—		—			360.6
	Residential Mortgage-Backed			—			6.4			—		—			6.4
	Other Asset-Backed			—			2,321.30			—		—			2,321.30
	Auction Rate			—			—			18.1		—			18.1
Other			—			155.8			—		—			155.8
Total Available for Sale			4,506.90			25,033.50			18.1		—			29,558.50
Trading Account			—			4.7			—		—			4.7
Total Available for Sale and Trading Securities			4,506.90			25,038.20			18.1		—			29,563.20
Other Assets
Derivative Assets
Foreign Exchange Contracts			—			4,275.20			—		—			4,275.20
Interest Rate Contracts			—			232.3			—		—			232.3
Total Derivative Assets			—			4,507.50			—		(2,257.1	)		2,250.40
Other Liabilities
Derivative Liabilities
Foreign Exchange Contracts			—			4,095.50			—		—			4,095.50
Interest Rate Swaps			—			131.8			—		—			131.8
Total Derivative Liabilities		$	—		$	4,227.30		$	—	$	(3,173.3	)	$	1,054.00
Note: Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of December 31, 2014, derivative assets and liabilities shown above also include reductions of $315.8 million and $1.2 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.
The following tables present the changes in Level 3 assets and liabilities for the three months ended March 31, 2015, and 2014.
Table 23: Changes in Level 3 Assets
Level 3 Assets (In Millions)		Auction Rate Securities
Three Months Ended March 31,		2015			2014
Fair Value at January 1		$	18.1		$	98.9
Total Gains (Losses):
Included in Earnings			—			—
Included in Other Comprehensive Income (1)			(0.3	)		(0.2	)
Purchases, Issues, Sales, and Settlements
Sales			—			0.1
Settlements			(0.3	)		(0.3	)
Fair Value at March 31		$	17.5		$	98.5
-1	Unrealized gains (losses) are included in net unrealized gains (losses) on securities available for sale in the consolidated statements of comprehensive income.
Table 24: Changes in Level 3 Liabilities
Level 3 Liabilities (In Millions)		Contingent Consideration
Three Months Ended March 31		2015			2014
Fair Value at January 1		$	—		$	55.4
Total (Gains) Losses:
Included in Earnings (1)			—			(0.1	)
Included in Other Comprehensive Income			—			—
Purchases, Issues, Sales, and Settlements			—			—
Purchases			—			—
Settlements			—			(55.3	)
Fair Value at March 31		$	—		$	—
Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held at March 31 (1)		$	—		$	—
-1	(Gains) losses are recorded in other operating income (expense) in the consolidated statements of income.
During the three months ended March 31, 2015, and 2014 there were no transfers into or out of Level 3 assets or liabilities.
Carrying values of assets and liabilities that are not measured at fair value on a recurring basis may be adjusted to fair value in periods subsequent to their initial recognition, for example, to record an impairment of an asset. GAAP requires entities to disclose separately these subsequent fair value measurements and to classify them under the fair value hierarchy.
Assets measured at fair value on a nonrecurring basis at March 31, 2015, and 2014, all of which were categorized as Level 3 under the fair value hierarchy, were comprised of impaired loans whose values were based on real estate and other available collateral, and of other real estate owned (OREO) properties. Fair values of real-estate loan collateral were estimated using a market approach typically supported by third-party valuations and property-specific fees and taxes, and were subject to adjustments to reflect management’s judgment as to realizable value. Other loan collateral, which typically consists of accounts receivable, inventory and equipment, is valued using a market approach adjusted for asset-specific characteristics and in limited instances third-party valuations are used. OREO assets are carried at the lower of cost or fair value less estimated costs to sell, with fair value typically based on third-party appraisals.
Collateral-based impaired loans and OREO assets that have been adjusted to fair value totaled $23.1 million and $0.3 million, respectively, at March 31, 2015, and $33.3 million and $1.5 million, respectively, at March 31, 2014. Assets measured at fair value on a nonrecurring basis reflect management’s judgment as to realizable value.
The following table provides the fair value of, and the valuation technique, significant unobservable inputs and quantitative information used to develop the significant unobservable inputs for, Northern Trust’s Level 3 assets that were measured at fair value on a nonrecurring basis as of March 31, 2015.
Table 25: Level 3 Nonrecurring Basis Significant Unobservable Inputs
Financial Instrument		Fair Value		Valuation	Unobservable Input		Range of Discounts
				Technique			Applied
Loans		$23.1 million		Market Approach	Discount to reflect realizable value		15% — 25%
OREO		$0.3 million		Market Approach	Discount to reflect realizable value		15% — 20%
Fair Value of Financial Instruments. GAAP requires disclosure of the estimated fair value of certain financial instruments and the methods and significant assumptions used to estimate fair value. It excludes from this requirement nonfinancial assets and liabilities, as well as a wide range of franchise, relationship and intangible values that add value to Northern Trust. Accordingly, the required fair value disclosures provide only a partial estimate of the fair value of Northern Trust. Financial instruments recorded at fair value in Northern Trust’s consolidated balance sheets are discussed above. The following methods and assumptions were used in estimating the fair values of financial instruments that are not carried at fair value.
Held to Maturity Securities. The fair values of held to maturity securities were modeled by external pricing vendors, or in limited cases internally, using widely accepted models which are based on an income approach (discounted cash flow) that incorporates current market yield curves.
Loans (excluding lease receivables). The fair value of the loan portfolio was estimated using an income approach (discounted cash flow) that incorporates current market rates offered by Northern Trust as of the date of the consolidated financial statements. The fair values of all loans were adjusted to reflect current assessments of loan collectability.
Federal Reserve and Federal Home Loan Bank Stock. The fair values of Federal Reserve and Federal Home Loan Bank stock are equal to their carrying values which represent redemption value.
Community Development Investments. The fair values of these instruments were estimated using an income approach (discounted cash flow) that incorporates current market rates.
Employee Benefit and Deferred Compensation. These assets include U.S. treasury securities and investments in mutual and collective trust funds held to fund certain supplemental employee benefit obligations and deferred compensation plans. Fair values of U.S. treasury securities were determined using quoted, active market prices for identical securities. The fair values of investments in mutual and collective trust funds were valued at the funds’ net asset values based on a market approach.
Savings Certificates and Other Time Deposits. The fair values of these instruments were estimated using an income approach (discounted cash flow) that incorporates market interest rates currently offered by Northern Trust for deposits with similar maturities.
Senior Notes, Subordinated Debt, and Floating Rate Capital Debt. Fair values were determined using a market approach based on quoted market prices, when available. If quoted market prices were not available, fair values were based on quoted market prices for comparable instruments.
Federal Home Loan Bank Borrowings. The fair values of these instruments were estimated using an income approach (discounted cash flow) that incorporates market interest rates available to Northern Trust.
Loan Commitments. The fair values of loan commitments represent the estimated costs to terminate or otherwise settle the obligations with a third party adjusted for any related allowance for credit losses.
Standby Letters of Credit. The fair values of standby letters of credit are measured as the amount of unamortized fees on these instruments, inclusive of the related allowance for credit losses. Fees are determined by applying basis points to the principal amounts of the letters of credit.
Financial Instruments Valued at Carrying Value. Due to their short maturity, the carrying values of certain financial instruments approximated their fair values. These financial instruments include: cash and due from banks; federal funds sold and securities purchased under agreements to resell; interest-bearing deposits with banks; Federal Reserve deposits; client security settlement receivables; non-U.S. offices interest-bearing deposits; federal funds purchased; securities sold under agreements to repurchase; and other borrowings (includes term federal funds purchased and other short-term borrowings). The fair values of demand, noninterest-bearing, savings, and money market deposits represent the amounts payable on demand as of the reporting date, although such deposits are typically priced at a premium in banking industry consolidations.
The following tables summarize the fair values of all financial instruments.
Table 26: Fair Value of Financial Instruments
(In Millions)		March 31, 2015
		Book			Total			Fair Value
		Value			Fair Value			Level 1		Level 2			Level 3
Assets
Cash and Due from Banks		$	4,074.80		$	4,074.80		$	4,074.80	$	—		$	—
Federal Funds Sold and Resell Agreements			1,113.90			1,113.90			—		1,113.90			—
Interest-Bearing Deposits with Banks			15,432.30			15,432.30			—		15,432.30			—
Federal Reserve Deposits			8,804.20			8,804.20			—		8,804.20			—
Securities
Available for Sale (1)			30,945.70			30,945.70			4,627.40		26,300.80			17.5
Held to Maturity			5,655.80			5,665.00			—		5,665.00			—
Trading Account			1.4			1.4			—		1.4			—
Loans (excluding Leases)
Held for Investment			31,503.80			31,747.70			—		—			31,747.70
Client Security Settlement Receivables			2,219.30			2,219.30			—		2,219.30			—
Other Assets
Federal Reserve and Federal Home Loan Bank Stock			207.5			207.5			—		207.5			—
Community Development Investments			198.4			198.7			—		198.7			—
Employee Benefit and Deferred Compensation			162.1			165.5			113.9		51.6			—
Liabilities
Deposits
Demand, Noninterest-Bearing, Savings and Money Market		$	39,016.40		$	39,016.40		$	39,016.40	$	—		$	—
Savings Certificates and Other Time			1,777.90			1,783.10			—		1,783.10			—
Non U.S. Offices Interest-Bearing			45,942.20			45,942.20			—		45,942.20			—
Federal Funds Purchased			410.7			410.7			—		410.7			—
Securities Sold under Agreements to Repurchase			613.2			613.2			—		613.2			—
Other Borrowings			2,874.10			2,875.70			—		2,875.70			—
Senior Notes			1,497.10			1,566.90			—		1,566.90			—
Long Term Debt (excluding Leases)
Subordinated Debt			1,368.60			1,374.90			—		1,374.90			—
Floating Rate Capital Debt			277.2			241.4			—		241.4			—
Other Liabilities
Standby Letters of Credit			58.3			58.3			—		—			58.3
Loan Commitments			26.8			26.8			—		—			26.8
Derivative Instruments
Asset/Liability Management
Foreign Exchange Contracts
Assets		$	67.4		$	67.4		$	—	$	67.4		$	—
Liabilities			27.7			27.7			—		27.7			—
Interest Rate Contracts
Assets			148.9			148.9			—		148.9			—
Liabilities			28.9			28.9			—		28.9			—
Client-Related and Trading
Foreign Exchange Contracts
Assets			4,731.20			4,731.20			—		4,731.20			—
Liabilities			4,686.40			4,686.40			—		4,686.40			—
Interest Rate Contracts
Assets			123.1			123.1			—		123.1			—
Liabilities			119.3			119.3			—		119.3			—
-1	Refer to the table located on page 31 for the disaggregation of available for sale securities.
(In Millions)		December 31, 2014
		Book			Total			Fair Value
		Value			Fair Value			Level 1		Level 2			Level 3
Assets
Cash and Due from Banks		$	3,050.60		$	3,050.60		$	3,050.60	$	—		$	—
Federal Funds Sold and Resell Agreements			1,062.70			1,062.70			—		1,062.70			—
Interest-Bearing Deposits with Banks			14,928.30			14,928.30			—		14,928.30			—
Federal Reserve Deposits			17,386.30			17,386.30			—		17,386.30			—
Securities
Available for Sale (1)			29,558.50			29,558.50			4,506.90		25,033.50			18.1
Held to Maturity			4,170.80			4,176.10			—		4,176.10			—
Trading Account			4.7			4.7			—		4.7			—
Loans (excluding Leases)
Held for Investment			30,458.00			30,600.40			—		—			30,600.40
Held for Sale			2.5			2.5			—		—			2.5
Client Security Settlement Receivables			1,568.80			1,568.80			—		1,568.80			—
Other Assets
Federal Reserve and Federal Home Loan Bank Stock			207.5			207.5			—		207.5			—
Community Development Investments			209.9			210.8			—		210.8			—
Employee Benefit and Deferred Compensation			143.2			146.7			96.7		50			—
Liabilities
Deposits
Demand, Noninterest-Bearing, Savings and Money Market		$	41,454.60		$	41,454.60		$	41,454.60	$	—		$	—
Savings Certificates and Other Time			1,757.40			1,757.40			—		1,757.40			—
Non U.S. Offices Interest-Bearing			47,545.00			47,545.00			—		47,545.00			—
Federal Funds Purchased			932.9			932.9			—		932.9			—
Securities Sold under Agreements to Repurchase			885.1			885.1			—		885.1			—
Other Borrowings			1,685.20			1,686.20			—		1,686.20			—
Senior Notes			1,497.00			1,541.80			—		1,541.80			—
Long Term Debt (excluding Leases)
Subordinated Debt			1,583.30			1,583.30			—		1,583.30			—
Floating Rate Capital Debt			277.1			242.8			—		242.8			—
Other Liabilities
Standby Letters of Credit			60.1			60.1			—		—			60.1
Loan Commitments			28.3			28.3			—		—			28.3
Derivative Instruments
Asset/Liability Management
Foreign Exchange Contracts
Assets		$	125.7		$	125.7		$	—	$	125.7		$	—
Liabilities			23.5			23.5			—		23.5			—
Interest Rate Contracts
Assets			126.8			126.8			—		126.8			—
Liabilities			30.5			30.5			—		30.5			—
Client-Related and Trading
Foreign Exchange Contracts
Assets			4,149.50			4,149.50			—		4,149.50			—
Liabilities			4,072.00			4,072.00			—		4,072.00			—
Interest Rate Contracts
Assets			105.5			105.5			—		105.5			—
Liabilities			101.3			101.3			—		101.3			—
-1	Refer to the table located on page 32 for the disaggregation of available for sale securities.

Securities

Securities	3 Months Ended
	Mar. 31, 2015
Securities	4. Securities — The following tables provide the amortized cost and fair values of securities at March 31, 2015, and December 31, 2014.
	Table 27: Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale
	Securities Available for Sale		March 31, 2015
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	U.S. Government		$	4,593.90	$	33.5		$	—	$	4,627.40
	Obligations of States and Political Subdivisions			4.5		—			—		4.5
	Government Sponsored Agency			16,536.90		107.6			10.3		16,634.20
	Non-U.S. Government			310.5		0.9			—		311.4
	Corporate Debt			3,661.50		6.7			16.9		3,651.30
	Covered Bonds			2,040.50		8.8			—		2,049.30
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			560		1.4			0.4		561
	Residential Mortgage-Backed			4.9		—			0.5		4.4
	Other Asset-Backed			2,948.70		1.1			0.5		2,949.30
	Auction Rate			18.1		0.5			1.1		17.5
	Other			135.1		0.3			—		135.4
	Total		$	30,814.60	$	160.8		$	29.7	$	30,945.70
	Securities Available for Sale		December 31, 2014
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	U.S. Government		$	4,493.50	$	15.1		$	1.7	$	4,506.90
	Obligations of States and Political Subdivisions			4.5		0.1			—		4.6
	Government Sponsored Agency			16,326.40		82.3			19.5		16,389.20
	Non-U.S. Government			309.5		0.9			—		310.4
	Corporate Debt			3,617.50		1.8			41.6		3,577.70
	Covered Bonds			1,899.90		7.9			0.3		1,907.50
	Supranational and Non-U.S. Agency Bonds			360		1.5			0.9		360.6
	Residential Mortgage-Backed			6.9		—			0.5		6.4
	Other Asset-Backed			2,321.80		0.5			1		2,321.30
	Auction Rate			18.4		0.5			0.8		18.1
	Other			155.7		0.3			0.2		155.8
	Total		$	29,514.10	$	110.9		$	66.5	$	29,558.50
	Table 28: Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity
	Securities Held to Maturity		March 31, 2015
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	Obligations of States and Political Subdivisions		$	112.6	$	7		$	—	$	119.6
	Government Sponsored Agency			15.5		1			—		16.5
	Non-U.S. Government			1,104.30		8			—		1,112.30
	Certificates of Deposit			2,575.70		0.2			0.3		2,575.60
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			1,769.70		12.5			0.2		1,782.00
	Other			78		—			19		59
	Total		$	5,655.80	$	28.7		$	19.5	$	5,665.00
	Securities Held to Maturity		December 31, 2014
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	Obligations of States and Political Subdivisions		$	121.9	$	7.4		$	—	$	129.3
	Government Sponsored Agency			18.4		1.1			—		19.5
	Non-U.S. Government			1,281.60		6.6			0.4		1,287.80
	Certificates of Deposit			924.3		0.1			0.1		924.3
	Supranational and Non-U.S. Agency Bonds			1,745.80		10.9			0.5		1,756.20
	Other			78.8		0.3			20.1		59
	Total		$	4,170.80	$	26.4		$	21.1	$	4,176.10
	Securities held to maturity consist of debt securities that management intends to, and Northern Trust has the ability to, hold until maturity.
	The following table provides the remaining maturity of securities as of March 31, 2015.
	Table 29: Remaining Maturity of Securities Available for Sale and Held to Maturity
			March 31, 2015
	(In Millions)		Amortized		Fair
			Cost		Value
	Available for Sale
	Due in One Year or Less		$	7,702.60	$	7,731.30
	Due After One Year Through Five Years			18,210.80		18,295.10
	Due After Five Years Through Ten Years			3,650.70		3,659.60
	Due After Ten Years			1,250.50		1,259.70
Total			30,814.60		30,945.70
Held to Maturity
Due in One Year or Less			3,219.30		3,220.70
Due After One Year Through Five Years			2,374.50		2,397.50
Due After Five Years Through Ten Years			21.4		19.4
Due After Ten Years			40.6		27.4
Total		$	5,655.80	$	5,665.00
Note: Mortgage-backed and asset-backed securities are included in the above table taking into account anticipated future prepayments.
Investment Security Gains and Losses. Net investment security gains of $0.1 million were recognized in the three months ended March 31, 2015, representing net realized gains from the sale of securities. Net investment security losses of $4.0 million were recognized in the three months ended March 31, 2014, and included $3.9 million of charges related to the OTTI of certain Community Reinvestment Act (CRA) eligible held to maturity securities. For the three months ended March 31, 2015, proceeds of $102.0 million were received from the sale of securities, resulting in gross realized gains of $0.1 million. For the three months ended March 31, 2014, proceeds of $199.7 million were received from the sale of securities, resulting in gross realized gains and losses of $0.3 million and $0.4 million, respectively.
Securities with Unrealized Losses. The following tables provide information regarding securities that had been in a continuous unrealized loss position for less than 12 months and for 12 months or longer as of March 31, 2015, and December 31, 2014.
Table 30: Securities with Unrealized Losses
Securities with Unrealized Losses as of		Less than 12 Months					12 Months or Longer				Total
March 31, 2015
(In Millions)		Fair Value		Unrealized			Fair Value		Unrealized		Fair Value		Unrealized
				Losses					Losses				Losses
Government Sponsored Agency		$	1,946.00	$	3.9		$	1,091.70	$	6.4	$	3,037.70	$	10.3
Corporate Debt			391.4		0.6			1,217.80		16.3		1,609.20		16.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			149.8		0.2			170.7		0.4		320.5		0.6
Residential Mortgage-Backed			—		—			4.4		0.5		4.4		0.5
Other Asset-Backed			1,071.10		0.5			—		—		1,071.10		0.5
Certificates of Deposit			1,395.30		0.3			—		—		1,395.30		0.3
Auction Rate			1.4		0.2			5.5		0.9		6.9		1.1
Other			17.7		10.8			48.3		8.2		66		19
Total		$	4,972.70	$	16.5		$	2,538.40	$	32.7	$	7,511.10	$	49.2
Securities with Unrealized Losses as of		Less than 12 Months					12 Months or Longer				Total
December 31, 2014
(In Millions)		Fair Value		Unrealized			Fair Value		Unrealized		Fair Value		Unrealized
				Losses					Losses				Losses
U.S. Government		$	998.2	$	1.7		$	—	$	—	$	998.2	$	1.7
Government Sponsored Agency			2,344.90		6.6			1,730.00		12.9		4,074.90		19.5
Non-U.S. Government			292.9		0.4			—		—		292.9		0.4
Corporate Debt			1,244.50		3.9			1,338.80		37.7		2,583.30		41.6
Covered Bonds			142.3		0.2			10		0.1		152.3		0.3
Supranational and Non-U.S. Agency Bonds			313.2		0.3			175.5		1.1		488.7		1.4
Residential Mortgage-Backed			—		—			4.5		0.5		4.5		0.5
Other Asset-Backed			1,297.60		1			—		—		1,297.60		1
Certificates of Deposit			438.6		0.1			—		—		438.6		0.1
Auction Rate			2.4		0.2			4.7		0.6		7.1		0.8
Other			27.1		12.1			45.6		8.2		72.7		20.3
Total		$	7,101.70	$	26.5		$	3,309.10	$	61.1	$	10,410.80	$	87.6
As of March 31, 2015, 391 securities with a combined fair value of $7.5 billion were in an unrealized loss position, with their unrealized losses totaling $49.2 million. Unrealized losses of $16.9 million within corporate debt securities primarily reflected higher market rates since purchase; 39% of the corporate debt portfolio is backed by guarantees provided by U.S. and non-U.S. governmental entities. Unrealized losses of $10.3 million related to government sponsored agency securities were primarily attributable to changes in market rates since their purchase.
Unrealized losses on residential mortgage-backed securities totaling $0.5 million reflected the impact of wider credit and liquidity spreads on the valuations of one residential mortgage-backed security since purchase, with it having been in an unrealized loss position for more than 12 months. Securities classified as “other asset-backed” had average lives less than 5 years, and 100% were rated triple-A.
The majority of the $19.0 million of unrealized losses in securities classified as “other” at March 31, 2015, related to securities primarily purchased at a premium or par by Northern Trust for compliance with the CRA. Unrealized losses on these CRA-related securities were attributable to yields that are below market rates for the purpose of supporting institutions and programs that benefit low- to moderate- income communities within Northern Trust’s market area. Unrealized losses of $1.1 million related to auction rate securities primarily reflected reduced market liquidity as a majority of auctions continued to fail preventing holders from liquidating their investments at par. The remaining unrealized losses on Northern Trust’s securities portfolio as of March 31, 2015, were attributable to changes in overall market interest rates, increased credit spreads or reduced market liquidity. As of March 31, 2015, Northern Trust did not intend to sell any investment in an unrealized loss position and it was not more likely than not that Northern Trust would be required to sell any such investment before the recovery of its amortized cost basis, which may be maturity.
Security impairment reviews are conducted quarterly to identify and evaluate securities that have indications of possible OTTI. A determination as to whether a security’s decline in market value is other-than-temporary takes into consideration numerous factors and the relative significance of any single factor can vary by security. Factors Northern Trust considers in determining whether impairment is other-than-temporary include, but are not limited to: the length of time the security has been impaired; the severity of the impairment; the cause of the impairment and the financial condition and near-term prospects of the issuer; activity in the market of the issuer which may indicate adverse credit conditions; Northern Trust’s intent regarding the sale of the security as of the balance sheet date; and the likelihood that it will not be required to sell the security for a period of time sufficient to allow for the recovery of the security’s amortized cost basis. For each security meeting the requirements of Northern Trust’s internal screening process, an extensive review is conducted to determine if OTTI has occurred.
While all securities are considered, the process for identifying credit impairment within CRA-eligible mortgage-backed securities, the security type for which Northern Trust recognized all of the OTTI in 2014, incorporates an expected loss approach using discounted cash flows on the underlying collateral pools. To evaluate whether an unrealized loss on CRA-eligible mortgage-backed securities is other-than-temporary, a calculation of the security’s present value is made using current pool data, the current delinquency pipeline, default rates and loan loss severities based on the historical performance of like collateral, and Northern Trust’s outlook for the housing market and the overall economy. If the present value of the collateral pools was found to be less than the current amortized cost of the security, a credit-related OTTI loss would be recorded in earnings equal to the difference between the two amounts.
Impairments of CRA-eligible mortgage-backed securities are influenced by a number of factors, including but not limited to, U.S. economic and housing market performance, pool credit enhancement level, year of origination and estimated credit quality of the collateral. The factors used in estimating losses related to CRA-eligible mortgage-backed securities vary by vintage of loan origination and collateral quality.
As of March 31, 2015, impairment estimates for CRA-eligible mortgage-backed securities were developed using default and loss severity rates sourced from industry mortgage data. Ultimate recovery value of the securities was determined by applying default and severity rates against remaining collateral balances in the pools. An expected loss amount was calculated by applying loss severity rates on defaulted amounts. Lastly, book values were compared against collateral values net of expected losses in order to determine OTTI.
There were no OTTI losses recognized in the three months ended March 31, 2015. There were $3.9 million of OTTI losses recognized during the three months ended March 31, 2014, related to CRA-eligible mortgage-backed securities.
Credit Losses on Debt Securities. The table below provides information regarding total other-than-temporarily-impaired securities, including noncredit-related amounts recognized in other comprehensive income and net impairment losses recognized in earnings, for the three months ended March 31, 2015, and 2014.
Table 31: Net Impairment Losses Recognized in Earnings
		Three Months Ended
		March 31,
(In Millions)		2015		2014
Changes in OTTI Losses*		$	—	$	(4.6	)
Noncredit-related Losses Recorded in / (Reclassified from) OCI**			—		0.7
Net Impairment Losses Recognized in Earnings		$	—	$	(3.9	)
*	For initial OTTI in the applicable period, the balance includes the excess of the amortized cost over the fair value of the impaired securities. For subsequent impairments of the same security, the balance includes any additional changes in fair value of the security subsequent to its most recently recorded OTTI.
**	For initial OTTI in the applicable period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in other comprehensive income (OCI). For subsequent impairments of the same security, the balance includes additional changes in OCI for that security subsequent to its most recently recorded OTTI.
Provided in the table below are the cumulative credit-related losses recognized in earnings on debt securities other-than-temporarily impaired.
Table 32: Cumulative Credit-Related Losses on Securities
		Three Months Ended
		March 31,
(In Millions)		2015		2014
Cumulative Credit-Related Losses on Securities Held — Beginning of Period		$	5.2	$	8.8
Plus: Losses on Newly Identified Impairments			—		1.8
Additional Losses on Previously Identified Impairments			—		2.1
Less: Current and Prior Period Losses on Securities Sold During the Period			—		—
Cumulative Credit-Related Losses on Securities Held — End of Period		$	5.2	$	12.7

Loans_and_Leases

Loans and Leases	3 Months Ended
	Mar. 31, 2015
Loans and Leases	5. Loans and Leases — Amounts outstanding for loans and leases, by segment and class, are shown below.
	Table 33: Loans and Leases
	(In Millions)		March 31,			December 31,
			2015			2014
	Commercial
	Commercial and Institutional		$	9,030.20		$	8,381.90
	Commercial Real Estate			3,500.90			3,333.30
	Lease Financing, net			870.6			916.3
	Non-U.S.			1,661.60			1,530.60
	Other			428			191.5
	Total Commercial			15,491.30			14,353.60
	Personal
	Residential Real Estate			9,440.20			9,782.60
	Private Client			7,615.80			7,466.90
	Other			82.9			37.1
	Total Personal			17,138.90			17,286.60
	Total Loans and Leases			32,630.20			31,640.20
	Allowance for Credit Losses Assigned to Loans and Leases			(259.0	)		(267.0	)
	Net Loans and Leases		$	32,371.20		$	31,373.20
	Residential real estate loans consist of traditional first lien mortgages and equity credit lines that generally require a loan-to-collateral value of no more than 65% to 80% at inception. Northern Trust’s equity credit line products generally have draw periods of up to 10 years and a balloon payment of any outstanding balance is due at maturity. Payments are interest only with variable interest rates. Northern Trust does not offer equity credit lines that include an option to convert the outstanding balance to an amortizing payment loan. As of March 31, 2015, and December 31, 2014, equity credit lines totaled $1.7 billion and $1.8 billion, respectively, and equity credit lines for which first liens were held by Northern Trust represented 89% of the total equity credit lines as of each of those dates.
	Included within the non-U.S., commercial-other and personal-other classes are short-duration advances primarily related to the processing of custodied client investments, that totaled $1.7 billion at March 31, 2015, and $1.5 billion at December 31, 2014. Demand deposits reclassified as loan balances totaled $77.6 million and $92.1 million at March 31, 2015, and December 31, 2014, respectively. There were no loans classified as held for sale at March 31, 2015. Loans classified as held for sale totaled $2.5 million at December 31, 2014.
	Credit Quality Indicators. Credit quality indicators are statistics, measurements or other metrics that provide information regarding the relative credit risk of loans and leases. Northern Trust utilizes a variety of credit quality indicators to assess the credit risk of loans and leases at the segment, class and individual credit exposure levels.
	As part of its credit process, Northern Trust utilizes an internal borrower risk rating system to support identification, approval and monitoring of credit risk. Borrower risk ratings are used in credit underwriting, management reporting and the calculation of credit loss allowances and economic capital.
	Risk ratings are used for ranking the credit risk of borrowers and the probability of their default. Each borrower is rated using one of a number of ratings models or other subjective assessment methodologies, which consider both quantitative and qualitative factors. The ratings models vary among classes of loans and leases in order to capture the unique risk characteristics inherent within each particular type of credit exposure. Provided below are the more significant performance indicator attributes considered within Northern Trust’s borrower rating models, by loan and lease class.
		•		Commercial and Institutional: leverage, profit margin, liquidity, asset size and capital levels;
		•		Commercial Real Estate: debt service coverage, loan-to-value ratio, leasing status and guarantor support;
		•		Lease Financing and Commercial-Other: leverage, profit margin, liquidity, asset size and capital levels;
		•		Non-U.S.: leverage, profit margin, liquidity, return on assets and capital levels;
		•		Residential Real Estate: payment history, credit bureau scores and loan-to-value ratio;
		•		Private Client: cash-flow-to-debt and net worth ratios, leverage and liquidity; and
		•		Personal-Other: cash-flow-to-debt and net worth ratios.
	While the criteria vary by model, the objective is for the borrower ratings to be consistent in both the measurement and ranking of risk. Each model is calibrated to a master rating scale to support this consistency. Ratings for borrowers not in default range from “1” for the strongest credits to “7” for the weakest non-defaulted credits. Ratings of “8” or “9” are used for defaulted borrowers. Borrower risk ratings are monitored and are revised when events or circumstances indicate a change is required. Risk ratings are validated at least annually.
	Loan and lease segment and class balances as of March 31, 2015, and December 31, 2014, are provided below, segregated by borrower ratings into “1 to 3,” “4 to 5” and “6 to 9” (watch list), categories.
	Table 34: Borrower Ratings
			March 31, 2015										December 31, 2014
	(In Millions)		1 to 3			4 to 5			6 to 9		Total		1 to 3		4 to 5		6 to 9		Total
			Category			Category			Category				Category		Category		Category
									(Watch List)								(Watch List)
	Commercial
	Commercial and Institutional		$	5,628.30		$	3,314.90		$	87	$	9,030.20	$	5,340.90	$	2,947.30	$	93.7	$	8,381.90
	Commercial Real Estate			1,452.50			1,949.90			98.5		3,500.90		1,371.70		1,861.80		99.8		3,333.30
	Lease Financing, net			510.6			324			36		870.6		552.5		360.3		3.5		916.3
	Non-U.S.			513.8			1,147.60			0.2		1,661.60		636.8		892.9		0.9		1,530.60
	Other			353			75			—		428		108.1		83.4		—		191.5
	Total Commercial			8,458.20			6,811.40			221.7		15,491.30		8,010.00		6,145.70		197.9		14,353.60
	Personal
	Residential Real Estate			2,734.20			6,304.90			401.1		9,440.20		3,148.00		6,207.00		427.6		9,782.60
	Private Client			4,907.10			2,699.40			9.3		7,615.80		5,143.80		2,311.70		11.4		7,466.90
	Other			68.5			14.4			—		82.9		21.1		16		—		37.1
	Total Personal			7,709.80			9,018.70			410.4		17,138.90		8,312.90		8,534.70		439		17,286.60
	Total Loans and Leases		$	16,168.00		$	15,830.10		$	632.1	$	32,630.20	$	16,322.90	$	14,680.40	$	636.9	$	31,640.20
	Loans and leases in the “1 to 3” category are expected to exhibit minimal to modest probabilities of default and are characterized by borrowers having the strongest financial qualities, including above average financial flexibility, cash flows and capital levels. Borrowers assigned these ratings are anticipated to experience very little to moderate financial pressure in adverse down cycle scenarios. As a result of these characteristics, borrowers within this category exhibit a minimal to modest likelihood of loss.
	Loans and leases in the “4 to 5” category are expected to exhibit moderate to acceptable probabilities of default and are characterized by borrowers with less financial flexibility than those in the “1 to 3” category. Cash flows and capital levels are generally sufficient to allow for borrowers to meet current requirements, but have reduced cushion in adverse down cycle scenarios. As a result of these characteristics, borrowers within this category exhibit a moderate likelihood of loss.
	Loans and leases in the watch list category have elevated credit risk profiles that are monitored through internal watch lists, and consist of credits with borrower ratings of “6 to 9.” These credits, which include all nonperforming credits, are expected to exhibit minimally acceptable probabilities of default, elevated risk of default, or are currently in default. Borrowers associated with these risk profiles that are not currently in default have limited financial flexibility. Cash flows and capital levels range from acceptable to potentially insufficient to meet current requirements, particularly in adverse down cycle scenarios. As a result of these characteristics, borrowers in this category exhibit an elevated to probable likelihood of loss.
	Recognition of Income. Interest income on loans is recorded on an accrual basis unless, in the opinion of management, there is a question as to the ability of the debtor to meet the terms of the loan agreement, or interest or principal is more than 90 days contractually past due and the loan is not well-secured and in the process of collection. Loans meeting such criteria are classified as nonperforming and interest income is recorded on a cash basis. At the time a loan is determined to be nonperforming, interest accrued but not collected is reversed against interest income in the current period. Interest collected on nonperforming loans is applied to principal unless, in the opinion of management, collectability of principal is not in doubt. Management’s assessment of the indicators of loan and lease collectability, and its policies relative to the recognition of interest income, including the suspension and subsequent resumption of income recognition, do not meaningfully vary between loan and lease classes. Nonperforming loans are returned to performing status when factors indicating doubtful collectability no longer exist. Factors considered in returning a loan to performing status are consistent across all classes of loans and leases and, in accordance with regulatory guidance, relate primarily to expected payment performance. Loans are eligible to be returned to performing status when: (i) no principal or interest that is due is unpaid and repayment of the remaining contractual principal and interest is expected or (ii) the loan has otherwise become well-secured (possessing realizable value sufficient to discharge the debt, including accrued interest, in full) and is in the process of collection (through action reasonably expected to result in debt repayment or restoration to a current status in the near future). A loan that has not been brought fully current may be restored to performing status provided there has been a sustained period of repayment performance (generally a minimum of six months) by the borrower in accordance with the contractual terms, and Northern Trust is reasonably assured of repayment within a reasonable period of time. Additionally, a loan that has been formally restructured so as to be reasonably assured of repayment and performance according to its modified terms may be returned to accrual status, provided there was a well-documented credit evaluation of the borrower’s financial condition and prospects of repayment under the revised terms and there has been a sustained period of repayment performance (generally a minimum of six months) under the revised terms.
	Past due status is based on how long since the contractual due date a principal or interest payment has been past due. For disclosure purposes, loans that are 29 days past due or less are reported as current. The following tables provide balances and delinquency status of performing and nonperforming loans and leases by segment and class, as well as the total OREO and nonperforming asset balances, as of March 31, 2015, and December 31, 2014.
	Table 35: Delinquency Status
	March 31, 2015
	(In Millions)		Current			30-59 Days			60-89 Days		90 Days or		Total		Nonperforming		Total Loans
						Past Due			Past Due		More Past		Performing				and Leases
											Due
	Commercial
	Commercial and Institutional		$	8,980.40		$	21.2		$	4.6	$	3.1	$	9,009.30	$	20.9	$	9,030.20
	Commercial Real Estate			3,442.90			10.2			4.8		3.1		3,461.00		39.9		3,500.90
Lease Financing, net			870.6			—			—		—		870.6		—		870.6
Non-U.S.			1,661.60			—			—		—		1,661.60		—		1,661.60
Other			428			—			—		—		428		—		428
Total Commercial			15,383.50			31.4			9.4		6.2		15,430.50		60.8		15,491.30
Personal
Residential Real Estate			9,240.20			37.5			2.6		2.2		9,282.50		157.7		9,440.20
Private Client			7,573.70			32.8			7.2		1		7,614.70		1.1		7,615.80
Other			82.9			—			—		—		82.9		—		82.9
Total Personal			16,896.80			70.3			9.8		3.2		16,980.10		158.8		17,138.90
Total Loans and Leases		$	32,280.30		$	101.7		$	19.2	$	9.4	$	32,410.60	$	219.6	$	32,630.20
											Other Real Estate Owned				8.5
											Total Nonperforming Assets			$	228.1
December 31, 2014
(In Millions)		Current			30-59 Days			60-89 Days		90 Days or		Total		Nonperforming		Total Loans
					Past Due			Past Due		More Past		Performing				and Leases
										Due
Commercial
Commercial and Institutional		$	8,340.50		$	14.5		$	4	$	7.9	$	8,366.90	$	15	$	8,381.90
Commercial Real Estate			3,274.30			9.6			9.8		2.5		3,296.20		37.1		3,333.30
Lease Financing, net			916.3			—			—		—		916.3		—		916.3
Non-U.S.			1,530.60			—			—		—		1,530.60		—		1,530.60
Other			191.5			—			—		—		191.5		—		191.5
Total Commercial			14,253.20			24.1			13.8		10.4		14,301.50		52.1		14,353.60
Personal
Residential Real Estate			9,556.30			49.5			9.9		4.5		9,620.20		162.4		9,782.60
Private Client			7,396.00			56			5.9		7.8		7,465.70		1.2		7,466.90
Other			37.1			—			—		—		37.1		—		37.1
Total Personal			16,989.40			105.5			15.8		12.3		17,123.00		163.6		17,286.60
Total Loans and Leases		$	31,242.60		$	129.6		$	29.6	$	22.7	$	31,424.50	$	215.7	$	31,640.20
											Other Real Estate Owned				16.6
											Total Nonperforming Assets			$	232.3
Impaired Loans. A loan is considered to be impaired when, based on current information and events, management determines that it is probable that Northern Trust will be unable to collect all amounts due according to the contractual terms of the loan agreement. Impaired loans are identified through ongoing credit management and risk rating processes, including the formal review of past due and watch list credits. Payment performance and delinquency status are critical factors in identifying impairment for all loans and leases, particularly those within the residential real estate, private client and personal-other classes. Other key factors considered in identifying impairment of loans and leases within the commercial and institutional, non-U.S., lease financing and commercial-other classes relate to the borrower’s ability to perform under the terms of the obligation as measured through the assessment of future cash flows, including consideration of collateral value, market value and other factors. A loan is also considered to be impaired if its terms have been modified as a concession resulting from the debtor’s financial difficulties, referred to as a troubled debt restructuring (TDR) and discussed in further detail below. Impairment is measured based upon the loan’s market price, the present value of expected future cash flows, discounted at the loan’s effective interest rate, or the fair value of the collateral if the loan is collateral dependent. If the loan valuation is less than the recorded value of the loan, based on the certainty of loss, either a specific allowance is established or a charge-off is recorded for the difference. Smaller balance (individually less than $250,000) homogeneous loans are collectively evaluated for impairment and excluded from impaired loan disclosures as allowed under applicable accounting standards. Northern Trust’s accounting policies for impaired loans is consistent across all classes of loans and leases.
The following tables provide information related to impaired loans by segment and class.
Table 36: Information about Impaired Loans as of the Period End
		As of March 31, 2015								As of December 31, 2014
(In Millions)		Recorded			Unpaid			Specific		Recorded		Unpaid		Specific
		Investment			Principal			Allowance		Investment		Principal		Allowance
					Balance							Balance
With No Related Specific Allowance
Commercial and Institutional		$	7.9		$	9.9		$	—	$	9	$	12	$	—
Commercial Real Estate			49.1			54.1			—		47		52.4		—
Lease Financing, net			3.4			3.4			—		4.2		4.2		—
Residential Real Estate			153.1			198.7			—		160.9		204.8		—
Private Client			0.2			0.2			—		0.2		0.5		—
With a Related Specific Allowance
Commercial and Institutional			12.5			9.9			4		6.5		6.6		2.9
Commercial Real Estate			12.4			18.7			2.8		12.2		18.3		2.9
Residential Real Estate			5.8			6			1.2		1.4		1.4		0.4
Private Client			0.8			0.8			0.4		0.8		0.8		0.4
Total
Commercial			85.3			96			6.8		78.9		93.5		5.8
Personal			159.9			205.7			1.6		163.3		207.5		0.8
Total		$	245.2		$	301.7		$	8.4	$	242.2	$	301	$	6.6
Table 37: Information about Impaired Loans for the Period
		Three Months Ended March 31,
		2015						2014
(In Millions)		Average			Interest			Average		Interest
		Recorded			Income			Recorded		Income
		Investment			Recognized			Investment		Recognized
With No Related Specific Allowance
Commercial and Institutional		$	9.2		$	—		$	11.4	$	—
Commercial Real Estate			48.5			0.2			44.8		0.3
Lease Financing, net			3.4			0.1			4.4		0.1
Residential Real Estate			161.3			0.3			189.2		0.6
Private Client			0.2			—			0.7		—
With a Related Specific Allowance
Commercial and Institutional			8.5			—			10		—
Commercial Real Estate			12.2			—			26.4		—
Residential Real Estate			2.8			—			7.1		—
Private Client			0.8			—			0.1		—
Total
Commercial			81.8			0.3			97		0.4
Personal			165.1			0.3			197.1		0.6
Total		$	246.9		$	0.6		$	294.1	$	1
Note: Average recorded investment in impaired loans is calculated as the average of the month-end impaired loan balances for the period.
Interest income that would have been recorded for nonperforming loans in accordance with their original terms was $2.1 million and $2.5 million, respectively, for the three months ended March 31, 2015, and 2014.
There were $2.3 million and $2.4 million of aggregate undrawn loan commitments and standby letters of credit at March 31, 2015, and December 31, 2014, respectively, issued to borrowers whose loans were classified as nonperforming or impaired.
Troubled Debt Restructurings (TDRs). Included within impaired loans were $85.9 million and $82.7 million of nonperforming TDRs, and $63.0 million and $68.6 million of performing TDRs as of March 31, 2015, and December 31, 2014, respectively. All TDRs are reported as impaired loans in the calendar year of their restructuring. In subsequent years, a TDR may cease being reported as impaired if the loan was modified at a market rate and has performed according to the modified terms for at least six months. A loan that has been modified at a below market rate will return to performing status if it satisfies the six-month performance requirement; however, it will remain reported as impaired.
The following tables provide, by segment and class, the number of loans and leases modified in TDRs during the three-month periods ended March 31, 2015, and 2014, and the recorded investments and unpaid principal balances as of March 31, 2015, and 2014.
Table 38: Troubled Debt Restructurings
($ In Millions)		Three Months Ended
		March 31, 2015
		Number of			Recorded			Unpaid Principal
		Loans and Leases			Investment			Balance
Commercial
Commercial and Institutional			1		$	0.1		$	0.1
Commercial Real Estate			1			0.7			0.7
Total Commercial			2			0.8			0.8
Personal
Residential Real Estate			57			7.1			10.2
Total Personal			57			7.1			10.2
Total Loans and Leases			59		$	7.9		$	11
Note: Period-end balances reflect all paydowns and charge-offs during the period.
($ In Millions)		Three Months Ended
		March 31, 2014
		Number of			Recorded			Unpaid Principal
		Loans and Leases			Investment			Balance
Commercial
Commercial Real Estate			1		$	0.7		$	0.7
Total Commercial			1			0.7			0.7
Personal
Residential Real Estate			34			3.5			3.8
Private Client			1			—			—
Total Personal			35			3.5			3.8
Total Loans and Leases			36		$	4.2		$	4.5
Note: Period-end balances reflect all paydowns and charge-offs during the period.
TDR modifications involve interest rate concessions, extensions of term, deferrals of principal and other modifications. Other modifications typically reflect other nonstandard terms which Northern Trust would not offer in non-troubled situations.
During the three months ended March 31, 2015, the TDR modification of the loan within commercial and institutional was an other modification and the loan within commercial real estate had modifications of deferred principal and extension of term. The majority of the TDR modifications within residential real estate were interest rate concessions, extensions of term or deferred principal. During the three months ended March 31, 2014, TDR modifications of loans within the commercial real estate, residential real estate, and private client classes were extensions of term.
There were 4 residential real estate loans modified as TDRs in the 12 months ended December 31, 2014 which subsequently became nonperforming during the three months ended March 31, 2015. The total recorded investment and unpaid principal balance for these loans was approximately $0.7 million as of March 31, 2015. There were no loans or leases modified as TDRs in the 12 months ended December 31, 2013, which subsequently became nonperforming during the three months ended March 31, 2014.
All loans and leases modified in troubled debt restructurings are evaluated for impairment. The nature and extent of impairment of TDRs, including those that have experienced a subsequent default, is considered in the determination of an appropriate level of allowance for credit losses.
Northern Trust may obtain physical possession of residential real estate collateralizing a consumer mortgage loan via foreclosure on an in-substance repossession. As of March 31, 2015, Northern Trust held foreclosed residential real estate properties with a carrying value of $5.7 million as a result of obtaining physical possession. In addition, as of March 31, 2015, Northern Trust had consumer loans with a carrying value of $29.3 million collateralized by residential real estate property for which formal foreclosure proceedings were in process.

Allowance_for_Credit_Losses

Allowance for Credit Losses	3 Months Ended
	Mar. 31, 2015
Allowance for Credit Losses	6. Allowance for Credit Losses — The allowance for credit losses, which represents management’s estimate of probable losses related to specific borrower relationships and inherent in the various loan and lease portfolios, undrawn commitments, and standby letters of credit, is determined by management through a disciplined credit review process. Northern Trust’s accounting policies related to the estimation of the allowance for credit losses and the charging off of loans, leases and other extensions of credit deemed uncollectible are consistent across both loan and lease segments.
	In establishing the inherent portion of the allowance for credit losses, Northern Trust’s Loan Loss Reserve Committee assesses a common set of qualitative factors applicable to both the commercial and personal loan segments. The risk characteristics underlying these qualitative factors, and management’s assessments as to the relative importance of a qualitative factor, can vary between loan segments and between classes within loan segments. Factors evaluated include those related to external matters, such as economic conditions and changes in collateral value, and those related to internal matters, such as changes in asset quality metrics and loan review activities. In addition to the factors noted above, risk characteristics such as portfolio delinquencies, percentage of portfolio on the watch list and on nonperforming status, and average borrower ratings are assessed in the determination of the inherent allowance.
	Loan-to-value levels are considered for collateral-secured loans and leases in both the personal and commercial segments. Borrower debt service coverage is evaluated in the personal segment, and cash flow coverage is analyzed in the commercial segment.
	Similar risk characteristics by type of exposure are analyzed when determining the allowance for undrawn commitments and standby letters of credit. These qualitative factors, together with historical loss rates, serve as the basis for the allowance for credit losses.
	Loans, leases and other extensions of credit deemed uncollectible are charged to the allowance for credit losses. Subsequent recoveries, if any, are credited to the allowance. Determinations as to whether an uncollectible loan is charged-off or a specific allowance is established are based on management’s assessment as to the level of certainty regarding the amount of loss.
	The following tables provide information regarding changes in the total allowance for credit losses by segment during the three months ended March 31, 2015, and 2014.
	Table 39: Changes in the Allowance for Credit Losses
		2015									2014
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Balance at Beginning of Period	$	169.7		$	126.2		$	295.9		$	168		$	139.9		$	307.9
	Charge-Offs		(2.7	)		(4.8	)		(7.5	)		(4.2	)		(7.3	)		(11.5	)
	Recoveries		1.7			1.2			2.9			8.5			1.5			10
	Net (Charge-Offs) Recoveries		(1.0	)		(3.6	)		(4.6	)		4.3			(5.8	)		(1.5	)
	Provision for Credit Losses		(4.3	)		(0.2	)		(4.5	)		0.8			2.2			3
	Effect of Foreign Exchange Rates		(0.1	)		—			(0.1	)		—			—			—
	Balance at End of Period	$	164.3		$	122.4		$	286.7		$	173.1		$	136.3		$	309.4
	The following table provides information regarding the balances of the recorded investments in loans and leases and the allowance for credit losses by segment as of March 31, 2015, and December 31, 2014.
	Table 40: Information about the Recorded Investments in Loans and Leases
		March 31, 2015									December 31, 2014
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Loans and Leases
	Specifically Evaluated for Impairment	$	85.3		$	159.9		$	245.2		$	78.9		$	163.3		$	242.2
	Evaluated for Inherent Impairment		15,406.00			16,979.00			32,385.00			14,274.70			17,123.30			31,398.00
	Total Loans and Leases		15,491.30			17,138.90			32,630.20			14,353.60			17,286.60			31,640.20
	Allowance for Loans and Leases
	Specifically Evaluated for Impairment		6.8			1.6			8.4			5.8			0.8			6.6
	Evaluated for Inherent Impairment		132.7			117.9			250.6			138			122.4			260.4
	Allowance Assigned to Loans and Leases		139.5			119.5			259			143.8			123.2			267
	Allowance for Unfunded Exposures
	Commitments and Standby Letters of Credit		24.8			2.9			27.7			25.9			3			28.9
	Total Allowance for Credit Losses	$	164.3		$	122.4		$	286.7		$	169.7		$	126.2		$	295.9

Pledged_Assets

Pledged Assets	3 Months Ended
	Mar. 31, 2015
Pledged Assets	7. Pledged Assets — Certain of Northern Trust’s subsidiaries, as required or permitted by law, pledge assets to secure public and trust deposits, repurchase agreements and Federal Home Loan Bank borrowings, as well as for other purposes, including support for securities settlement, primarily related to client activities, and for potential Federal Reserve Bank discount window borrowings. As of March 31, 2015, securities and loans totaling $33.2 billion ($23.9 billion of government-sponsored agency and other securities, $113.0 million of obligations of states and political subdivisions and $9.2 billion of loans) were pledged. This compares to $32.3 billion ($23.1 billion of government-sponsored agency and other securities, $122.9 million of obligations of states and political subdivisions and $9.1 billion of loans) at December 31, 2014. Collateral required for these purposes totaled $5.6 billion and $5.9 billion at March 31, 2015, and December 31, 2014, respectively. Included in the total pledged assets at March 31, 2015, and December 31, 2014, were available for sale securities with a total fair value of $611.4 million and $884.8 million, respectively, which were pledged as collateral for agreements to repurchase securities sold transactions. The secured parties to these transactions have the right to repledge or sell these securities.
	Northern Trust is not permitted, by contract or custom, to repledge or sell collateral from agreements to resell securities purchased transactions. The total fair value of accepted collateral was $1.0 billion as of each of March 31, 2015 and December 31, 2014. There was no repledged or sold collateral at March 31, 2015 or December 31, 2014.
	Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $1.5 billion and $1.2 billion for the three months ended March 31, 2015, and 2014, respectively.

Goodwill_and_Other_Intangibles

Goodwill and Other Intangibles	3 Months Ended
	Mar. 31, 2015
Goodwill and Other Intangibles	8. Goodwill and Other Intangibles — The carrying amounts of goodwill, reflecting the effect of foreign exchange rates on non-U.S.-dollar-denominated balances, by reporting segment at March 31, 2015, and December 31, 2014, were as follows:
	Table 41: Goodwill by Reporting Segment
	(In Millions)	March 31,		December 31,
		2015		2014
	Corporate & Institutional Services	$	456	$	461.8
	Wealth Management		71.3		71.4
	Total Goodwill	$	527.3	$	533.2
	The gross carrying amount and accumulated amortization of other intangible assets subject to amortization as of March 31, 2015, and December 31, 2014, were as follows:
	Table 42: Other Intangible Assets
	(In Millions)	March 31,		December 31,
		2015		2014
	Gross Carrying Amount	$	182.3	$	189.5
	Less: Accumulated Amortization		129.7		129.5
	Net Book Value	$	52.6	$	60
	Other intangible assets consist primarily of the value of acquired client relationships and are included within other assets in the consolidated balance sheets. Amortization expense related to other intangible assets totaled $4.6 million and $4.9 million for the three months ended March 31, 2015, and 2014, respectively. Amortization for the remainder of 2015 and for the years 2016, 2017, 2018, and 2019 is estimated to be $6.3 million, $8.4 million, $8.3 million, $7.7 million and $7.5 million, respectively.

Reporting_Segments

Reporting Segments	3 Months Ended
	Mar. 31, 2015
Reporting Segments	9. Reporting Segments — The following tables show the earnings contributions of Northern Trust’s reporting segments for the three-month periods ended March 31, 2015, and 2014.
	Table 43: Results of Reporting Segments
	Three Months Ended		Corporate &						Wealth						Treasury and						Total
	March 31,		Institutional Services						Management						Other						Consolidated
	($ In Millions)		2015			2014			2015			2014			2015			2014			2015			2014
	Noninterest Income
	Trust, Investment and Other Servicing Fees		$	407.3		$	379.2		$	320.2		$	300.3		$	—		$	—		$	727.5		$	679.5
	Foreign Exchange Trading Income			67.5			48.1			4.1			2			—			—			71.6			50.1
	Other Noninterest Income			41.9			44.4			28.8			22.7			4.1			(1.9	)		74.8			65.2
	Net Interest Income*			96			73.7			138.3			135			32.5			45.7			266.8			254.4
	Revenue*			612.7			545.4			491.4			460			36.6			43.8			1,140.70			1,049.20
	Provision for Credit Losses			(2.2	)		1.2			(2.3	)		1.8			—			—			(4.5	)		3
	Noninterest Expense			434.9			423.5			321.9			318.8			32.2			25.7			789			768
	Income before Income Taxes*			180			120.7			171.8			139.4			4.4			18.1			356.2			278.2
	Provision for Income Taxes*			57.3			34.8			64.6			52.6			3.6			9.4			125.5			96.8
	Net Income		$	122.7		$	85.9		$	107.2		$	86.8		$	0.8		$	8.7		$	230.7		$	181.4
	Percentage of Consolidated Net Income			53	%		47	%		47	%		48	%		—	%		5	%		100	%		100	%
	Average Assets		$	39,762.40		$	57,252.40		$	24,308.80		$	22,528.70		$	43,442.00		$	20,462.40		$	107,513.20		$	100,243.50
	*	Stated on an FTE basis. Total consolidated includes FTE adjustments of $6.2 million for 2015 and $8.7 million for 2014.
	In the current quarter, the presentation of certain assets was changed from C&IS to Treasury and Other to reflect better the internal management responsibility for these assets. In addition to the transfer of assets, the Corporation’s internal funds pricing treatment of deposits that fund the transferred assets was updated to reflect the economics of these deposits.
	Further discussion of reporting segment results is provided within the “Reporting Segments” section of Management’s Discussion and Analysis of Financial Condition and Results of Operations.

Stockholders_Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2015
Stockholders' Equity	10. Stockholders’ Equity
	Preferred Stock. The Corporation is authorized to issue 10 million shares of preferred stock without par value. The Board of Directors is authorized to fix the particular designations, preferences and relative, participating, optional and other special rights and qualifications, limitations or restrictions for each series of preferred stock issued. As of March 31, 2015, and December 31, 2014, preferred stock totaled $388.5 million, related to the issuance of Series C Non-Cumulative Perpetual Preferred Stock (Series C Preferred Stock) in August 2014. As of March 31, 2015, and December 31, 2014, 16 million depositary shares, each representing 1/1000 ownership interest in a share of Series C Preferred Stock, were issued and outstanding. Series C Preferred Stock has no par value and has a liquidation preference of $25,000 ($25 per depositary share).
	Dividends on the Series C Preferred Stock will accrue and be payable on the liquidation preference amount, on a non-cumulative basis, quarterly in arrears on the first day of January, April, July and October of each year, at a rate per annum equal to 5.85%. On January 21, 2015, the Corporation declared a cash dividend of $365.625 per share of Series C Preferred Stock payable on April 1, 2015, to stockholders on record as of March 15, 2015.
	Common Stock. During the three months ended March 31, 2015, the Corporation repurchased 1.6 million shares of its common stock at an average cost of $68.93 per share and an aggregate cost of approximately $107.2 million, under a program approved by the Board of Directors in April 2014. On April 21, 2015, this program was terminated and replaced with a new repurchase authorization, announced on April 22, 2015, pursuant to which the Corporation may repurchase up to 15 million shares. The new repurchase authorization has no expiration date.
	Under the Corporation’s capital plan submitted in January 2015, which was reviewed without objection by the Federal Reserve, the Corporation may repurchase up to $675 million of common stock between April 2015 and June 2016.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss)	11. Accumulated Other Comprehensive Income (Loss) — The following tables summarize the components of accumulated other comprehensive income (loss) (AOCI) at March 31, 2015, and 2014, and changes during the three-month periods then ended.
	Table 44: Summary of Changes in Accumulated Other Comprehensive Income (Loss)
	(In Millions)	Balance at				Net Change			Balance at
		March 31,							December 31,
		2015							2014
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	81.7			$	54.1		$	27.6
	Net Unrealized Gains (Losses) on Cash Flow Hedges		(5.8	)			(1.1	)		(4.7	)
	Net Foreign Currency Adjustments		(7.6	)			(5.9	)		(1.7	)
	Net Pension and Other Postretirement Benefit Adjustments		(334.7	)			6.2			(340.9	)
	Total	$	(266.4	)		$	53.3		$	(319.7	)
	(In Millions)	Balance at				Net Change			Balance at
		March 31,							December 31,
		2014							2013
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	10.5			$	4.5		$	6
	Net Unrealized Gains (Losses) on Cash Flow Hedges		4.5				1.6			2.9
	Net Foreign Currency Adjustments		5.8				(1.3	)		7.1
	Net Pension and Other Postretirement Benefit Adjustments		(256.9	)			3.4			(260.3	)
	Total	$	(236.1	)		$	8.2		$	(244.3	)
	Table 45: Details of Changes in Accumulated Other Comprehensive Income (Loss)
		Three Months Ended March 31,
		2015										2014
	(In Millions)	Before Tax				Tax Effect			After Tax			Before Tax			Tax Effect			After Tax
	Unrealized Gains (Losses) on Securities Available for Sale
	Noncredit-Related Unrealized Gains (Losses) on Securities OTTI	$	—			$	—		$	—		$	1.5		$	(0.6	)	$	0.9
	Other Unrealized Gains (Losses) on Securities Available for Sale		86.8				(32.7	)		54.1			5.7			(2.2	)		3.5
	Reclassification Adjustment for (Gains) Losses Included in Net Income		(0.1	)			0.1			—			0.1			—			0.1
	Net Change		86.7				(32.6	)		54.1			7.3			(2.8	)		4.5
	Unrealized Gains (Losses) on Cash Flow Hedges
	Unrealized Gains (Losses) on Cash Flow Hedges		(4.7	)			3			(1.7	)		2.4			(0.9	)		1.5
	Reclassification Adjustment for (Gains) Losses Included in Net Income		1				(0.4	)		0.6			0.2			(0.1	)		0.1
	Net Change		(3.7	)			2.6			(1.1	)		2.6			(1.0	)		1.6
	Foreign Currency Adjustments
	Foreign Currency Translation Adjustments		(90.1	)			12.2			(77.9	)		6.5			(0.9	)		5.6
	Long-Term Intra-Entity Foreign Currency Transaction Losses		(1.0	)			0.3			(0.7	)		(0.6	)		0.2			(0.4	)
	Net Investment Hedge Gains (Losses)		116.6				(43.9	)		72.7			(10.6	)		4.1			(6.5	)
	Net Change		25.5				(31.4	)		(5.9	)		(4.7	)		3.4			(1.3	)
	Pension and Other Postretirement Benefit Adjustments
	Net Actuarial Gains (Losses)		—				—			—			—			—			—
	Reclassification Adjustment for (Gains) Losses Included in Net Income		9.5				(3.3	)		6.2			6.4			(3.0	)		3.4
	Net Change	$	9.5			$	(3.3	)	$	6.2		$	6.4		$	(3.0	)	$	3.4
	The following table provides the location and before-tax amounts of reclassifications out of accumulated other comprehensive income (loss) during the three months ended March 31, 2015.
	Table 46: Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
	(In Millions)	Location of		Amount of Reclassification
		Reclassification Adjustments		Adjustments Recognized
		Recognized in Income		in Income
				Three Months Ended
				March 31, 2015
	Securities Available for Sale
	Realized (Gains) Losses on Securities Available for Sale	Investment Security Gains (Losses), net		$	(0.1	)
	Realized (Gains) Losses on Cash Flow Hedges
	Foreign Exchange Contracts	Other Operating Income/Expense			1
	Pension and Other Postretirement Benefit Adjustments
	Amortization of Net Actuarial (Gains) Losses	Employee Benefits			9.6
	Amortization of Prior Service Cost	Employee Benefits			(0.1	)
	Gross Reclassification Adjustment			$	9.5

Net_Income_Per_Common_Share_Co

Net Income Per Common Share Computations	3 Months Ended
	Mar. 31, 2015
Net Income Per Common Share Computations	12. Net Income Per Common Share Computations — The computations of net income per common share are presented in the following table.
	Table 47: Net Income per Common Share
		Three Months Ended
		March 31,
	($ In Millions Except Per Common Share Information)	2015		2014
	Basic Net Income Per Common Share
	Average Number of Common Shares Outstanding		233,381,168		237,208,151
	Net Income	$	230.7	$	181.4
	Less: Dividends on Preferred Stock		5.9		—
	Net Income Applicable to Common Stock		224.8		181.4
	Less: Earnings Allocated to Participating Securities		3.7		2.9
	Earnings Allocated to Common Shares Outstanding		221.1		178.5
	Basic Net Income Per Common Share	$	0.95	$	0.75
	Diluted Net Income Per Common Share
	Average Number of Common Shares Outstanding		233,381,168		237,208,151
	Plus: Dilutive Effect of Share-based Compensation		1,907,527		1,842,563
	Average Common and Potential Common Shares		235,288,695		239,050,714
	Earnings Allocated to Common and Potential Common Shares	$	221.1	$	178.5
	Diluted Net Income Per Common Share		0.94		0.75
	Note: Common stock equivalents totaling 1,504,850 and 1,997,337 for the three months ended March 31, 2015, and 2014, respectively, were not included in the computation of diluted net income per common share because their inclusion would have been antidilutive.

Net_Interest_Income

Net Interest Income	3 Months Ended
	Mar. 31, 2015
Net Interest Income	13. Net Interest Income — The components of net interest income were as follows:
	Table 48: Net Interest Income
		Three Months Ended
		March 31,
	(In Millions)	2015		2014
	Interest Income
	Loans and Leases	$	183.9	$	182.9
	Securities — Taxable		76.6		69.5
	— Non-Taxable		1.3		2.2
	Interest-Bearing Due from and Deposits with Banks (Note)		26.6		32.1
	Federal Reserve Deposits and Other		10.4		8.7
	Total Interest Income		298.8		295.4
	Interest Expense
	Deposits		16.7		20.1
	Federal Funds Purchased		0.2		0.3
	Securities Sold Under Agreements to Repurchase		0.1		0.1
	Other Borrowings		1.2		0.7
	Senior Notes		11.6		17.5
	Long-Term Debt		7.8		10.4
	Floating Rate Capital Debt		0.6		0.6
	Total Interest Expense		38.2		49.7
	Net Interest Income	$	260.6	$	245.7
	Note: Interest income for the three months ended March 31, 2015, was earned on cash and due from banks of $2.2 billion and interest-bearing deposits with banks of $15.4 billion as of March 31, 2015.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Taxes	14. Income Taxes — Income tax expense for the three months ended March 31, 2015, of $119.3 million was recorded, representing an effective tax rate of 34.1%. The provision for income taxes in the comparable period of 2014 was $88.1 million, representing an effective tax rate of 32.7%.

Pension_and_Postretirement_Hea

Pension and Postretirement Health Care	3 Months Ended
	Mar. 31, 2015
Pension and Postretirement Health Care	15. Pension and Postretirement Health Care — The following tables set forth the net periodic pension and postretirement benefit expense for Northern Trust’s U.S. and non-U.S. pension plans, supplemental pension plan, and postretirement health care plan for the three months ended March 31, 2015, and 2014.
	Table 49: Net Periodic Pension Expense (Benefit)
	Net Periodic Pension Expense	Three Months Ended
		March 31,
	U.S. Plan
	(In Millions)	2015			2014
	Service Cost	$	9.5		$	8.2
	Interest Cost		11.2			11.1
	Expected Return on Plan Assets		(24.1	)		(24.4	)
	Amortization
	Net Actuarial Loss		7.4			5.4
	Prior Service Cost		(0.1	)		(0.1	)
	Net Periodic Pension Expense	$	3.9		$	0.2
	Net Periodic Pension Expense (Benefit)	Three Months Ended
		March 31,
	Non U.S. Plans
	(In Millions)	2015			2014
	Interest Cost	$	1.4		$	1.7
	Expected Return on Plan Assets		(1.5	)		(1.7	)
	Net Actuarial Loss Amortization		0.4			(0.4	)
	Net Periodic Pension Expense (Benefit)	$	0.3		$	(0.4	)
	Net Periodic Pension Expense	Three Months Ended
		March 31,
	Supplemental Plan
	(In Millions)	2015			2014
	Service Cost	$	0.9		$	0.8
	Interest Cost		1.3			1.2
	Amortization
	Net Actuarial Loss		1.8			1.5
	Prior Service Cost		—			0.1
	Net Periodic Pension Expense	$	4		$	3.6
	Net Periodic Postretirement Expense	Three Months Ended
		March 31,
	Postretirement Health Care Plan
	(In Millions)	2015			2014
	Interest Cost		0.3			0.4
	Amortization
	Net Actuarial (Gain)		—			(0.1	)
	Net Periodic Postretirement Expense	$	0.3		$	0.3

ShareBased_Compensation_Plans

Share-Based Compensation Plans	3 Months Ended
	Mar. 31, 2015
Share-Based Compensation Plans	16. Share-Based Compensation Plans — The Northern Trust Corporation 2012 Stock Plan provides for the grant of nonqualified stock options, incentive stock options, stock appreciation rights, stock awards, restricted stock units and performance stock units.
	In the first quarter of 2015, the Corporation granted 453,862 nonqualified stock options with a total grant-date fair value of $8.5 million, 886,452 stock unit awards with a total grant-date fair value of $62.2 million, and 272,319 performance stock units with a total grant-date fair value of $19.1 million. Stock unit award compensation expense for the three months ended March 31, 2015 and 2014 includes $1.5 million and $1.4 million, respectively, attributable to restricted stock units which vested in full and were expensed in their entirety upon their date of grant. Compensation expense for the three months ended March 31, 2015 and 2014 includes $4.3 million and $3.0 million, respectively, attributable to stock options granted to retirement-eligible employees that were expensed in their entirety on their date of grant.
	Total compensation expense for share-based payment arrangements and the associated tax impacts were as follows for the three months ended March 31, 2015, and 2014.
	Table 50: Total Compensation Expense for Share-Based Payment Arrangements
		Three Months Ended
		March 31,
	(In Millions)	2015		2014
	Restricted Stock Unit Awards	$	13.5	$	14.1
	Stock Options		6		6.4
	Performance Stock Units		3.4		2.6
	Total Share-Based Compensation Expense		22.9		23.1
	Tax Benefits Recognized	$	8.6	$	8.7

Variable_Interest_Entities

Variable Interest Entities	3 Months Ended
	Mar. 31, 2015
Variable Interest Entities	17. Variable Interest Entities — Variable Interest Entities (VIEs) are defined within GAAP as entities that either have a total equity investment that is insufficient to permit the entity to finance its activities without additional subordinated financial support or whose equity investors lack the characteristics of a controlling financial interest. Investors that finance a VIE through debt or equity interests, or other counterparties that provide other forms of support, such as guarantees, subordinated fee arrangements, or certain types of derivative contracts, are variable interest holders in the entity. The variable interest holder, if any, that has both the power to direct the activities that most significantly impact the entity and a variable interest that could potentially be significant to the entity is deemed to be the VIE’s primary beneficiary and is required to consolidate the VIE.
	Leveraged Leases. In leveraged leasing transactions, Northern Trust acts as lessor of the underlying asset subject to the lease and typically funds 20-30% of the asset’s cost via an equity ownership in a trust with the remaining 70-80% provided by third-party non-recourse debt holders. In such transactions, the trusts, which are VIEs, are created to provide the lessee use of the property with substantially all of the rights and obligations of ownership. The lessee’s maintenance and operation of the leased property has a direct effect on the fair value of the underlying property, and the lessee also has the ability to increase the benefits it can receive and limit the losses it can suffer by the manner in which it uses the property. As a result, Northern Trust has determined that it is not the primary beneficiary of these VIEs given it lacks the power to direct the activities that most significantly impact the economic performance of the VIEs.
	Northern Trust’s maximum exposure to loss as a result of its involvement with the leveraged lease trust VIEs is limited to the carrying amounts of its leveraged lease investments. As of March 31, 2015, and December 31, 2014, the carrying amounts of these investments, which are included in loans and leases in the consolidated balance sheets, were $516.4 million and $547.6 million, respectively. Northern Trust’s funding requirements relative to the VIEs are limited to its invested capital. Northern Trust has no other liquidity arrangements or obligations to purchase assets of the VIEs that would expose Northern Trust to a loss.
	Tax Credit Structures. Northern Trust invests in qualified affordable housing projects and community development entities (collectively, community development projects) that are designed to generate a return primarily through the realization of tax credits. The community development projects are formed as limited partnerships and LLCs in which Northern Trust invests as a limited partner/investor member through equity contributions. The economic performance of the community development projects, which are VIEs, is subject to the performance of their underlying investment and their ability to operate in compliance with the rules and regulations necessary for the qualification of tax credits generated by equity investments. Northern Trust has determined that it is not the primary beneficiary of any community development projects as it lacks the power to direct the activities that most significantly impact the economic performance of the underlying investments or to affect their ability to operate in compliance with the rules and regulations necessary for the qualification of tax credits generated by equity investments. This power is held by the general partners and managing members who exercise full and exclusive control of the operations of the VIEs.
	Northern Trust’s maximum exposure to loss as a result of its involvement with community development projects is limited to the carrying amount of its investments, including any undrawn commitments. As of March 31, 2015, and December 31, 2014, the carrying amount of investments in community development projects that generate tax credits, included in other assets in the consolidated balance sheets, totaled $198.2 million and $208.9 million, respectively. As of March 31, 2015, and December 31, 2014, liabilities related to undrawn commitments on investments in tax credit community development projects, included in other liabilities in the consolidated balance sheets, totaled $13.9 million and $15.6 million, respectively. Northern Trust’s funding requirements are limited to its invested capital and undrawn commitments for future equity contributions. Northern Trust has no exposure to loss from liquidity arrangements and no obligation to purchase assets of the community development projects.
	Affordable housing tax credits and other tax benefits attributable to community development projects totaled $13.2 million and $14.8 million for the three months ended March 31, 2015, and March 31, 2014, respectively.
	Trust Preferred Securities. In 1997, Northern Trust issued Floating Rate Capital Securities, Series A and Series B, through NTC Capital I and NTC Capital II, respectively, statutory business trusts wholly-owned by the Corporation. The sole assets of the trusts are Subordinated Debentures of the Corporation that have the same interest rates and maturity dates as the corresponding distribution rates and redemption dates of the Floating Rate Capital Securities. NTC Capital I and NTC Capital II are considered VIEs; however, as the sole asset of each trust is a receivable from the Corporation and proceeds to the Corporation from the receivable exceed the Corporation’s investment in the VIEs’ equity shares, the Corporation is not permitted to consolidate the trusts, even though the Corporation owns all of the voting equity shares of the trusts, has fully guaranteed the trusts’ obligations and has the right to redeem the preferred securities in certain circumstances. Northern Trust recognizes the subordinated debentures in its consolidated balance sheets as long-term liabilities.
	Investment Funds. Northern Trust acts as asset manager for various funds in which clients of Northern Trust are investors. As an asset manager of funds, the Corporation earns a competitively priced fee that is based on assets managed and varies with each fund’s investment objective. Based on its analysis, Northern Trust has determined that it is not the primary beneficiary of these VIEs under GAAP.

Contingent_Liabilities

Contingent Liabilities	3 Months Ended
	Mar. 31, 2015
Contingent Liabilities	18. Contingent Liabilities
	Commitments, Letters of Credit and Indemnifications. Northern Trust, in the normal course of business, enters into various types of commitments and issues letters of credit to meet the liquidity and credit enhancement needs of its clients.
	Legally binding commitments to extend credit generally have fixed expiration dates or other termination clauses. Since a significant portion of the commitments are expected to expire without being drawn upon, the total commitment amount does not necessarily represent future loans or liquidity requirements. Legally binding commitments to extend credit totaled $35.6 billion and $35.1 billion as of March 31, 2015, and December 31, 2014, respectively, excluding $477.2 million and $481.4 million of commitments participated to others as of March 31, 2015, and December 31, 2014, respectively.
	Standby letters of credit obligate Northern Trust to meet certain financial obligations of its clients, if, under the contractual terms of the agreement, the clients are unable to do so. These instruments are primarily issued to support public and private financial commitments, including commercial paper, bond financing, initial margin requirements on futures exchanges and similar transactions. Northern Trust is obligated to meet the entire financial obligation of these agreements and in certain cases is able to recover the amounts paid through recourse against collateral received or other participants. Standby letters of credit outstanding were $4.4 billion and $4.5 billion as of March 31, 2015, and December 31, 2014, respectively.
	As part of its securities custody activities and at the direction of its clients, Northern Trust lends securities owned by clients to borrowers who are reviewed and approved by the Northern Trust Counterparty Risk Management Committee. In connection with these activities, Northern Trust has issued indemnifications to certain clients against certain losses that are a direct result of a borrower’s failure to return securities when due, should the value of such securities exceed the value of the collateral required to be posted. Borrowers are required to collateralize fully securities received with cash or marketable securities. As securities are loaned, collateral is maintained at a minimum of 100% of the fair value of the securities plus accrued interest. The collateral is revalued on a daily basis. The amount of securities loaned as of March 31, 2015, and December 31, 2014, subject to indemnification was $105.5 billion and $98.1 billion, respectively. Because of the credit quality of the borrowers and the requirement to collateralize fully securities borrowed, management believes that the exposure to credit loss from this activity is not significant and no liability was recorded as of March 31, 2015, or December 31, 2014, related to these indemnifications.
	Legal Proceedings. In the normal course of business, the Corporation and its subsidiaries are routinely defendants in or parties to a number of pending and threatened legal actions, including, but not limited to, actions brought on behalf of various claimants or classes of claimants, regulatory matters, employment matters and challenges from tax authorities regarding the amount of taxes due. In certain of these actions and proceedings, claims for substantial monetary damages or adjustments to recorded tax liabilities are asserted.
	Based on current knowledge, after consultation with legal counsel and after taking into account current accruals, management does not believe that losses, if any, arising from pending litigation or threatened legal actions or regulatory matters will have a material adverse effect on the consolidated financial position or liquidity of the Corporation, although such matters could have a material adverse effect on the Corporation’s operating results for a particular period.
	Under GAAP, (i) an event is “probable” if the “future event or events are likely to occur”; (ii) an event is “reasonably possible” if “the chance of the future event or events occurring is more than remote but less than likely”; and (iii) an event is “remote” if “the chance of the future event or events occurring is slight.”
	For the reasons set out in this paragraph, the outcome of some matters is inherently difficult to predict and/or the range of loss cannot be reasonably estimated. This may be the case in matters that (i) will be decided by a jury, (ii) are in early stages, (iii) involve uncertainty as to the likelihood of a class being certified or the ultimate size of the class, (iv) are subject to appeals or motions, (v) involve significant factual issues to be resolved, including with respect to the amount of damages, (vi) do not specify the amount of damages sought or (vii) seek very large damages based on novel and complex damage and liability legal theories. Accordingly, the Corporation cannot reasonably estimate the eventual outcome of these pending matters, the timing of their ultimate resolution or what the eventual loss, fines or penalties, if any, related to each pending matter will be.
	In accordance with applicable accounting guidance, the Corporation records accruals for litigation and regulatory matters when those matters present loss contingencies that are both probable and reasonably estimable. When loss contingencies are not both probable and reasonably estimable, the Corporation does not record accruals. No material accruals have been recorded for pending litigation or threatened legal actions or regulatory matters.
	For a limited number of the matters for which a loss is reasonably possible in future periods, whether in excess of an accrued liability or where there is no accrued liability, the Corporation is able to estimate a range of possible loss. As of March 31, 2015, the Corporation has estimated the upper end of the range of reasonably possible losses for these matters to be approximately $125 million in the aggregate. This aggregate amount of reasonably possible loss is based upon currently available information and is subject to significant judgment and a variety of assumptions and known and unknown uncertainties. The matters underlying the estimated range will change from time to time, and actual results will vary significantly from the current estimate.
	In certain other pending matters, there may be a range of reasonably possible losses (including reasonably possible losses in excess of amounts accrued) that cannot be reasonably estimated for the reasons described above. Such matters are not included in the estimate of reasonably possible losses identified above.
	A number of participants in Northern Trust’s securities lending program, which is associated with its asset servicing business, have commenced either individual lawsuits or purported class actions in which they claim, among other things, that Northern Trust failed to exercise prudence in the investment management of the collateral received from the borrowers of the securities, resulting in losses that they seek to recover. The cases assert various contractual, statutory and common law claims, including claims for breach of fiduciary duty under common law and under the Employee Retirement Income Security Act (ERISA). In 2013, Northern Trust recorded a $19.2 million pre-tax charge in connection with an agreement to resolve certain claims related to two of these lawsuits, the settlement of which remains pending while awaiting court approval. Other lawsuits and claims related to securities lending are not part of the proposed settlement, and remain pending.
	As previously disclosed in April 2014, Northern Trust remains subject to an investigation by the U.S. Securities and Exchange Commission (SEC) related to Northern Trust’s securities lending activities. Northern Trust continues to cooperate with the SEC in this investigation.
	In January 2015, the Public Prosecutor’s Office of France recommended that certain charges be brought against Northern Trust Fiduciary Services (Guernsey) Limited (“NTFS”), an indirect subsidiary of the Corporation, relating to the administration of two trusts for which NTFS serves as trustee. In April 2015, a French investigating magistrate judge charged NTFS with complicity in estate tax fraud. NTFS will contest the criminal charge in the French court. As trustee, NTFS provided no tax advice and had no involvement in the preparation or filing of the challenged estate tax filings.
	Visa Membership. Northern Trust, as a member of Visa U.S.A. Inc. and in connection with the initial public offering of Visa Inc. (Visa), received shares of restricted stock in Visa. As of March 31, 2015, the Visa shares held by Northern Trust are recorded at their original cost basis of zero and have restrictions as to their sale or transfer.
	Northern Trust is obligated to indemnify Visa for losses resulting from certain indemnified litigation relating to interchange fees and has been required to recognize, at its estimated fair value in accordance with GAAP, a guarantee liability arising from such litigation that has not yet settled. During 2007, Northern Trust recorded liabilities relating to Visa indemnified litigation. Subsequently, Visa established an escrow account to cover the settlements of, or judgments in, indemnified litigation. The fundings by Visa of its escrow account have resulted in reductions of Northern Trust’s indemnification liability. Northern Trust’s indemnification liability was fully eliminated as of December 31, 2011. On October 19, 2012, Visa signed a settlement agreement with plaintiff representatives for binding settlement of the indemnified litigation. On January 14, 2014, the trial court entered a final judgment order approving the settlement with the class plaintiffs, which is subject to appeal. A number of objectors have appealed from that order and more than 30 opt-out cases have been filed by merchants in various federal district courts.
	While the ultimate resolution of the indemnified litigation and the timing for removal of selling restrictions on the Visa shares are highly uncertain, Northern Trust anticipates that the value of its Visa shares will be adequate to offset any remaining indemnification obligations related to Visa litigation.

Derivative_Financial_Instrumen

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2015
Derivative Financial Instruments	19. Derivative Financial Instruments — Northern Trust is a party to various derivative financial instruments that are used in the normal course of business to meet the needs of its clients, as part of its trading activity for its own account and as part of its risk management activities. These instruments include foreign exchange contracts and interest rate contracts.
	Northern Trust’s primary risks associated with these instruments is the possibility that interest rates, foreign exchange rates or credit spreads could change in an unanticipated manner, resulting in higher costs or a loss in the underlying value of the instrument. These risks are mitigated by establishing limits, monitoring the level of actual positions taken against such established limits and monitoring the level of any interest rate sensitivity gaps created by such positions. When establishing position limits, market liquidity and volatility, as well as experience in each market, are taken into account.
	Credit risk associated with derivative instruments relates to the failure of the counterparty to pay based on the contractual terms of the agreement, and is generally limited to the unrealized fair value gains on these instruments, net of any cash collateral received. The amount of credit risk will increase or decrease during the lives of the instruments as interest rates, foreign exchange rates or credit spreads fluctuate. Northern Trust’s risk is controlled by limiting such activity to an approved list of counterparties and by subjecting such activity to the same credit and quality controls as are followed in lending and investment activities. Credit Support Annexes and other similar agreements are currently in place with a number of Northern Trust’s counterparties which mitigate the aforementioned credit risk associated with derivative activity conducted with those counterparties by requiring that significant net unrealized fair value gains be supported by collateral placed with Northern Trust.
	All derivative financial instruments, whether designated as hedges or not, are recorded in the consolidated balance sheets at fair value within other assets or other liabilities. As noted in the discussions below, the manner in which changes in the fair value of a derivative is accounted for in the consolidated statements of income depends on whether the contract has been designated as a hedge and qualifies for hedge accounting under GAAP. Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. Derivative assets and liabilities recorded in the consolidated balance sheets were each reduced by $2.4 billion and $1.9 billion as of March 31, 2015, and December 31, 2014, respectively, as a result of master netting arrangements and similar agreements in place. Derivative assets and liabilities recorded at March 31, 2015, also reflect reductions of $440.8 million and $1.1 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties, respectively. This compares with reductions of derivative assets and liabilities of $315.8 million and $1.2 billion, respectively, at December 31, 2014. Additional cash collateral received from and deposited with derivative counterparties totaling $29.4 million and $124.3 million, respectively, as of March 31, 2015, and $19.6 million and $153.2 million, respectively, as of December 31, 2014, were not offset against derivative assets and liabilities in the consolidated balance sheets as the amounts exceeded the net derivative positions with those counterparties. Northern Trust centrally clears eligible interest rate derivative instruments as required under Title VII of the Dodd-Frank Wall Street Reform and Consumer Protection Act. Securities posted as collateral for these transactions totaled $27.3 million and $27.4 million at March 31, 2015, and December 31, 2014, respectively, are not offset against derivative assets and liabilities in the consolidated balance sheets, and the counterparty receiving the securities as collateral does not have the right to repledge or sell the securities.
	Certain master netting arrangements Northern Trust enters into with derivative counterparties contain credit-risk-related contingent features in which the counterparty has the option to declare Northern Trust in default and accelerate cash settlement of net derivative liabilities with the counterparty in the event Northern Trust’s credit rating falls below specified levels. The aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position was $245.4 million and $299.5 million at March 31, 2015, and December 31, 2014, respectively. Cash collateral amounts deposited with derivative counterparties on those dates included $201.7 million and $272.9 million, respectively, posted against these liabilities, resulting in a net maximum amount of termination payments that could have been required at March 31, 2015, and December 31, 2014, of $43.7 million and $26.6 million, respectively. Accelerated settlement of these liabilities would not have a material effect on the consolidated financial position or liquidity of Northern Trust.
	Foreign exchange contracts are agreements to exchange specific amounts of currencies at a future date, at a specified rate of exchange. Foreign exchange contracts are entered into primarily to meet the foreign exchange needs of clients. Foreign exchange contracts are also used for trading purposes and risk management. For risk management purposes, Northern Trust uses foreign exchange contracts to reduce its exposure to changes in foreign exchange rates relating to certain forecasted non-functional-currency-denominated revenue and expenditure transactions, foreign-currency-denominated assets and liabilities and net investments in non-U.S. affiliates.
	Interest rate contracts include swap and option contracts. Interest rate swap contracts involve the exchange of fixed and floating rate interest payment obligations without the exchange of the underlying principal amounts. Northern Trust enters into interest rate swap contracts with its clients and offsetting contracts with outside counterparties. It also may utilize such contracts to reduce or eliminate the exposure to changes in the cash flows or fair value of hedged assets or liabilities due to changes in interest rates. Interest rate option contracts may include caps, floors, collars and swaptions, and provide for the transfer or reduction of interest rate risk, typically in exchange for a fee. Northern Trust enters into option contracts primarily as a seller of interest rate protection to clients. Northern Trust receives a fee at the outset of the agreement for the assumption of the risk of an unfavorable change in interest rates. This assumed interest rate risk is then mitigated by entering into an offsetting position with an outside counterparty. Northern Trust may also purchase or enter into option contracts for risk management purposes including to reduce the exposure to changes in the cash flows of hedged assets due to changes in interest rates.
	Client-Related and Trading Derivative Instruments. Approximately 97% of Northern Trust’s derivatives outstanding at March 31, 2015, and December 31, 2014, measured on a notional value basis, relate to client-related and trading activities. These activities consist principally of providing foreign exchange services to clients in connection with Northern Trust’s global custody business. However, in the normal course of business, Northern Trust also engages in trading of currencies for its own account.
	The following table shows the notional and fair values of client-related and trading derivative financial instruments. Notional amounts of derivative financial instruments do not represent credit risk, and are not recorded in the consolidated balance sheets. They are used merely to express the volume of this activity. Northern Trust’s credit-related risk of loss is limited to the positive fair value of the derivative instrument, which is significantly less than the notional amount.
	Table 51: Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments
		March 31, 2015											December 31, 2014
		Notional				Fair Value							Notional		Fair Value
	(In Millions)	Value				Asset				Liability			Value		Asset		Liability
	Foreign Exchange Contracts	$	270,476.90			$	4,731.20			$	4,686.40		$	257,568.70	$	4,149.50	$	4,072.00
	Interest Rate Contracts		5,662.20				123.1				119.3			5,353.80		105.5		101.3
	Total	$	276,139.10			$	4,854.30			$	4,805.70		$	262,922.50	$	4,255.00	$	4,173.30
	Changes in the fair value of client-related and trading derivative instruments are recognized currently in income. The following table shows the location and amount of gains and losses recorded in the consolidated statements of income for the three months ended March 31, 2015, and 2014.
	Table 52: Location and Amount of Gains and Losses Recorded in Income
		Location of Derivative Gain/(Loss)		Amount of Derivative
		Recognized in Income		Gain/(Loss) Recognized in Income
				Three Months Ended
				March 31,
	(In Millions)			2015				2014
	Foreign Exchange Contracts	Foreign Exchange Trading Income		$	71.6			$	50.1
	Interest Rate Contracts	Security Commissions and Trading Income			4.9				1.9
	Total			$	76.5			$	52
	Risk Management Instruments. Northern Trust uses derivative instruments to hedge its exposure to foreign currency, interest rate and credit risk. Certain hedging relationships are formally designated and qualify for hedge accounting under GAAP as fair value, cash flow or net investment hedges. Other derivatives that are entered into for risk management purposes as economic hedges are not formally designated as hedges and changes in fair value are recognized currently in other operating income.
	In order to qualify for hedge accounting, a formal assessment is performed on a calendar-quarter basis to verify that derivatives used in designated hedging transactions continue to be highly effective in offsetting the changes in fair value or cash flows of the hedged item. If a derivative ceases to be highly effective, matures, is sold or is terminated, or if a hedged forecasted transaction is no longer probable of occurring, hedge accounting is terminated and the derivative is treated as if it were a trading instrument.
	The following table identifies the types and classifications of derivative instruments formally designated as hedges under GAAP and used by Northern Trust to manage risk, their notional and fair values, and the respective risks addressed.
	Table 53: Notional and Fair Values of Designated Risk Management Derivative Financial Instruments
						March 31, 2015									December 31, 2014
	(In Millions)	Derivative		Risk		Notional				Fair Value					Notional		Fair Value
		Instrument		Classification		Value				Asset			Liability		Value		Asset		Liability
	Fair Value Hedges
	Available for Sale Investment Securities	Interest Rate Swap Contracts		Interest		$	2,815.70			$	6		$	26.9	$	2,859.50	$	12.7	$	28.5
				Rate
	Senior Notes and Long-Term Subordinated Debt	Interest Rate Swap Contracts		Interest			1,250.00				138.4			2		1,250.00		112.8		2
				Rate
	Cash Flow Hedges
	Forecasted Foreign Currency Denominated Transactions	Foreign Exchange Contracts		Foreign			340.5				8.8			24.4		344.9		6		14.8
				Currency
	Available for Sale Investment Securities	Interest Rate Swap Contracts		Interest			10				—			—		10		—		—
				Rate
	Available for Sale Investment Securities	Interest Rate Option Contracts		Interest			725				4.5			—		625		1.3		—
				Rate
	Net Investment Hedges
	Net Investments in Non-U.S. Affiliates	Foreign Exchange Contracts		Foreign			1,874.20				57.7			0.3		1,795.40		118.9		3.4
				Currency
	Total					$	7,015.40			$	215.4		$	53.6	$	6,884.80	$	251.7	$	48.7
	In addition to the above, Sterling denominated debt, totaling $243.9 million at December 31, 2014, was designated as a hedge of the foreign exchange associated with the net investment in certain non-U.S. affiliates. This debt matured during the quarter ended March 31, 2015.
	Derivatives are designated as fair value hedges to limit Northern Trust’s exposure to changes in the fair value of assets and liabilities due to movements in interest rates. For a fair value hedge, changes in the fair value of the derivative instrument and changes in the fair value of the hedged asset or liability attributable to the hedged risk are recorded currently in income. The following table shows the location and amount of derivative gains and losses recorded in the consolidated statements of income related to fair value hedges for the three months ended March 31, 2015, and 2014.
	Table 54: Location and Amount of Derivative Gains and Losses Recorded in Income
	(In Millions)	Derivative		Location of		Amount of Derivative
		Instrument		Derivative		Gain/(Loss)
				Gain/(Loss)		Recognized in Income
				Recognized in		Three Months Ended
				Income		March 31,
						2015				2014
	Available for Sale Investment Securities	Interest Rate Swap Contracts		Interest		$	(17.3	)		$	(12.1	)
				Income
	Senior Notes and Long-Term Subordinated Debt	Interest Rate Swap Contracts		Interest			29.8				32.5
				Expense
	Total					$	12.5			$	20.4
	Northern Trust applies the “shortcut” method of accounting, available under GAAP, to substantially all of its fair value hedges, which assumes there is no ineffectiveness in a hedge. As a result, changes recorded in the fair value of the hedged item are equal to the offsetting gain or loss on the derivative and are reflected in the same line item. For fair value hedges that do not qualify for the “shortcut” method of accounting, Northern Trust utilizes regression analysis, the “long-haul” method of accounting, in assessing whether the hedging relationships are highly effective at inception and quarterly thereafter. Ineffectiveness resulting from fair value hedges is recorded in either interest income or interest expense. There was no ineffectiveness or changes in the fair value of hedged items recognized in earnings for fair value hedges accounted for under the “long-haul” method of accounting during the three month periods ended March 31, 2015, and 2014.
Derivatives are also designated as cash flow hedges in order to minimize the variability in cash flows of earning assets or forecasted transactions caused by movements in interest or foreign exchange rates. The effective portion of changes in the fair value of such derivatives is recognized in AOCI, a component of stockholders’ equity, and there is no change in the accounting for the hedged item. Balances in AOCI are reclassified to earnings when the hedged forecasted transaction impacts earnings. Northern Trust applies the “shortcut” method of accounting for cash flow hedges of certain available for sale investment securities. For cash flow hedges of certain other available for sale investment securities and forecasted foreign currency denominated revenue and expenditure transactions, Northern Trust closely matches all terms of the hedged item and the hedging derivative at inception and on an ongoing basis which limits hedge ineffectiveness. For cash flow hedges of available for sale investment securities, to the extent all terms are not perfectly matched, effectiveness is assessed using regression analysis and any ineffectiveness is measured using the hypothetical derivative method. For cash flow hedges of forecasted foreign currency denominated revenue and expenditure transactions, to the extent all terms are not perfectly matched, effectiveness is assessed using the dollar-offset method and any ineffectiveness is measured using the hypothetical derivative method. There was no ineffectiveness recognized in earnings for cash flow hedges during the three-month periods ended March 31, 2015, and 2014. As of March 31, 2015, twenty-three months was the maximum length of time over which the exposure to variability in future cash flows of forecasted foreign-currency-denominated transactions was being hedged.
The following table provides cash flow hedge derivative gains and losses recognized in AOCI and the amounts reclassified to earnings during the three months ended March 31, 2015, and 2014.
Table 55: Cash Flow Hedge Derivative Gains and Losses Recognized in AOCI and Reclassified to Earnings
(In Millions)	Foreign Exchange								Interest Rate Option
	Contracts (Before Tax)								Contracts (Before Tax)
Three Months Ended March 31,	2015				2014				2015			2014
Net Gain/(Loss) Recognized in AOCI	$	(7.9	)		$	2.4			$	3.2		$	—
Net Gain/(Loss) Reclassified from AOCI to Net Income
Other Operating Income		(1.6	)			0.8				—			—
Interest Income		—				—				1.1			—
Other Operating Expense		(0.5	)			(0.2	)			—			—
Total	$	(2.1	)		$	0.6			$	1.1		$	—
During the three months ended March 31, 2015, and 2014, there were no transactions discontinued due to the original forecasted transactions no longer being probable of occurring. It is estimated that a net loss of $11.1 million will be reclassified into net income within the next twelve months relating to cash flow hedges of foreign-currency-denominated transactions. It is estimated that a net gain of $4.1 million will be reclassified into net income upon the receipt of interest payments on earning assets within the next twelve months relating to cash flow hedges of available for sale investment securities.
Certain foreign exchange contracts and qualifying nonderivative instruments are designated as net investment hedges to minimize Northern Trust’s exposure to variability in the foreign currency translation of net investments in non-U.S. branches and subsidiaries. The effective portion of changes in the fair value of the hedging instrument is recognized in AOCI consistent with the related translation gains and losses of the hedged net investment. For net investment hedges, all critical terms of the hedged item and the hedging instrument are matched at inception and on an ongoing basis to minimize the risk of hedge ineffectiveness. To the extent all terms are not perfectly matched, any ineffectiveness is measured using the hypothetical derivative method. Ineffectiveness resulting from net investment hedges is recorded in other operating income. There was no ineffectiveness recorded during the three months ended March 31, 2015, and 2014. Amounts recorded in AOCI are reclassified to net income only upon the sale or liquidation of an investment in a non-U.S. branch or subsidiary.
The following table provides net investment hedge gains and losses recognized in AOCI during the three months ended March 31, 2015, and 2014.
Table 56: Net Investment Hedge Gains and Losses Recognized in AOCI
	Hedging Gain/(Loss)
	Recognized in OCI (Before Tax)
	Three Months Ended March 31,
(In Millions)	2015				2014
Foreign Exchange Contracts	$	111.6			$	(9.0	)
Sterling Denominated Subordinated Debt		5				(1.6	)
Total	$	116.6			$	(10.6	)
Derivatives that are not formally designated as hedges under GAAP are entered into for risk management purposes. Foreign exchange contracts are entered into to manage the foreign currency risk of non-U.S.-dollar-denominated assets and liabilities, the net investment in certain non-U.S. affiliates, commercial loans and forecasted foreign-currency-denominated transactions. The following table identifies the types of risk management derivative instruments not formally designated as hedges and their notional amounts and fair values.
Table 57: Notional and Fair Values of Non-Designated Risk Management Derivative Instruments
(In Millions)	March 31, 2015											December 31, 2014
	Notional				Fair Value							Notional		Fair Value
	Value				Asset				Liability			Value		Asset		Liability
Foreign Exchange Contracts	$	257			$	0.9			$	3		$	246.3	$	0.8	$	5.3
Changes in the fair value of derivative instruments not formally designated as hedges are recognized currently in income. The following table provides the location and amount of gains and losses recorded in the consolidated statements of income for the three months ended March 31, 2015, and 2014.
Table 58: Location and Amount of Gains and Losses Recorded in Income for Non-Designated Risk Management Derivative Instruments
(In Millions)	Location of		Amount of Derivative
	Derivative Gain/		Gain/(Loss)
	(Loss) Recognized		Recognized in Income
	in Income		Three Months Ended
			March 31,
			2015				2014
Foreign Exchange Contracts	Other Operating Income			(8.7	)			(0.7	)

Offsetting_of_Assets_and_Liabi

Offsetting of Assets and Liabilities	3 Months Ended
	Mar. 31, 2015
Offsetting of Assets and Liabilities	20. Offsetting of Assets and Liabilities
	The following tables provide information regarding the offsetting of derivative assets and securities purchased under agreements to resell within the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
	Table 59: Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell
	March 31, 2015
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (3)
			Assets		Offset		Presented		Not Offset
	Derivative Assets (1)
	Foreign Exchange Contracts Over the Counter (OTC)		$	3,705.10	$	2,360.50	$	1,344.60	$	—	$	1,344.60
	Interest Rate Swaps OTC			197.9		31.8		166.1		—		166.1
	Interest Rate Swaps Exchange Cleared			74.1		25.7		48.4		—		48.4
	Cross Product Netting Adjustment			—		25.4		—		—		—
	Cross Product Collateral Adjustment			—		440.8		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,977.10		2,884.20		1,092.90		—		1,092.90
	Total Derivatives Not Subject to a Master Netting Arrangement			1,093.50		—		1,093.50		—		1,093.50
	Total Derivatives			5,070.60		2,884.20		2,186.40		—		2,186.40
	Securities Purchased under Agreements to Resell (2)		$	1,000.00	$	—	$	1,000.00	$	1,000.00	$	—
	December 31, 2014
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (3)
			Assets		Offset		Presented		Not Offset
	Derivative Assets (1)
	Foreign Exchange Contracts OTC		$	3,442.80	$	1,889.80	$	1,553.00	$	—	$	1,553.00
	Interest Rate Swaps OTC			183.9		32.1		151.8		—		151.8
	Interest Rate Swaps Exchange Cleared			48.4		13.1		35.3		—		35.3
	Cross Product Netting Adjustment			—		6.3		—		—		—
	Cross Product Collateral Adjustment			—		315.8		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,675.10		2,257.10		1,418.00		—		1,418.00
	Total Derivatives Not Subject to a Master Netting Arrangement			832.4		—		832.4		—		832.4
	Total Derivatives			4,507.50		2,257.10		2,250.40		—		2,250.40
	Securities Purchased under Agreements to Resell (2)		$	1,000.00	$	—	$	1,000.00	$	1,000.00	$	—
	-1	Derivative assets are reported in other assets in the consolidated balance sheets. Other assets (excluding derivative assets) totaled $3,177.3 million and $3,614.7 million as of March 31, 2015, and December 31, 2014, respectively.
	-2	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheets. Federal funds sold totaled $113.9 million and $62.7 million as of March 31, 2015, and December 31, 2014, respectively.
	-3	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
	The following tables provide information regarding the offsetting of derivative liabilities and securities sold under agreements to repurchase within the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
	Table 60: Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase
	March 31, 2015
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (2)
			Liabilities		Offset		Presented		Not
									Offset
	Derivative Liabilities (1)
	Foreign Exchange Contracts OTC		$	3,895.10	$	2,360.50	$	1,534.60	$	—	$	1,534.60
	Interest Rate Swaps OTC			122.5		31.8		90.7		—		90.7
	Interest Rate Swaps Exchange Cleared			25.7		25.7		—		—		—
	Cross Product Netting Adjustment			—		25.4		—		—		—
	Cross Product Collateral Adjustment			—		1,067.50		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			4,043.30		3,510.90		532.4		—		532.4
	Total Derivatives Not Subject to a Master Netting Arrangement			819		—		819		—		819
	Total Derivatives			4,862.30		3,510.90		1,351.40		—		1,351.40
	Securities Sold under Agreements to Repurchase		$	613.2	$	—	$	613.2	$	613.2	$	—
	December 31, 2014
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (2)
			Liabilities		Offset		Presented		Not
									Offset
	Derivative Liabilities (1)
	Foreign Exchange Contracts OTC		$	3,431.00	$	1,889.80	$	1,541.20	$	—	$	1,541.20
	Interest Rate Swaps OTC			118.7		32.1		86.6		—		86.6
	Interest Rate Swaps Exchange Cleared			13.1		13.1		—		—		—
	Cross Product Netting Adjustment			—		6.3		—		—		—
	Cross Product Collateral Adjustment			—		1,232.00		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,562.80		3,173.30		389.5		—		389.5
	Total Derivatives Not Subject to a Master Netting Arrangement			664.5		—		664.5		—		664.5
Total Derivatives			4,227.30		3,173.30		1,054.00		—		1,054.00
Securities Sold under Agreements to Repurchase		$	885.1	$	—	$	885.1	$	885.1	$	—
-1	Derivative liabilities are reported in other liabilities in the consolidated balance sheets. Other liabilities (excluding derivative liabilities) totaled $3,185.1 million and $2,794.1 million as of March 31, 2015, and December 31, 2014, respectively.
-2	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
All of Northern Trust’s securities sold under agreements to repurchase (repurchase agreements) and securities purchased under agreements to resell (reverse repurchase agreements) involve the transfer of financial assets in exchange for cash subject to a right and obligation to repurchase those assets for an agreed upon amount. In the event of a repurchase failure, the cash or financial assets are available for offset. All of Northern Trust’s repurchase agreements and reverse repurchase agreements are subject to a master netting arrangement, which sets forth the rights and obligations for repurchase and offset. Under the master netting arrangement, Northern Trust is entitled to set off receivables from and collateral placed with a single counterparty against obligations owed to that counterparty. In addition, collateral held by Northern Trust can be offset against receivables from that counterparty.
Derivative asset and liability positions with a single counterparty can be offset against each other in cases where legally enforceable master netting arrangements or similar agreements exist. Derivative assets and liabilities can be further offset by cash collateral received from, and deposited with, the transacting counterparty. The basis for this view is that, upon termination of transactions subject to a master netting arrangement or similar agreement, the individual derivative receivables do not represent resources to which general creditors have rights and individual derivative payables do not represent claims that are equivalent to the claims of general creditors. Northern Trust centrally clears those interest rate derivative instruments addressed under Title VII of the Dodd-Frank Wall Street Reform and Consumer Protection Act. These transactions are subject to an agreement similar to a master netting arrangement which has the same rights of offset as described above.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level	The following tables present assets and liabilities measured at fair value on a recurring basis as of March 31, 2015, and December 31, 2014, segregated by fair value hierarchy level.
	Table 22: Recurring Basis Hierarchy Leveling
	(In Millions)		Level 1			Level 2			Level 3		Netting			Assets/Liabilities
														at Fair Value
	March 31, 2015
	Securities
	Available for Sale
	U.S. Government		$	4,627.40		$	—		$	—	$	—		$	4,627.40
	Obligations of States and Political Subdivisions			—			4.5			—		—			4.5
	Government Sponsored Agency			—			16,634.20			—		—			16,634.20
	Non-U.S. Government			—			311.4			—		—			311.4
	Corporate Debt			—			3,651.30			—		—			3,651.30
	Covered Bonds			—			2,049.30			—		—			2,049.30
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			—			561			—		—			561
	Residential Mortgage-Backed			—			4.4			—		—			4.4
	Other Asset-Backed			—			2,949.30			—		—			2,949.30
	Auction Rate			—			—			17.5		—			17.5
	Other			—			135.4			—		—			135.4
	Total Available for Sale			4,627.40			26,300.80			17.5		—			30,945.70
	Trading Account			—			1.4			—		—			1.4
	Total Available for Sale and Trading Securities			4,627.40			26,302.20			17.5		—			30,947.10
	Other Assets
	Derivative Assets
	Foreign Exchange Contracts			—			4,798.60			—		—			4,798.60
	Interest Rate Contracts			—			272			—		—			272
	Total Derivative Assets			—			5,070.60			—		(2,884.2	)		2,186.40
	Other Liabilities
	Derivative Liabilities
	Foreign Exchange Contracts			—			4,714.10			—		—			4,714.10
	Interest Rate Swaps			—			148.2			—		—			148.2
	Total Derivative Liabilities		$	—		$	4,862.30		$	—	$	(3,510.9	)	$	1,351.40
	Note: Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of March 31, 2015, derivative assets and liabilities shown above also include reductions of $440.8 million and $1.1 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.
	(In Millions)		Level 1			Level 2			Level 3		Netting			Assets/Liabilities
														at Fair Value
	December 31, 2014
	Securities
	Available for Sale
	U.S. Government		$	4,506.90		$	—		$	—	$	—		$	4,506.90
	Obligations of States and Political Subdivisions			—			4.6			—		—			4.6
	Government Sponsored Agency			—			16,389.20			—		—			16,389.20
	Non-U.S. Government			—			310.4			—		—			310.4
	Corporate Debt			—			3,577.70			—		—			3,577.70
	Covered Bonds			—			1,907.50			—		—			1,907.50
	Supranational and Non-U.S. Agency Bonds			—			360.6			—		—			360.6
	Residential Mortgage-Backed			—			6.4			—		—			6.4
	Other Asset-Backed			—			2,321.30			—		—			2,321.30
	Auction Rate			—			—			18.1		—			18.1
	Other			—			155.8			—		—			155.8
	Total Available for Sale			4,506.90			25,033.50			18.1		—			29,558.50
	Trading Account			—			4.7			—		—			4.7
	Total Available for Sale and Trading Securities			4,506.90			25,038.20			18.1		—			29,563.20
	Other Assets
	Derivative Assets
	Foreign Exchange Contracts			—			4,275.20			—		—			4,275.20
	Interest Rate Contracts			—			232.3			—		—			232.3
	Total Derivative Assets			—			4,507.50			—		(2,257.1	)		2,250.40
	Other Liabilities
	Derivative Liabilities
	Foreign Exchange Contracts			—			4,095.50			—		—			4,095.50
	Interest Rate Swaps			—			131.8			—		—			131.8
	Total Derivative Liabilities		$	—		$	4,227.30		$	—	$	(3,173.3	)	$	1,054.00
	Note: Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of December 31, 2014, derivative assets and liabilities shown above also include reductions of $315.8 million and $1.2 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.
Changes in Level 3 Assets	The following tables present the changes in Level 3 assets and liabilities for the three months ended March 31, 2015, and 2014.
	Table 23: Changes in Level 3 Assets
	Level 3 Assets (In Millions)		Auction Rate Securities
	Three Months Ended March 31,		2015			2014
	Fair Value at January 1		$	18.1		$	98.9
	Total Gains (Losses):
	Included in Earnings			—			—
	Included in Other Comprehensive Income (1)			(0.3	)		(0.2	)
	Purchases, Issues, Sales, and Settlements
	Sales			—			0.1
	Settlements			(0.3	)		(0.3	)
	Fair Value at March 31		$	17.5		$	98.5
	-1	Unrealized gains (losses) are included in net unrealized gains (losses) on securities available for sale in the consolidated statements of comprehensive income.
Changes in Level 3 Liabilities	Table 24: Changes in Level 3 Liabilities
	Level 3 Liabilities (In Millions)		Contingent Consideration
	Three Months Ended March 31		2015			2014
	Fair Value at January 1		$	—		$	55.4
	Total (Gains) Losses:
	Included in Earnings (1)			—			(0.1	)
	Included in Other Comprehensive Income			—			—
	Purchases, Issues, Sales, and Settlements			—			—
	Purchases			—			—
	Settlements			—			(55.3	)
	Fair Value at March 31		$	—		$	—
	Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held at March 31 (1)		$	—		$	—
	-1	(Gains) losses are recorded in other operating income (expense) in the consolidated statements of income.
Book and Fair Values of All Financial Instruments	The following tables summarize the fair values of all financial instruments.
	Table 26: Fair Value of Financial Instruments
	(In Millions)		March 31, 2015
			Book			Total			Fair Value
			Value			Fair Value			Level 1		Level 2			Level 3
	Assets
	Cash and Due from Banks		$	4,074.80		$	4,074.80		$	4,074.80	$	—		$	—
	Federal Funds Sold and Resell Agreements			1,113.90			1,113.90			—		1,113.90			—
	Interest-Bearing Deposits with Banks			15,432.30			15,432.30			—		15,432.30			—
	Federal Reserve Deposits			8,804.20			8,804.20			—		8,804.20			—
	Securities
	Available for Sale (1)			30,945.70			30,945.70			4,627.40		26,300.80			17.5
	Held to Maturity			5,655.80			5,665.00			—		5,665.00			—
	Trading Account			1.4			1.4			—		1.4			—
	Loans (excluding Leases)
	Held for Investment			31,503.80			31,747.70			—		—			31,747.70
	Client Security Settlement Receivables			2,219.30			2,219.30			—		2,219.30			—
	Other Assets
	Federal Reserve and Federal Home Loan Bank Stock			207.5			207.5			—		207.5			—
	Community Development Investments			198.4			198.7			—		198.7			—
	Employee Benefit and Deferred Compensation			162.1			165.5			113.9		51.6			—
	Liabilities
	Deposits
	Demand, Noninterest-Bearing, Savings and Money Market		$	39,016.40		$	39,016.40		$	39,016.40	$	—		$	—
	Savings Certificates and Other Time			1,777.90			1,783.10			—		1,783.10			—
	Non U.S. Offices Interest-Bearing			45,942.20			45,942.20			—		45,942.20			—
	Federal Funds Purchased			410.7			410.7			—		410.7			—
	Securities Sold under Agreements to Repurchase			613.2			613.2			—		613.2			—
	Other Borrowings			2,874.10			2,875.70			—		2,875.70			—
	Senior Notes			1,497.10			1,566.90			—		1,566.90			—
	Long Term Debt (excluding Leases)
	Subordinated Debt			1,368.60			1,374.90			—		1,374.90			—
	Floating Rate Capital Debt			277.2			241.4			—		241.4			—
	Other Liabilities
	Standby Letters of Credit			58.3			58.3			—		—			58.3
	Loan Commitments			26.8			26.8			—		—			26.8
	Derivative Instruments
	Asset/Liability Management
	Foreign Exchange Contracts
	Assets		$	67.4		$	67.4		$	—	$	67.4		$	—
	Liabilities			27.7			27.7			—		27.7			—
	Interest Rate Contracts
	Assets			148.9			148.9			—		148.9			—
	Liabilities			28.9			28.9			—		28.9			—
	Client-Related and Trading
	Foreign Exchange Contracts
	Assets			4,731.20			4,731.20			—		4,731.20			—
	Liabilities			4,686.40			4,686.40			—		4,686.40			—
	Interest Rate Contracts
	Assets			123.1			123.1			—		123.1			—
	Liabilities			119.3			119.3			—		119.3			—
	-1	Refer to the table located on page 31 for the disaggregation of available for sale securities.
	(In Millions)		December 31, 2014
			Book			Total			Fair Value
			Value			Fair Value			Level 1		Level 2			Level 3
	Assets
	Cash and Due from Banks		$	3,050.60		$	3,050.60		$	3,050.60	$	—		$	—
	Federal Funds Sold and Resell Agreements			1,062.70			1,062.70			—		1,062.70			—
	Interest-Bearing Deposits with Banks			14,928.30			14,928.30			—		14,928.30			—
	Federal Reserve Deposits			17,386.30			17,386.30			—		17,386.30			—
	Securities
	Available for Sale (1)			29,558.50			29,558.50			4,506.90		25,033.50			18.1
	Held to Maturity			4,170.80			4,176.10			—		4,176.10			—
	Trading Account			4.7			4.7			—		4.7			—
Loans (excluding Leases)
Held for Investment			30,458.00			30,600.40			—		—			30,600.40
Held for Sale			2.5			2.5			—		—			2.5
Client Security Settlement Receivables			1,568.80			1,568.80			—		1,568.80			—
Other Assets
Federal Reserve and Federal Home Loan Bank Stock			207.5			207.5			—		207.5			—
Community Development Investments			209.9			210.8			—		210.8			—
Employee Benefit and Deferred Compensation			143.2			146.7			96.7		50			—
Liabilities
Deposits
Demand, Noninterest-Bearing, Savings and Money Market		$	41,454.60		$	41,454.60		$	41,454.60	$	—		$	—
Savings Certificates and Other Time			1,757.40			1,757.40			—		1,757.40			—
Non U.S. Offices Interest-Bearing			47,545.00			47,545.00			—		47,545.00			—
Federal Funds Purchased			932.9			932.9			—		932.9			—
Securities Sold under Agreements to Repurchase			885.1			885.1			—		885.1			—
Other Borrowings			1,685.20			1,686.20			—		1,686.20			—
Senior Notes			1,497.00			1,541.80			—		1,541.80			—
Long Term Debt (excluding Leases)
Subordinated Debt			1,583.30			1,583.30			—		1,583.30			—
Floating Rate Capital Debt			277.1			242.8			—		242.8			—
Other Liabilities
Standby Letters of Credit			60.1			60.1			—		—			60.1
Loan Commitments			28.3			28.3			—		—			28.3
Derivative Instruments
Asset/Liability Management
Foreign Exchange Contracts
Assets		$	125.7		$	125.7		$	—	$	125.7		$	—
Liabilities			23.5			23.5			—		23.5			—
Interest Rate Contracts
Assets			126.8			126.8			—		126.8			—
Liabilities			30.5			30.5			—		30.5			—
Client-Related and Trading
Foreign Exchange Contracts
Assets			4,149.50			4,149.50			—		4,149.50			—
Liabilities			4,072.00			4,072.00			—		4,072.00			—
Interest Rate Contracts
Assets			105.5			105.5			—		105.5			—
Liabilities			101.3			101.3			—		101.3			—
-1	Refer to the table located on page 32 for the disaggregation of available for sale securities.
Fair Value, Measurements, Recurring
Valuation Techniques, Significant Unobservable Inputs, and Quantitative Information	The following presents the fair values of, and the valuation techniques, significant unobservable inputs and quantitative information used to develop significant unobservable inputs for, Northern Trust’s Level 3 assets as of March 31, 2015.
	Table 21: Level 3 Significant Unobservable Inputs
	Financial Instrument		Fair Value			Valuation		Unobservable Inputs	Range of Lives
						Technique			and Rates
	Auction Rate Securities		$	17.5 million		Discounted Cash		Remaining lives	1.2 — 8.6 years
						Flow		Discount rates	0.1% — 8.4%
Fair Value, Measurements, Nonrecurring
Valuation Techniques, Significant Unobservable Inputs, and Quantitative Information	The following table provides the fair value of, and the valuation technique, significant unobservable inputs and quantitative information used to develop the significant unobservable inputs for, Northern Trust’s Level 3 assets that were measured at fair value on a nonrecurring basis as of March 31, 2015.
	Table 25: Level 3 Nonrecurring Basis Significant Unobservable Inputs
	Financial Instrument		Fair Value		Valuation	Unobservable Input		Range of Discounts
					Technique			Applied
	Loans		$23.1 million		Market Approach	Discount to reflect realizable value		15% — 25%
	OREO		$0.3 million		Market Approach	Discount to reflect realizable value		15% — 20%

Securities_Tables

Securities (Tables)	3 Months Ended
	Mar. 31, 2015
Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale	The following tables provide the amortized cost and fair values of securities at March 31, 2015, and December 31, 2014.
	Table 27: Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale
	Securities Available for Sale		March 31, 2015
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	U.S. Government		$	4,593.90	$	33.5		$	—	$	4,627.40
	Obligations of States and Political Subdivisions			4.5		—			—		4.5
	Government Sponsored Agency			16,536.90		107.6			10.3		16,634.20
	Non-U.S. Government			310.5		0.9			—		311.4
	Corporate Debt			3,661.50		6.7			16.9		3,651.30
	Covered Bonds			2,040.50		8.8			—		2,049.30
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			560		1.4			0.4		561
	Residential Mortgage-Backed			4.9		—			0.5		4.4
	Other Asset-Backed			2,948.70		1.1			0.5		2,949.30
	Auction Rate			18.1		0.5			1.1		17.5
	Other			135.1		0.3			—		135.4
	Total		$	30,814.60	$	160.8		$	29.7	$	30,945.70
	Securities Available for Sale		December 31, 2014
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	U.S. Government		$	4,493.50	$	15.1		$	1.7	$	4,506.90
	Obligations of States and Political Subdivisions			4.5		0.1			—		4.6
	Government Sponsored Agency			16,326.40		82.3			19.5		16,389.20
	Non-U.S. Government			309.5		0.9			—		310.4
	Corporate Debt			3,617.50		1.8			41.6		3,577.70
	Covered Bonds			1,899.90		7.9			0.3		1,907.50
	Supranational and Non-U.S. Agency Bonds			360		1.5			0.9		360.6
	Residential Mortgage-Backed			6.9		—			0.5		6.4
	Other Asset-Backed			2,321.80		0.5			1		2,321.30
	Auction Rate			18.4		0.5			0.8		18.1
	Other			155.7		0.3			0.2		155.8
	Total		$	29,514.10	$	110.9		$	66.5	$	29,558.50
Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity	Table 28: Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity
	Securities Held to Maturity		March 31, 2015
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	Obligations of States and Political Subdivisions		$	112.6	$	7		$	—	$	119.6
	Government Sponsored Agency			15.5		1			—		16.5
	Non-U.S. Government			1,104.30		8			—		1,112.30
	Certificates of Deposit			2,575.70		0.2			0.3		2,575.60
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			1,769.70		12.5			0.2		1,782.00
	Other			78		—			19		59
	Total		$	5,655.80	$	28.7		$	19.5	$	5,665.00
	Securities Held to Maturity		December 31, 2014
			Amortized		Gross Unrealized					Fair
	(In Millions)		Cost		Gains			Losses		Value
	Obligations of States and Political Subdivisions		$	121.9	$	7.4		$	—	$	129.3
	Government Sponsored Agency			18.4		1.1			—		19.5
	Non-U.S. Government			1,281.60		6.6			0.4		1,287.80
	Certificates of Deposit			924.3		0.1			0.1		924.3
	Supranational and Non-U.S. Agency Bonds			1,745.80		10.9			0.5		1,756.20
	Other			78.8		0.3			20.1		59
	Total		$	4,170.80	$	26.4		$	21.1	$	4,176.10
Securities Continuous Unrealized Loss Position	The following tables provide information regarding securities that had been in a continuous unrealized loss position for less than 12 months and for 12 months or longer as of March 31, 2015, and December 31, 2014.
	Table 30: Securities with Unrealized Losses
	Securities with Unrealized Losses as of		Less than 12 Months					12 Months or Longer				Total
	March 31, 2015
	(In Millions)		Fair Value		Unrealized			Fair Value		Unrealized		Fair Value		Unrealized
					Losses					Losses				Losses
	Government Sponsored Agency		$	1,946.00	$	3.9		$	1,091.70	$	6.4	$	3,037.70	$	10.3
	Corporate Debt			391.4		0.6			1,217.80		16.3		1,609.20		16.9
	Sub-Sovereign, Supranational and Non-U.S. Agency Bonds			149.8		0.2			170.7		0.4		320.5		0.6
	Residential Mortgage-Backed			—		—			4.4		0.5		4.4		0.5
	Other Asset-Backed			1,071.10		0.5			—		—		1,071.10		0.5
	Certificates of Deposit			1,395.30		0.3			—		—		1,395.30		0.3
	Auction Rate			1.4		0.2			5.5		0.9		6.9		1.1
	Other			17.7		10.8			48.3		8.2		66		19
	Total		$	4,972.70	$	16.5		$	2,538.40	$	32.7	$	7,511.10	$	49.2
	Securities with Unrealized Losses as of		Less than 12 Months					12 Months or Longer				Total
	December 31, 2014
	(In Millions)		Fair Value		Unrealized			Fair Value		Unrealized		Fair Value		Unrealized
					Losses					Losses				Losses
	U.S. Government		$	998.2	$	1.7		$	—	$	—	$	998.2	$	1.7
	Government Sponsored Agency			2,344.90		6.6			1,730.00		12.9		4,074.90		19.5
	Non-U.S. Government			292.9		0.4			—		—		292.9		0.4
	Corporate Debt			1,244.50		3.9			1,338.80		37.7		2,583.30		41.6
	Covered Bonds			142.3		0.2			10		0.1		152.3		0.3
	Supranational and Non-U.S. Agency Bonds			313.2		0.3			175.5		1.1		488.7		1.4
	Residential Mortgage-Backed			—		—			4.5		0.5		4.5		0.5
	Other Asset-Backed			1,297.60		1			—		—		1,297.60		1
	Certificates of Deposit			438.6		0.1			—		—		438.6		0.1
	Auction Rate			2.4		0.2			4.7		0.6		7.1		0.8
	Other			27.1		12.1			45.6		8.2		72.7		20.3
	Total		$	7,101.70	$	26.5		$	3,309.10	$	61.1	$	10,410.80	$	87.6
Credit Related Impairment Losses Recognized in Earnings on Other Than Temporarily Impaired Securities	The table below provides information regarding total other-than-temporarily-impaired securities, including noncredit-related amounts recognized in other comprehensive income and net impairment losses recognized in earnings, for the three months ended March 31, 2015, and 2014.
	Table 31: Net Impairment Losses Recognized in Earnings
			Three Months Ended
			March 31,
	(In Millions)		2015		2014
	Changes in OTTI Losses*		$	—	$	(4.6	)
	Noncredit-related Losses Recorded in / (Reclassified from) OCI**			—		0.7
	Net Impairment Losses Recognized in Earnings		$	—	$	(3.9	)
	*	For initial OTTI in the applicable period, the balance includes the excess of the amortized cost over the fair value of the impaired securities. For subsequent impairments of the same security, the balance includes any additional changes in fair value of the security subsequent to its most recently recorded OTTI.
	**	For initial OTTI in the applicable period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in other comprehensive income (OCI). For subsequent impairments of the same security, the balance includes additional changes in OCI for that security subsequent to its most recently recorded OTTI.
Cumulative Credit-Related Losses Recognized in Earnings on Debt Securities Other-Than-Temporarily Impaired	Provided in the table below are the cumulative credit-related losses recognized in earnings on debt securities other-than-temporarily impaired.
	Table 32: Cumulative Credit-Related Losses on Securities
			Three Months Ended
			March 31,
	(In Millions)		2015		2014
	Cumulative Credit-Related Losses on Securities Held — Beginning of Period		$	5.2	$	8.8
	Plus: Losses on Newly Identified Impairments			—		1.8
	Additional Losses on Previously Identified Impairments			—		2.1
	Less: Current and Prior Period Losses on Securities Sold During the Period			—		—
	Cumulative Credit-Related Losses on Securities Held — End of Period		$	5.2	$	12.7
Available for Sale Investment Securities
Remaining Maturity of Securities	Remaining Maturity of Securities Available for Sale
	March 31, 2015
	(In Millions)		Amortized		Fair
			Cost		Value
	Available for Sale
	Due in One Year or Less		$	7,702.60	$	7,731.30
	Due After One Year Through Five Years			18,210.80		18,295.10
	Due After Five Years Through Ten Years			3,650.70		3,659.60
	Due After Ten Years			1,250.50		1,259.70
	Total			30,814.60		30,945.70
	Note: Mortgage-backed and asset-backed securities are included in the above table taking into account anticipated future prepayments.
Held-to-maturity Securities
Remaining Maturity of Securities	Remaining Maturity of Securities Held to Maturity
	Held to Maturity
	Due in One Year or Less			3,219.30		3,220.70
	Due After One Year Through Five Years			2,374.50		2,397.50
	Due After Five Years Through Ten Years			21.4		19.4
	Due After Ten Years			40.6		27.4
	Total		$	5,655.80	$	5,665.00
	Note: Mortgage-backed and asset-backed securities are included in the above table taking into account anticipated future prepayments.

Loans_and_Leases_Tables

Loans and Leases (Tables)	3 Months Ended
	Mar. 31, 2015
Amounts Outstanding for Loans and Leases by Segment and Class	Amounts outstanding for loans and leases, by segment and class, are shown below.
	Table 33: Loans and Leases
	(In Millions)	March 31,			December 31,
		2015			2014
	Commercial
	Commercial and Institutional	$	9,030.20		$	8,381.90
	Commercial Real Estate		3,500.90			3,333.30
	Lease Financing, net		870.6			916.3
	Non-U.S.		1,661.60			1,530.60
	Other		428			191.5
	Total Commercial		15,491.30			14,353.60
	Personal
	Residential Real Estate		9,440.20			9,782.60
	Private Client		7,615.80			7,466.90
	Other		82.9			37.1
	Total Personal		17,138.90			17,286.60
	Total Loans and Leases		32,630.20			31,640.20
	Allowance for Credit Losses Assigned to Loans and Leases		(259.0	)		(267.0	)
	Net Loans and Leases	$	32,371.20		$	31,373.20
Loan and Lease Segment and Class Balances Segregated by Borrower Ratings into "1 to 3", "4 to 5" and "6 to 9" (Watch List) Categories	Loan and lease segment and class balances as of March 31, 2015, and December 31, 2014, are provided below, segregated by borrower ratings into “1 to 3,” “4 to 5” and “6 to 9” (watch list), categories.
	Table 34: Borrower Ratings
		March 31, 2015										December 31, 2014
	(In Millions)	1 to 3			4 to 5			6 to 9		Total		1 to 3		4 to 5		6 to 9		Total
		Category			Category			Category				Category		Category		Category
								(Watch List)								(Watch List)
	Commercial
	Commercial and Institutional	$	5,628.30		$	3,314.90		$	87	$	9,030.20	$	5,340.90	$	2,947.30	$	93.7	$	8,381.90
	Commercial Real Estate		1,452.50			1,949.90			98.5		3,500.90		1,371.70		1,861.80		99.8		3,333.30
	Lease Financing, net		510.6			324			36		870.6		552.5		360.3		3.5		916.3
	Non-U.S.		513.8			1,147.60			0.2		1,661.60		636.8		892.9		0.9		1,530.60
	Other		353			75			—		428		108.1		83.4		—		191.5
	Total Commercial		8,458.20			6,811.40			221.7		15,491.30		8,010.00		6,145.70		197.9		14,353.60
	Personal
	Residential Real Estate		2,734.20			6,304.90			401.1		9,440.20		3,148.00		6,207.00		427.6		9,782.60
	Private Client		4,907.10			2,699.40			9.3		7,615.80		5,143.80		2,311.70		11.4		7,466.90
	Other		68.5			14.4			—		82.9		21.1		16		—		37.1
	Total Personal		7,709.80			9,018.70			410.4		17,138.90		8,312.90		8,534.70		439		17,286.60
	Total Loans and Leases	$	16,168.00		$	15,830.10		$	632.1	$	32,630.20	$	16,322.90	$	14,680.40	$	636.9	$	31,640.20
Balances and Delinquency Status of Performing and Nonperforming Loans and Leases by Segment and Class as well as Total OREO and Nonperforming Asset Balances	The following tables provide balances and delinquency status of performing and nonperforming loans and leases by segment and class, as well as the total OREO and nonperforming asset balances, as of March 31, 2015, and December 31, 2014.
	Table 35: Delinquency Status
	March 31, 2015
	(In Millions)	Current			30-59 Days			60-89 Days		90 Days or		Total		Nonperforming		Total Loans
					Past Due			Past Due		More Past		Performing				and Leases
										Due
	Commercial
	Commercial and Institutional	$	8,980.40		$	21.2		$	4.6	$	3.1	$	9,009.30	$	20.9	$	9,030.20
	Commercial Real Estate		3,442.90			10.2			4.8		3.1		3,461.00		39.9		3,500.90
	Lease Financing, net		870.6			—			—		—		870.6		—		870.6
	Non-U.S.		1,661.60			—			—		—		1,661.60		—		1,661.60
	Other		428			—			—		—		428		—		428
	Total Commercial		15,383.50			31.4			9.4		6.2		15,430.50		60.8		15,491.30
	Personal
	Residential Real Estate		9,240.20			37.5			2.6		2.2		9,282.50		157.7		9,440.20
	Private Client		7,573.70			32.8			7.2		1		7,614.70		1.1		7,615.80
	Other		82.9			—			—		—		82.9		—		82.9
	Total Personal		16,896.80			70.3			9.8		3.2		16,980.10		158.8		17,138.90
	Total Loans and Leases	$	32,280.30		$	101.7		$	19.2	$	9.4	$	32,410.60	$	219.6	$	32,630.20
											Other Real Estate Owned				8.5
											Total Nonperforming Assets			$	228.1
	December 31, 2014
	(In Millions)	Current			30-59 Days			60-89 Days		90 Days or		Total		Nonperforming		Total Loans
					Past Due			Past Due		More Past		Performing				and Leases
										Due
	Commercial
	Commercial and Institutional	$	8,340.50		$	14.5		$	4	$	7.9	$	8,366.90	$	15	$	8,381.90
	Commercial Real Estate		3,274.30			9.6			9.8		2.5		3,296.20		37.1		3,333.30
	Lease Financing, net		916.3			—			—		—		916.3		—		916.3
	Non-U.S.		1,530.60			—			—		—		1,530.60		—		1,530.60
	Other		191.5			—			—		—		191.5		—		191.5
	Total Commercial		14,253.20			24.1			13.8		10.4		14,301.50		52.1		14,353.60
	Personal
	Residential Real Estate		9,556.30			49.5			9.9		4.5		9,620.20		162.4		9,782.60
	Private Client		7,396.00			56			5.9		7.8		7,465.70		1.2		7,466.90
	Other		37.1			—			—		—		37.1		—		37.1
	Total Personal		16,989.40			105.5			15.8		12.3		17,123.00		163.6		17,286.60
	Total Loans and Leases	$	31,242.60		$	129.6		$	29.6	$	22.7	$	31,424.50	$	215.7	$	31,640.20
											Other Real Estate Owned				16.6
											Total Nonperforming Assets			$	232.3
Impaired Loans By Segment and Class	The following tables provide information related to impaired loans by segment and class.
	Table 36: Information about Impaired Loans as of the Period End
		As of March 31, 2015								As of December 31, 2014
	(In Millions)	Recorded			Unpaid			Specific		Recorded		Unpaid		Specific
		Investment			Principal			Allowance		Investment		Principal		Allowance
					Balance							Balance
	With No Related Specific Allowance
	Commercial and Institutional	$	7.9		$	9.9		$	—	$	9	$	12	$	—
	Commercial Real Estate		49.1			54.1			—		47		52.4		—
	Lease Financing, net		3.4			3.4			—		4.2		4.2		—
	Residential Real Estate		153.1			198.7			—		160.9		204.8		—
	Private Client		0.2			0.2			—		0.2		0.5		—
	With a Related Specific Allowance
	Commercial and Institutional		12.5			9.9			4		6.5		6.6		2.9
	Commercial Real Estate		12.4			18.7			2.8		12.2		18.3		2.9
	Residential Real Estate		5.8			6			1.2		1.4		1.4		0.4
	Private Client		0.8			0.8			0.4		0.8		0.8		0.4
	Total
	Commercial		85.3			96			6.8		78.9		93.5		5.8
	Personal		159.9			205.7			1.6		163.3		207.5		0.8
	Total	$	245.2		$	301.7		$	8.4	$	242.2	$	301	$	6.6
	Table 37: Information about Impaired Loans for the Period
		Three Months Ended March 31,
		2015						2014
	(In Millions)	Average			Interest			Average		Interest
		Recorded			Income			Recorded		Income
		Investment			Recognized			Investment		Recognized
	With No Related Specific Allowance
	Commercial and Institutional	$	9.2		$	—		$	11.4	$	—
	Commercial Real Estate		48.5			0.2			44.8		0.3
	Lease Financing, net		3.4			0.1			4.4		0.1
	Residential Real Estate		161.3			0.3			189.2		0.6
	Private Client		0.2			—			0.7		—
	With a Related Specific Allowance
	Commercial and Institutional		8.5			—			10		—
	Commercial Real Estate		12.2			—			26.4		—
	Residential Real Estate		2.8			—			7.1		—
	Private Client		0.8			—			0.1		—
	Total
	Commercial		81.8			0.3			97		0.4
	Personal		165.1			0.3			197.1		0.6
	Total	$	246.9		$	0.6		$	294.1	$	1
Number of Loans and Leases Modified in TDRs and Total Recorded Investments and Unpaid Principal Balances	The following tables provide, by segment and class, the number of loans and leases modified in TDRs during the three-month periods ended March 31, 2015, and 2014, and the recorded investments and unpaid principal balances as of March 31, 2015, and 2014.
	Table 38: Troubled Debt Restructurings
	($ In Millions)	Three Months Ended
		March 31, 2015
		Number of			Recorded			Unpaid Principal
		Loans and Leases			Investment			Balance
	Commercial
	Commercial and Institutional		1		$	0.1		$	0.1
	Commercial Real Estate		1			0.7			0.7
	Total Commercial		2			0.8			0.8
	Personal
	Residential Real Estate		57			7.1			10.2
	Total Personal		57			7.1			10.2
	Total Loans and Leases		59		$	7.9		$	11
	Note: Period-end balances reflect all paydowns and charge-offs during the period.
	($ In Millions)	Three Months Ended
		March 31, 2014
		Number of			Recorded			Unpaid Principal
		Loans and Leases			Investment			Balance
	Commercial
	Commercial Real Estate		1		$	0.7		$	0.7
	Total Commercial		1			0.7			0.7
	Personal
	Residential Real Estate		34			3.5			3.8
	Private Client		1			—			—
	Total Personal		35			3.5			3.8
	Total Loans and Leases		36		$	4.2		$	4.5
	Note: Period-end balances reflect all paydowns and charge-offs during the period.

Allowance_for_Credit_Losses_Ta

Allowance for Credit Losses (Tables)	3 Months Ended
	Mar. 31, 2015
Changes in Allowance for Credit Losses by Segment	The following tables provide information regarding changes in the total allowance for credit losses by segment during the three months ended March 31, 2015, and 2014.
	Table 39: Changes in the Allowance for Credit Losses
		2015									2014
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Balance at Beginning of Period	$	169.7		$	126.2		$	295.9		$	168		$	139.9		$	307.9
	Charge-Offs		(2.7	)		(4.8	)		(7.5	)		(4.2	)		(7.3	)		(11.5	)
	Recoveries		1.7			1.2			2.9			8.5			1.5			10
	Net (Charge-Offs) Recoveries		(1.0	)		(3.6	)		(4.6	)		4.3			(5.8	)		(1.5	)
	Provision for Credit Losses		(4.3	)		(0.2	)		(4.5	)		0.8			2.2			3
	Effect of Foreign Exchange Rates		(0.1	)		—			(0.1	)		—			—			—
	Balance at End of Period	$	164.3		$	122.4		$	286.7		$	173.1		$	136.3		$	309.4
Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment	The following table provides information regarding the balances of the recorded investments in loans and leases and the allowance for credit losses by segment as of March 31, 2015, and December 31, 2014.
	Table 40: Information about the Recorded Investments in Loans and Leases
		March 31, 2015									December 31, 2014
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Loans and Leases
	Specifically Evaluated for Impairment	$	85.3		$	159.9		$	245.2		$	78.9		$	163.3		$	242.2
	Evaluated for Inherent Impairment		15,406.00			16,979.00			32,385.00			14,274.70			17,123.30			31,398.00
	Total Loans and Leases		15,491.30			17,138.90			32,630.20			14,353.60			17,286.60			31,640.20
	Allowance for Loans and Leases
	Specifically Evaluated for Impairment		6.8			1.6			8.4			5.8			0.8			6.6
	Evaluated for Inherent Impairment		132.7			117.9			250.6			138			122.4			260.4
	Allowance Assigned to Loans and Leases		139.5			119.5			259			143.8			123.2			267
	Allowance for Unfunded Exposures
	Commitments and Standby Letters of Credit		24.8			2.9			27.7			25.9			3			28.9
	Total Allowance for Credit Losses	$	164.3		$	122.4		$	286.7		$	169.7		$	126.2		$	295.9

Goodwill_and_Other_Intangibles1

Goodwill and Other Intangibles (Tables)	3 Months Ended
	Mar. 31, 2015
Changes by Reporting Segment in Carrying Amounts of Goodwill, including Effect of Foreign Exchange Rates on Non-U.S.-Dollar-Denominated Balances	The carrying amounts of goodwill, reflecting the effect of foreign exchange rates on non-U.S.-dollar-denominated balances, by reporting segment at March 31, 2015, and December 31, 2014, were as follows:
	Table 41: Goodwill by Reporting Segment
	(In Millions)	March 31,		December 31,
		2015		2014
	Corporate & Institutional Services	$	456	$	461.8
	Wealth Management		71.3		71.4
	Total Goodwill	$	527.3	$	533.2
Other Intangible Assets Subject to Amortization	The gross carrying amount and accumulated amortization of other intangible assets subject to amortization as of March 31, 2015, and December 31, 2014, were as follows:
	Table 42: Other Intangible Assets
	(In Millions)	March 31,		December 31,
		2015		2014
	Gross Carrying Amount	$	182.3	$	189.5
	Less: Accumulated Amortization		129.7		129.5
	Net Book Value	$	52.6	$	60

Reporting_Segments_Tables

Reporting Segments (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Contribution of Northern Trust's Reporting Segments	The following tables show the earnings contributions of Northern Trust’s reporting segments for the three-month periods ended March 31, 2015, and 2014.
	Table 43: Results of Reporting Segments
	Three Months Ended		Corporate &						Wealth						Treasury and						Total
	March 31,		Institutional Services						Management						Other						Consolidated
	($ In Millions)		2015			2014			2015			2014			2015			2014			2015			2014
	Noninterest Income
	Trust, Investment and Other Servicing Fees		$	407.3		$	379.2		$	320.2		$	300.3		$	—		$	—		$	727.5		$	679.5
	Foreign Exchange Trading Income			67.5			48.1			4.1			2			—			—			71.6			50.1
	Other Noninterest Income			41.9			44.4			28.8			22.7			4.1			(1.9	)		74.8			65.2
	Net Interest Income*			96			73.7			138.3			135			32.5			45.7			266.8			254.4
	Revenue*			612.7			545.4			491.4			460			36.6			43.8			1,140.70			1,049.20
	Provision for Credit Losses			(2.2	)		1.2			(2.3	)		1.8			—			—			(4.5	)		3
	Noninterest Expense			434.9			423.5			321.9			318.8			32.2			25.7			789			768
	Income before Income Taxes*			180			120.7			171.8			139.4			4.4			18.1			356.2			278.2
	Provision for Income Taxes*			57.3			34.8			64.6			52.6			3.6			9.4			125.5			96.8
	Net Income		$	122.7		$	85.9		$	107.2		$	86.8		$	0.8		$	8.7		$	230.7		$	181.4
	Percentage of Consolidated Net Income			53	%		47	%		47	%		48	%		—	%		5	%		100	%		100	%
	Average Assets		$	39,762.40		$	57,252.40		$	24,308.80		$	22,528.70		$	43,442.00		$	20,462.40		$	107,513.20		$	100,243.50
	*	Stated on an FTE basis. Total consolidated includes FTE adjustments of $6.2 million for 2015 and $8.7 million for 2014.

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2015
Components of Accumulated Other Comprehensive Income (Loss) (AOCI)	The following tables summarize the components of accumulated other comprehensive income (loss) (AOCI) at March 31, 2015, and 2014, and changes during the three-month periods then ended.
	Table 44: Summary of Changes in Accumulated Other Comprehensive Income (Loss)
	(In Millions)	Balance at				Net Change			Balance at
		March 31,							December 31,
		2015							2014
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	81.7			$	54.1		$	27.6
	Net Unrealized Gains (Losses) on Cash Flow Hedges		(5.8	)			(1.1	)		(4.7	)
	Net Foreign Currency Adjustments		(7.6	)			(5.9	)		(1.7	)
	Net Pension and Other Postretirement Benefit Adjustments		(334.7	)			6.2			(340.9	)
	Total	$	(266.4	)		$	53.3		$	(319.7	)
	(In Millions)	Balance at				Net Change			Balance at
		March 31,							December 31,
		2014							2013
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	10.5			$	4.5		$	6
	Net Unrealized Gains (Losses) on Cash Flow Hedges		4.5				1.6			2.9
	Net Foreign Currency Adjustments		5.8				(1.3	)		7.1
	Net Pension and Other Postretirement Benefit Adjustments		(256.9	)			3.4			(260.3	)
	Total	$	(236.1	)		$	8.2		$	(244.3	)
Components of Changes in Accumulated Other Comprehensive Income (Loss)	Table 45: Details of Changes in Accumulated Other Comprehensive Income (Loss)
		Three Months Ended March 31,
		2015										2014
	(In Millions)	Before Tax				Tax Effect			After Tax			Before Tax			Tax Effect			After Tax
	Unrealized Gains (Losses) on Securities Available for Sale
	Noncredit-Related Unrealized Gains (Losses) on Securities OTTI	$	—			$	—		$	—		$	1.5		$	(0.6	)	$	0.9
	Other Unrealized Gains (Losses) on Securities Available for Sale		86.8				(32.7	)		54.1			5.7			(2.2	)		3.5
	Reclassification Adjustment for (Gains) Losses Included in Net Income		(0.1	)			0.1			—			0.1			—			0.1
	Net Change		86.7				(32.6	)		54.1			7.3			(2.8	)		4.5
	Unrealized Gains (Losses) on Cash Flow Hedges
	Unrealized Gains (Losses) on Cash Flow Hedges		(4.7	)			3			(1.7	)		2.4			(0.9	)		1.5
	Reclassification Adjustment for (Gains) Losses Included in Net Income		1				(0.4	)		0.6			0.2			(0.1	)		0.1
	Net Change		(3.7	)			2.6			(1.1	)		2.6			(1.0	)		1.6
	Foreign Currency Adjustments
	Foreign Currency Translation Adjustments		(90.1	)			12.2			(77.9	)		6.5			(0.9	)		5.6
	Long-Term Intra-Entity Foreign Currency Transaction Losses		(1.0	)			0.3			(0.7	)		(0.6	)		0.2			(0.4	)
	Net Investment Hedge Gains (Losses)		116.6				(43.9	)		72.7			(10.6	)		4.1			(6.5	)
	Net Change		25.5				(31.4	)		(5.9	)		(4.7	)		3.4			(1.3	)
	Pension and Other Postretirement Benefit Adjustments
	Net Actuarial Gains (Losses)		—				—			—			—			—			—
	Reclassification Adjustment for (Gains) Losses Included in Net Income		9.5				(3.3	)		6.2			6.4			(3.0	)		3.4
	Net Change	$	9.5			$	(3.3	)	$	6.2		$	6.4		$	(3.0	)	$	3.4
Reclassification Out of Accumulated Other Comprehensive Income (Loss)	Table 46: Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
	(In Millions)	Location of		Amount of Reclassification
		Reclassification Adjustments		Adjustments Recognized
		Recognized in Income		in Income
				Three Months Ended
				March 31, 2015
	Securities Available for Sale
	Realized (Gains) Losses on Securities Available for Sale	Investment Security Gains (Losses), net		$	(0.1	)
	Realized (Gains) Losses on Cash Flow Hedges
	Foreign Exchange Contracts	Other Operating Income/Expense			1
	Pension and Other Postretirement Benefit Adjustments
	Amortization of Net Actuarial (Gains) Losses	Employee Benefits			9.6
	Amortization of Prior Service Cost	Employee Benefits			(0.1	)
	Gross Reclassification Adjustment			$	9.5

Net_Income_Per_Common_Share_Co1

Net Income Per Common Share Computations (Tables)	3 Months Ended
	Mar. 31, 2015
Computations of Net Income per Common Share	The computations of net income per common share are presented in the following table.
	Table 47: Net Income per Common Share
		Three Months Ended
		March 31,
	($ In Millions Except Per Common Share Information)	2015		2014
	Basic Net Income Per Common Share
	Average Number of Common Shares Outstanding		233,381,168		237,208,151
	Net Income	$	230.7	$	181.4
	Less: Dividends on Preferred Stock		5.9		—
	Net Income Applicable to Common Stock		224.8		181.4
	Less: Earnings Allocated to Participating Securities		3.7		2.9
	Earnings Allocated to Common Shares Outstanding		221.1		178.5
	Basic Net Income Per Common Share	$	0.95	$	0.75
	Diluted Net Income Per Common Share
	Average Number of Common Shares Outstanding		233,381,168		237,208,151
	Plus: Dilutive Effect of Share-based Compensation		1,907,527		1,842,563
	Average Common and Potential Common Shares		235,288,695		239,050,714
	Earnings Allocated to Common and Potential Common Shares	$	221.1	$	178.5
	Diluted Net Income Per Common Share		0.94		0.75
	Note: Common stock equivalents totaling 1,504,850 and 1,997,337 for the three months ended March 31, 2015, and 2014, respectively, were not included in the computation of diluted net income per common share because their inclusion would have been antidilutive.

Net_Interest_Income_Tables

Net Interest Income (Tables)	3 Months Ended
	Mar. 31, 2015
Components of Net Interest Income	The components of net interest income were as follows:
	Table 48: Net Interest Income
		Three Months Ended
		March 31,
	(In Millions)	2015		2014
	Interest Income
	Loans and Leases	$	183.9	$	182.9
	Securities — Taxable		76.6		69.5
	— Non-Taxable		1.3		2.2
	Interest-Bearing Due from and Deposits with Banks (Note)		26.6		32.1
	Federal Reserve Deposits and Other		10.4		8.7
	Total Interest Income		298.8		295.4
	Interest Expense
	Deposits		16.7		20.1
	Federal Funds Purchased		0.2		0.3
	Securities Sold Under Agreements to Repurchase		0.1		0.1
	Other Borrowings		1.2		0.7
	Senior Notes		11.6		17.5
	Long-Term Debt		7.8		10.4
	Floating Rate Capital Debt		0.6		0.6
	Total Interest Expense		38.2		49.7
	Net Interest Income	$	260.6	$	245.7
	Note: Interest income for the three months ended March 31, 2015, was earned on cash and due from banks of $2.2 billion and interest-bearing deposits with banks of $15.4 billion as of March 31, 2015.

Pension_and_Postretirement_Hea1

Pension and Postretirement Health Care (Tables)	3 Months Ended
	Mar. 31, 2015
Components of Company's Net Periodic Benefit Cost	The following tables set forth the net periodic pension and postretirement benefit expense for Northern Trust’s U.S. and non-U.S. pension plans, supplemental pension plan, and postretirement health care plan for the three months ended March 31, 2015, and 2014.
	Table 49: Net Periodic Pension Expense (Benefit)
	Net Periodic Pension Expense	Three Months Ended
		March 31,
	U.S. Plan
	(In Millions)	2015			2014
	Service Cost	$	9.5		$	8.2
	Interest Cost		11.2			11.1
	Expected Return on Plan Assets		(24.1	)		(24.4	)
	Amortization
	Net Actuarial Loss		7.4			5.4
	Prior Service Cost		(0.1	)		(0.1	)
	Net Periodic Pension Expense	$	3.9		$	0.2
	Net Periodic Pension Expense (Benefit)	Three Months Ended
		March 31,
	Non U.S. Plans
	(In Millions)	2015			2014
	Interest Cost	$	1.4		$	1.7
	Expected Return on Plan Assets		(1.5	)		(1.7	)
	Net Actuarial Loss Amortization		0.4			(0.4	)
	Net Periodic Pension Expense (Benefit)	$	0.3		$	(0.4	)
	Net Periodic Pension Expense	Three Months Ended
		March 31,
	Supplemental Plan
	(In Millions)	2015			2014
	Service Cost	$	0.9		$	0.8
	Interest Cost		1.3			1.2
	Amortization
	Net Actuarial Loss		1.8			1.5
	Prior Service Cost		—			0.1
	Net Periodic Pension Expense	$	4		$	3.6
	Net Periodic Postretirement Expense	Three Months Ended
		March 31,
	Postretirement Health Care Plan
	(In Millions)	2015			2014
	Interest Cost		0.3			0.4
	Amortization
	Net Actuarial (Gain)		—			(0.1	)
	Net Periodic Postretirement Expense	$	0.3		$	0.3

ShareBased_Compensation_Plans_

Share-Based Compensation Plans (Tables)	3 Months Ended
	Mar. 31, 2015
Compensation Expense for Share-Based Payment Arrangements and Associated Tax Impacts	Total compensation expense for share-based payment arrangements and the associated tax impacts were as follows for the three months ended March 31, 2015, and 2014.
	Table 50: Total Compensation Expense for Share-Based Payment Arrangements
		Three Months Ended
		March 31,
	(In Millions)	2015		2014
	Restricted Stock Unit Awards	$	13.5	$	14.1
	Stock Options		6		6.4
	Performance Stock Units		3.4		2.6
	Total Share-Based Compensation Expense		22.9		23.1
	Tax Benefits Recognized	$	8.6	$	8.7

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Notional and Fair Value Amounts of Client-related and Trading Derivative Financial Instruments	The following table shows the notional and fair values of client-related and trading derivative financial instruments. Notional amounts of derivative financial instruments do not represent credit risk, and are not recorded in the consolidated balance sheets. They are used merely to express the volume of this activity. Northern Trust’s credit-related risk of loss is limited to the positive fair value of the derivative instrument, which is significantly less than the notional amount.
	Table 51: Notional and Fair Values of Client-Related and Trading Derivative Financial Instruments
		March 31, 2015											December 31, 2014
		Notional				Fair Value							Notional		Fair Value
	(In Millions)	Value				Asset				Liability			Value		Asset		Liability
	Foreign Exchange Contracts	$	270,476.90			$	4,731.20			$	4,686.40		$	257,568.70	$	4,149.50	$	4,072.00
	Interest Rate Contracts		5,662.20				123.1				119.3			5,353.80		105.5		101.3
	Total	$	276,139.10			$	4,854.30			$	4,805.70		$	262,922.50	$	4,255.00	$	4,173.30
Location and Amount of Gains and Losses Recorded in Consolidated Statements of Income	The following table shows the location and amount of gains and losses recorded in the consolidated statements of income for the three months ended March 31, 2015, and 2014.
	Table 52: Location and Amount of Gains and Losses Recorded in Income
		Location of Derivative Gain/(Loss)		Amount of Derivative
		Recognized in Income		Gain/(Loss) Recognized in Income
				Three Months Ended
				March 31,
	(In Millions)			2015				2014
	Foreign Exchange Contracts	Foreign Exchange Trading Income		$	71.6			$	50.1
	Interest Rate Contracts	Security Commissions and Trading Income			4.9				1.9
	Total			$	76.5			$	52
Types and Classifications of Derivative Instruments	The following table identifies the types and classifications of derivative instruments formally designated as hedges under GAAP and used by Northern Trust to manage risk, their notional and fair values, and the respective risks addressed.
	Table 53: Notional and Fair Values of Designated Risk Management Derivative Financial Instruments
						March 31, 2015									December 31, 2014
	(In Millions)	Derivative		Risk		Notional				Fair Value					Notional		Fair Value
		Instrument		Classification		Value				Asset			Liability		Value		Asset		Liability
	Fair Value Hedges
	Available for Sale Investment Securities	Interest Rate Swap Contracts		Interest		$	2,815.70			$	6		$	26.9	$	2,859.50	$	12.7	$	28.5
				Rate
	Senior Notes and Long-Term Subordinated Debt	Interest Rate Swap Contracts		Interest			1,250.00				138.4			2		1,250.00		112.8		2
				Rate
	Cash Flow Hedges
	Forecasted Foreign Currency Denominated Transactions	Foreign Exchange Contracts		Foreign			340.5				8.8			24.4		344.9		6		14.8
				Currency
	Available for Sale Investment Securities	Interest Rate Swap Contracts		Interest			10				—			—		10		—		—
				Rate
	Available for Sale Investment Securities	Interest Rate Option Contracts		Interest			725				4.5			—		625		1.3		—
				Rate
	Net Investment Hedges
	Net Investments in Non-U.S. Affiliates	Foreign Exchange Contracts		Foreign			1,874.20				57.7			0.3		1,795.40		118.9		3.4
				Currency
	Total					$	7,015.40			$	215.4		$	53.6	$	6,884.80	$	251.7	$	48.7
Gains/Losses of Derivative Financial Instruments	The following table shows the location and amount of derivative gains and losses recorded in the consolidated statements of income related to fair value hedges for the three months ended March 31, 2015, and 2014.
	Table 54: Location and Amount of Derivative Gains and Losses Recorded in Income
	(In Millions)	Derivative		Location of		Amount of Derivative
		Instrument		Derivative		Gain/(Loss)
				Gain/(Loss)		Recognized in Income
				Recognized in		Three Months Ended
				Income		March 31,
						2015				2014
	Available for Sale Investment Securities	Interest Rate Swap Contracts		Interest		$	(17.3	)		$	(12.1	)
				Income
	Senior Notes and Long-Term Subordinated Debt	Interest Rate Swap Contracts		Interest			29.8				32.5
				Expense
	Total					$	12.5			$	20.4
Cash Flow Hedge Derivative Gains and Losses Recognized in AOCI and the Amounts Reclassified to Earnings	The following table provides cash flow hedge derivative gains and losses recognized in AOCI and the amounts reclassified to earnings during the three months ended March 31, 2015, and 2014.
	Table 55: Cash Flow Hedge Derivative Gains and Losses Recognized in AOCI and Reclassified to Earnings
	(In Millions)	Foreign Exchange								Interest Rate Option
		Contracts (Before Tax)								Contracts (Before Tax)
	Three Months Ended March 31,	2015				2014				2015			2014
	Net Gain/(Loss) Recognized in AOCI	$	(7.9	)		$	2.4			$	3.2		$	—
	Net Gain/(Loss) Reclassified from AOCI to Net Income
	Other Operating Income		(1.6	)			0.8				—			—
	Interest Income		—				—				1.1			—
	Other Operating Expense		(0.5	)			(0.2	)			—			—
	Total	$	(2.1	)		$	0.6			$	1.1		$	—
Net Investment Hedge Gains and Losses Recognized in AOCI	The following table provides net investment hedge gains and losses recognized in AOCI during the three months ended March 31, 2015, and 2014.
	Table 56: Net Investment Hedge Gains and Losses Recognized in AOCI
		Hedging Gain/(Loss)
		Recognized in OCI (Before Tax)
		Three Months Ended March 31,
	(In Millions)	2015				2014
	Foreign Exchange Contracts	$	111.6			$	(9.0	)
	Sterling Denominated Subordinated Debt		5				(1.6	)
	Total	$	116.6			$	(10.6	)
Types of Risk Management Derivative Instruments Not Formally Designated as Hedges, Including Notional Amounts and Fair Values	The following table identifies the types of risk management derivative instruments not formally designated as hedges and their notional amounts and fair values.
	Table 57: Notional and Fair Values of Non-Designated Risk Management Derivative Instruments
	(In Millions)	March 31, 2015											December 31, 2014
		Notional				Fair Value							Notional		Fair Value
		Value				Asset				Liability			Value		Asset		Liability
	Foreign Exchange Contracts	$	257			$	0.9			$	3		$	246.3	$	0.8	$	5.3
Gains/Losses of Derivative Financial Instruments	The following table provides the location and amount of gains and losses recorded in the consolidated statements of income for the three months ended March 31, 2015, and 2014.
	Table 58: Location and Amount of Gains and Losses Recorded in Income for Non-Designated Risk Management Derivative Instruments
	(In Millions)	Location of		Amount of Derivative
		Derivative Gain/		Gain/(Loss)
		(Loss) Recognized		Recognized in Income
		in Income		Three Months Ended
				March 31,
				2015				2014
	Foreign Exchange Contracts	Other Operating Income			(8.7	)			(0.7	)

Offsetting_of_Assets_and_Liabi1

Offsetting of Assets and Liabilities (Tables)	3 Months Ended
	Mar. 31, 2015
Securities Purchased Under Agreements to Resell and Derivative Assets in the Consolidated Balance Sheets	The following tables provide information regarding the offsetting of derivative assets and securities purchased under agreements to resell within the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
	Table 59: Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell
	March 31, 2015
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (3)
			Assets		Offset		Presented		Not Offset
	Derivative Assets (1)
	Foreign Exchange Contracts Over the Counter (OTC)		$	3,705.10	$	2,360.50	$	1,344.60	$	—	$	1,344.60
	Interest Rate Swaps OTC			197.9		31.8		166.1		—		166.1
	Interest Rate Swaps Exchange Cleared			74.1		25.7		48.4		—		48.4
	Cross Product Netting Adjustment			—		25.4		—		—		—
	Cross Product Collateral Adjustment			—		440.8		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,977.10		2,884.20		1,092.90		—		1,092.90
	Total Derivatives Not Subject to a Master Netting Arrangement			1,093.50		—		1,093.50		—		1,093.50
	Total Derivatives			5,070.60		2,884.20		2,186.40		—		2,186.40
	Securities Purchased under Agreements to Resell (2)		$	1,000.00	$	—	$	1,000.00	$	1,000.00	$	—
	December 31, 2014
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (3)
			Assets		Offset		Presented		Not Offset
	Derivative Assets (1)
	Foreign Exchange Contracts OTC		$	3,442.80	$	1,889.80	$	1,553.00	$	—	$	1,553.00
	Interest Rate Swaps OTC			183.9		32.1		151.8		—		151.8
	Interest Rate Swaps Exchange Cleared			48.4		13.1		35.3		—		35.3
	Cross Product Netting Adjustment			—		6.3		—		—		—
	Cross Product Collateral Adjustment			—		315.8		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,675.10		2,257.10		1,418.00		—		1,418.00
	Total Derivatives Not Subject to a Master Netting Arrangement			832.4		—		832.4		—		832.4
	Total Derivatives			4,507.50		2,257.10		2,250.40		—		2,250.40
	Securities Purchased under Agreements to Resell (2)		$	1,000.00	$	—	$	1,000.00	$	1,000.00	$	—
	-1	Derivative assets are reported in other assets in the consolidated balance sheets. Other assets (excluding derivative assets) totaled $3,177.3 million and $3,614.7 million as of March 31, 2015, and December 31, 2014, respectively.
	-2	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheets. Federal funds sold totaled $113.9 million and $62.7 million as of March 31, 2015, and December 31, 2014, respectively.
	-3	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
Securities Sold Under Agreements to Repurchase and Derivative Liabilities in the Consolidated Balance Sheets	The following tables provide information regarding the offsetting of derivative liabilities and securities sold under agreements to repurchase within the consolidated balance sheets as of March 31, 2015, and December 31, 2014.
	Table 60: Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase
	March 31, 2015
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (2)
			Liabilities		Offset		Presented		Not
									Offset
	Derivative Liabilities (1)
	Foreign Exchange Contracts OTC		$	3,895.10	$	2,360.50	$	1,534.60	$	—	$	1,534.60
	Interest Rate Swaps OTC			122.5		31.8		90.7		—		90.7
	Interest Rate Swaps Exchange Cleared			25.7		25.7		—		—		—
	Cross Product Netting Adjustment			—		25.4		—		—		—
	Cross Product Collateral Adjustment			—		1,067.50		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			4,043.30		3,510.90		532.4		—		532.4
	Total Derivatives Not Subject to a Master Netting Arrangement			819		—		819		—		819
	Total Derivatives			4,862.30		3,510.90		1,351.40		—		1,351.40
	Securities Sold under Agreements to Repurchase		$	613.2	$	—	$	613.2	$	613.2	$	—
	December 31, 2014
	(In Millions)		Gross		Gross		Net		Gross		Net
			Recognized		Amounts		Amounts		Amounts		Amount (2)
			Liabilities		Offset		Presented		Not
									Offset
	Derivative Liabilities (1)
	Foreign Exchange Contracts OTC		$	3,431.00	$	1,889.80	$	1,541.20	$	—	$	1,541.20
	Interest Rate Swaps OTC			118.7		32.1		86.6		—		86.6
	Interest Rate Swaps Exchange Cleared			13.1		13.1		—		—		—
	Cross Product Netting Adjustment			—		6.3		—		—		—
	Cross Product Collateral Adjustment			—		1,232.00		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,562.80		3,173.30		389.5		—		389.5
	Total Derivatives Not Subject to a Master Netting Arrangement			664.5		—		664.5		—		664.5
	Total Derivatives			4,227.30		3,173.30		1,054.00		—		1,054.00
	Securities Sold under Agreements to Repurchase		$	885.1	$	—	$	885.1	$	885.1	$	—
	-1	Derivative liabilities are reported in other liabilities in the consolidated balance sheets. Other liabilities (excluding derivative liabilities) totaled $3,185.1 million and $2,794.1 million as of March 31, 2015, and December 31, 2014, respectively.
	-2	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended
	Apr. 30, 2014	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities portfolio, value		$30,945,700,000		$29,558,500,000
Trading account securities		1,400,000		4,700,000
Settlement of contingent consideration	55,300,000
Transfers into Level 3 assets		0	0
Transfers out of Level 3 assets		0	0
Transfers into Level 3 liabilities		0	0
Transfers out of Level 3 liabilities		0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio		924		881
Available for sale securities portfolio, value		26,300,000,000		25,000,000,000
Loans | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value		23,100,000	33,300,000
Other Real Estate Owned | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value		300,000	1,500,000
External Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio		924		881
Trading account securities		$1,400,000		$4,700,000

Valuation_Techniques_Significa

Valuation Techniques, Significant Unobservable Inputs, and Quantitative Information (Detail) (Fair Value, Inputs, Level 3, Auction Rate Securities, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument, fair value	17.5
Group One
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique	Discounted Cash Flow
Unobservable Inputs	Remaining lives
Group Two
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique	Discounted Cash Flow
Unobservable Inputs	Discount rates
Lower Limit
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument remaining lives	1 year 2 months 12 days
Financial instrument discount rate	0.10%
Upper Limit
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument remaining lives	8 years 7 months 6 days
Financial instrument discount rate	8.40%

Assets_and_Liabilities_Measure

Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$30,945.70		$29,558.50
Trading Account	1.4		4.7
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	30,947.10		29,563.20
Fair Value, Measurements, Recurring | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	30,945.70		29,558.50
Trading Account	1.4		4.7
Fair Value, Measurements, Recurring | Securities Investment | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4,627.40		4,506.90
Fair Value, Measurements, Recurring | Securities Investment | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4.5		4.6
Fair Value, Measurements, Recurring | Securities Investment | U.S. Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	16,634.20		16,389.20
Fair Value, Measurements, Recurring | Securities Investment | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	311.4		310.4
Fair Value, Measurements, Recurring | Securities Investment | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,651.30		3,577.70
Fair Value, Measurements, Recurring | Securities Investment | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,049.30		1,907.50
Fair Value, Measurements, Recurring | Securities Investment | Sub-Sovereign, Supranational and Non US Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	561
Fair Value, Measurements, Recurring | Securities Investment | Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale			360.6
Fair Value, Measurements, Recurring | Securities Investment | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4.4		6.4
Fair Value, Measurements, Recurring | Securities Investment | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,949.30		2,321.30
Fair Value, Measurements, Recurring | Securities Investment | Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	17.5		18.1
Fair Value, Measurements, Recurring | Securities Investment | Others
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	135.4		155.8
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	2,186.40		2,250.40
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	4,798.60		4,275.20
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	272		232.3
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1,351.40		1,054
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	4,714.10		4,095.50
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	148.2		131.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	4,627.40		4,506.90
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4,627.40		4,506.90
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Securities Investment | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4,627.40		4,506.90
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	26,302.20		25,038.20
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	26,300.80		25,033.50
Trading Account	1.4		4.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4.5		4.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | U.S. Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	16,634.20		16,389.20
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	311.4		310.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,651.30		3,577.70
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,049.30		1,907.50
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Sub-Sovereign, Supranational and Non US Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	561
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale			360.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	4.4		6.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,949.30		2,321.30
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Others
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	135.4		155.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	5,070.60		4,507.50
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	4,798.60		4,275.20
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	272		232.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	4,862.30		4,227.30
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	4,714.10		4,095.50
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	148.2		131.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	17.5		18.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	17.5		18.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Securities Investment | Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	17.5		18.1
Fair Value, Measurements, Recurring | Subject to master netting arrangement or similar arrangement | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	-2,884.20	[1]	-2,257.10	[2]
Fair Value, Measurements, Recurring | Subject to master netting arrangement or similar arrangement | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	($3,510.90)	[1]	($3,173.30)	[2]
[1]	Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of March 31, 2015, derivative assets and liabilities shown above also include reductions of $440.8 million and $1.1 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.
[2]	Northern Trust has elected to net derivative assets and liabilities when legally enforceable master netting arrangements or similar agreements exist between Northern Trust and the counterparty. As of December 31, 2014, derivative assets and liabilities shown above also include reductions of $315.8 million and $1.2 billion, respectively, as a result of cash collateral received from and deposited with derivative counterparties.

Assets_and_Liabilities_Measure1

Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level (Parenthetical) (Detail) (Subject to master netting arrangement or similar arrangement, Fair Value, Measurements, Recurring, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Derivative Financial Instruments, Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$1,100	$1,200
Derivative Financial Instruments, Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$440.80	$315.80

Changes_in_Level_3_Assets_Deta

Changes in Level 3 Assets (Detail) (Auction Rate Securities, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assets Beginning Balance	$18.10		$98.90
Total Gains (Losses):
Included in Earnings	0		0
Included in Other Comprehensive Income	-0.3	[1]	-0.2	[1]
Purchases, Issues, Sales, and Settlements:
Sales			0.1
Settlements	-0.3		-0.3
Fair Value Assets Ending Balance	$17.50		$98.50
[1]	Unrealized gains (losses) are included in net unrealized gains (losses) on securities available for sale in the consolidated statements of comprehensive income.

Changes_in_Level_3_Liabilities

Changes in Level 3 Liabilities (Detail) (Contingent Consideration, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Contingent Consideration
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Liabilities Beginning Balance			$55.40
Total (Gains) Losses:
Included in Earnings			-0.1	[1]
Included in Other Comprehensive Income	0		0
Purchases, Issues, Sales, and Settlements
Purchases	0		0
Settlements			-55.3
Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held at March 31	$0	[1]	$0	[1]
[1]	(Gains) losses are recorded in other operating income (expense) in the consolidated statements of income.

Valuation_Techniques_Significa1

Valuation Techniques, Significant Unobservable Inputs, Quantitative Information Non-recurring Basis (Detail) (Fair Value, Inputs, Level 3, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instrument, fair value	$23.10
Valuation Technique	Market Approach
Unobservable Input	Discount to reflect realizable value
Other Real Estate Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instrument, fair value	$0.30
Valuation Technique	Market Approach
Unobservable Input	Discount to reflect realizable value
Lower Limit | Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Range of Discounts Applied	15.00%
Lower Limit | Other Real Estate Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Range of Discounts Applied	15.00%
Upper Limit | Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Range of Discounts Applied	25.00%
Upper Limit | Other Real Estate Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Range of Discounts Applied	20.00%

Book_and_Fair_Values_of_All_Fi

Book and Fair Values of All Financial Instruments (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets
Cash and Due from Banks	$4,074.80		$3,050.60		$3,676.30	$3,162.40
Federal Funds Sold and Resell Agreements	1,113.90		1,062.70
Interest-Bearing Deposits with Banks	15,432.30		14,928.30
Federal Reserve Deposits	8,804.20		17,386.30
Securities
Available for Sale	30,945.70		29,558.50
Held to Maturity	5,655.80		4,170.80
Trading Account	1.4		4.7
Loans (excluding Leases)
Client Security Settlement Receivables	2,219.30		1,568.80
Deposits
Savings Certificates and Other Time	1,777.90		1,757.40
Non U.S. Offices Interest-Bearing	45,942.20		47,545
Federal Funds Purchased	410.7		932.9
Securities Sold under Agreements to Repurchase	613.2		885.1
Other Borrowings	2,874.10		1,685.20
Senior Notes	1,497.10		1,497
Long Term Debt (excluding Leases)
Floating Rate Capital Debt	277.2		277.2
Asset And Liability Management
Derivative Instruments
Assets	215.4		251.7
Liabilities	53.6		48.7
Client Related and Trading
Derivative Instruments
Assets	4,854.30		4,255
Liabilities	4,805.70		4,173.30
Carrying (Reported) Amount, Fair Value Disclosure
Assets
Cash and Due from Banks	4,074.80		3,050.60
Federal Funds Sold and Resell Agreements	1,113.90		1,062.70
Interest-Bearing Deposits with Banks	15,432.30		14,928.30
Federal Reserve Deposits	8,804.20		17,386.30
Securities
Available for Sale	30,945.70	[1]	29,558.50	[2]
Held to Maturity	5,655.80		4,170.80
Trading Account	1.4		4.7
Loans (excluding Leases)
Held for Investment	31,503.80		30,458
Held for Sale			2.5
Client Security Settlement Receivables	2,219.30		1,568.80
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	207.5		207.5
Community Development Investments	198.4		209.9
Employee Benefit and Deferred Compensation	162.1		143.2
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	39,016.40		41,454.60
Savings Certificates and Other Time	1,777.90		1,757.40
Non U.S. Offices Interest-Bearing	45,942.20		47,545
Federal Funds Purchased	410.7		932.9
Securities Sold under Agreements to Repurchase	613.2		885.1
Other Borrowings	2,874.10		1,685.20
Senior Notes	1,497.10		1,497
Long Term Debt (excluding Leases)
Subordinated Debt	1,368.60		1,583.30
Floating Rate Capital Debt	277.2		277.1
Other Liabilities
Standby Letters of Credit	58.3		60.1
Loan Commitments	26.8		28.3
Carrying (Reported) Amount, Fair Value Disclosure | Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	67.4		125.7
Liabilities	27.7		23.5
Carrying (Reported) Amount, Fair Value Disclosure | Asset And Liability Management | Interest Rate Swaps
Derivative Instruments
Assets			126.8
Liabilities			30.5
Carrying (Reported) Amount, Fair Value Disclosure | Asset And Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	148.9
Liabilities	28.9
Carrying (Reported) Amount, Fair Value Disclosure | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	4,731.20		4,149.50
Liabilities	4,686.40		4,072
Carrying (Reported) Amount, Fair Value Disclosure | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	123.1		105.5
Liabilities	119.3		101.3
Estimate of Fair Value, Fair Value Disclosure
Assets
Cash and Due from Banks	4,074.80		3,050.60
Federal Funds Sold and Resell Agreements	1,113.90		1,062.70
Interest-Bearing Deposits with Banks	15,432.30		14,928.30
Federal Reserve Deposits	8,804.20		17,386.30
Securities
Available for Sale	30,945.70	[1]	29,558.50	[2]
Held to Maturity	5,665		4,176.10
Trading Account	1.4		4.7
Loans (excluding Leases)
Held for Investment	31,747.70		30,600.40
Held for Sale			2.5
Client Security Settlement Receivables	2,219.30		1,568.80
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	207.5		207.5
Community Development Investments	198.7		210.8
Employee Benefit and Deferred Compensation	165.5		146.7
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	39,016.40		41,454.60
Savings Certificates and Other Time	1,783.10		1,757.40
Non U.S. Offices Interest-Bearing	45,942.20		47,545
Federal Funds Purchased	410.7		932.9
Securities Sold under Agreements to Repurchase	613.2		885.1
Other Borrowings	2,875.70		1,686.20
Senior Notes	1,566.90		1,541.80
Long Term Debt (excluding Leases)
Subordinated Debt	1,374.90		1,583.30
Floating Rate Capital Debt	241.4		242.8
Other Liabilities
Standby Letters of Credit	58.3		60.1
Loan Commitments	26.8		28.3
Estimate of Fair Value, Fair Value Disclosure | Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	67.4		125.7
Liabilities	27.7		23.5
Estimate of Fair Value, Fair Value Disclosure | Asset And Liability Management | Interest Rate Swaps
Derivative Instruments
Assets			126.8
Liabilities			30.5
Estimate of Fair Value, Fair Value Disclosure | Asset And Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	148.9
Liabilities	28.9
Estimate of Fair Value, Fair Value Disclosure | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	4,731.20		4,149.50
Liabilities	4,686.40		4,072
Estimate of Fair Value, Fair Value Disclosure | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	123.1		105.5
Liabilities	119.3		101.3
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 1
Assets
Cash and Due from Banks	4,074.80		3,050.60
Securities
Available for Sale	4,627.40	[1]	4,506.90	[2]
Other Assets
Employee Benefit and Deferred Compensation	113.9		96.7
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	39,016.40		41,454.60
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2
Assets
Federal Funds Sold and Resell Agreements	1,113.90		1,062.70
Interest-Bearing Deposits with Banks	15,432.30		14,928.30
Federal Reserve Deposits	8,804.20		17,386.30
Securities
Available for Sale	26,300.80	[1]	25,033.50	[2]
Held to Maturity	5,665		4,176.10
Trading Account	1.4		4.7
Loans (excluding Leases)
Client Security Settlement Receivables	2,219.30		1,568.80
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	207.5		207.5
Community Development Investments	198.7		210.8
Employee Benefit and Deferred Compensation	51.6		50
Deposits
Savings Certificates and Other Time	1,783.10		1,757.40
Non U.S. Offices Interest-Bearing	45,942.20		47,545
Federal Funds Purchased	410.7		932.9
Securities Sold under Agreements to Repurchase	613.2		885.1
Other Borrowings	2,875.70		1,686.20
Senior Notes	1,566.90		1,541.80
Long Term Debt (excluding Leases)
Subordinated Debt	1,374.90		1,583.30
Floating Rate Capital Debt	241.4		242.8
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	67.4		125.7
Liabilities	27.7		23.5
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Asset And Liability Management | Interest Rate Swaps
Derivative Instruments
Assets			126.8
Liabilities			30.5
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Asset And Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	148.9
Liabilities	28.9
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	4,731.20		4,149.50
Liabilities	4,686.40		4,072
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	123.1		105.5
Liabilities	119.3		101.3
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 3
Securities
Available for Sale	17.5	[1]	18.1	[2]
Loans (excluding Leases)
Held for Investment	31,747.70		30,600.40
Held for Sale			2.5
Other Liabilities
Standby Letters of Credit	58.3		60.1
Loan Commitments	$26.80		$28.30
[1]	Refer to the table located on page 31 for the disaggregation of available for sale securities.
[2]	Refer to the table located on page 32 for the disaggregation of available for sale securities.

Reconciliation_of_Amortized_Co

Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Investment Holdings [Line Items]
Amortized Cost	$30,814.60	$29,514.10
Gross Unrealized Gains	160.8	110.9
Gross Unrealized Losses	29.7	66.5
Fair Value	30,945.70	29,558.50
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	4,593.90	4,493.50
Gross Unrealized Gains	33.5	15.1
Gross Unrealized Losses		1.7
Fair Value	4,627.40	4,506.90
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	4.5	4.5
Gross Unrealized Gains		0.1
Fair Value	4.5	4.6
U.S. Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,536.90	16,326.40
Gross Unrealized Gains	107.6	82.3
Gross Unrealized Losses	10.3	19.5
Fair Value	16,634.20	16,389.20
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	310.5	309.5
Gross Unrealized Gains	0.9	0.9
Fair Value	311.4	310.4
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,661.50	3,617.50
Gross Unrealized Gains	6.7	1.8
Gross Unrealized Losses	16.9	41.6
Fair Value	3,651.30	3,577.70
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	2,040.50	1,899.90
Gross Unrealized Gains	8.8	7.9
Gross Unrealized Losses		0.3
Fair Value	2,049.30	1,907.50
Sub-Sovereign, Supranational and Non US Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	560
Gross Unrealized Gains	1.4
Gross Unrealized Losses	0.4
Fair Value	561
Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost		360
Gross Unrealized Gains		1.5
Gross Unrealized Losses		0.9
Fair Value		360.6
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	4.9	6.9
Gross Unrealized Losses	0.5	0.5
Fair Value	4.4	6.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,948.70	2,321.80
Gross Unrealized Gains	1.1	0.5
Gross Unrealized Losses	0.5	1
Fair Value	2,949.30	2,321.30
Auction Rate Securities
Investment Holdings [Line Items]
Amortized Cost	18.1	18.4
Gross Unrealized Gains	0.5	0.5
Gross Unrealized Losses	1.1	0.8
Fair Value	17.5	18.1
Others
Investment Holdings [Line Items]
Amortized Cost	135.1	155.7
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses		0.2
Fair Value	$135.40	$155.80

Reconciliation_of_Amortized_Co1

Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$5,655.80	$4,170.80
Gross Unrealized Gains	28.7	26.4
Gross Unrealized Losses	19.5	21.1
Fair Value	5,665	4,176.10
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	112.6	121.9
Gross Unrealized Gains	7	7.4
Fair Value	119.6	129.3
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	15.5	18.4
Gross Unrealized Gains	1	1.1
Fair Value	16.5	19.5
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,104.30	1,281.60
Gross Unrealized Gains	8	6.6
Gross Unrealized Losses		0.4
Fair Value	1,112.30	1,287.80
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,575.70	924.3
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	0.3	0.1
Fair Value	2,575.60	924.3
Sub-Sovereign, Supranational and Non US Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,769.70
Gross Unrealized Gains	12.5
Gross Unrealized Losses	0.2
Fair Value	1,782
Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		1,745.80
Gross Unrealized Gains		10.9
Gross Unrealized Losses		0.5
Fair Value		1,756.20
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	78	78.8
Gross Unrealized Gains		0.3
Gross Unrealized Losses	19	20.1
Fair Value	$59	$59

Remaining_Maturity_of_Securiti

Remaining Maturity of Securities Available for Sale and Held to Maturity (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Available for Sale - Cost
Due in One Year or Less	$7,702.60
Due After One Year Through Five Years	18,210.80
Due After Five Years Through Ten Years	3,650.70
Due After Ten Years	1,250.50
Total	30,814.60
Held to Maturity - Amortized Cost
Due in One Year or Less	3,219.30
Due After One Year Through Five Years	2,374.50
Due After Five Years Through Ten Years	21.4
Due After Ten Years	40.6
Amortized Cost	5,655.80	4,170.80
Available for Sale - Value
Due in One Year or Less	7,731.30
Due After One Year Through Five Years	18,295.10
Due After Five Years Through Ten Years	3,659.60
Due After Ten Years	1,259.70
Total	30,945.70
Held to Maturity - Fair Value
Due in One Year or Less	3,220.70
Due After One Year Through Five Years	2,397.50
Due After Five Years Through Ten Years	19.4
Due After Ten Years	27.4
Total	$5,665	$4,176.10

Securities_Additional_Informat

Securities - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
	Investment
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net	$0.10	[1]	($4)	[1]
Net impairment losses recognized in earnings	0		3.9
Gross proceeds from sale of securities	102		199.7
Gross realized securities gains	0.1		0.3
Gross realized securities losses			0.4
Number of securities in an unrealized loss position	391
Total Fair Value	7,511.10				10,410.80
Total Unrealized Losses	49.2				87.6
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	16.9
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	19
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	1.1
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	10.3
Percent of corporate debt portfolio	39.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	$0.50
Number of securities in an unrealized loss position for more than 12 months	1
Other Asset-Backed | Floating Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of "other asset-backed" securities rated triple-A	100.00%
Credit Rating	AAA
Other Asset-Backed | Floating Rate Securities | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average life	5 years
[1]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ (4.6 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - 0.7 Other Security Gains (Losses), net 0.1 (0.1 ) Investment Security Gains (Losses), net $ 0.1 $ (4.0 )

Securities_Continuous_Unrealiz

Securities Continuous Unrealized Loss Position (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$4,972.70	$7,101.70
Less than 12 Months Unrealized Losses	16.5	26.5
12 Months or Longer Fair Value	2,538.40	3,309.10
12 Months or Longer Unrealized Losses	32.7	61.1
Total Fair Value	7,511.10	10,410.80
Total Unrealized Losses	49.2	87.6
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		998.2
Less than 12 Months Unrealized Losses		1.7
Total Fair Value		998.2
Total Unrealized Losses		1.7
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,946	2,344.90
Less than 12 Months Unrealized Losses	3.9	6.6
12 Months or Longer Fair Value	1,091.70	1,730
12 Months or Longer Unrealized Losses	6.4	12.9
Total Fair Value	3,037.70	4,074.90
Total Unrealized Losses	10.3	19.5
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		292.9
Less than 12 Months Unrealized Losses		0.4
Total Fair Value		292.9
Total Unrealized Losses		0.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	391.4	1,244.50
Less than 12 Months Unrealized Losses	0.6	3.9
12 Months or Longer Fair Value	1,217.80	1,338.80
12 Months or Longer Unrealized Losses	16.3	37.7
Total Fair Value	1,609.20	2,583.30
Total Unrealized Losses	16.9	41.6
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		142.3
Less than 12 Months Unrealized Losses		0.2
12 Months or Longer Fair Value		10
12 Months or Longer Unrealized Losses		0.1
Total Fair Value		152.3
Total Unrealized Losses		0.3
Sub-Sovereign, Supranational and Non US Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	149.8
Less than 12 Months Unrealized Losses	0.2
12 Months or Longer Fair Value	170.7
12 Months or Longer Unrealized Losses	0.4
Total Fair Value	320.5
Total Unrealized Losses	0.6
Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		313.2
Less than 12 Months Unrealized Losses		0.3
12 Months or Longer Fair Value		175.5
12 Months or Longer Unrealized Losses		1.1
Total Fair Value		488.7
Total Unrealized Losses		1.4
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	4.4	4.5
12 Months or Longer Unrealized Losses	0.5	0.5
Total Fair Value	4.4	4.5
Total Unrealized Losses	0.5	0.5
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,071.10	1,297.60
Less than 12 Months Unrealized Losses	0.5	1
Total Fair Value	1,071.10	1,297.60
Total Unrealized Losses	0.5	1
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,395.30	438.6
Less than 12 Months Unrealized Losses	0.3	0.1
Total Fair Value	1,395.30	438.6
Total Unrealized Losses	0.3	0.1
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1.4	2.4
Less than 12 Months Unrealized Losses	0.2	0.2
12 Months or Longer Fair Value	5.5	4.7
12 Months or Longer Unrealized Losses	0.9	0.6
Total Fair Value	6.9	7.1
Total Unrealized Losses	1.1	0.8
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	17.7	27.1
Less than 12 Months Unrealized Losses	10.8	12.1
12 Months or Longer Fair Value	48.3	45.6
12 Months or Longer Unrealized Losses	8.2	8.2
Total Fair Value	66	72.7
Total Unrealized Losses	$19	$20.30

Credit_Related_Impairment_Loss

Credit Related Impairment Losses Recognized in Earnings on Other Than Temporarily Impaired Securities (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Changes in OTTI Losses		($4.60)	[1]
Noncredit-related Losses Recorded in / (Reclassified from) OCI		0.7	[2]
Net Impairment Losses Recognized in Earnings	$0	($3.90)
[1]	For initial OTTI in the applicable period, the balance includes the excess of the amortized cost over the fair value of the impaired securities. For subsequent impairments of the same security, the balance includes any additional changes in fair value of the security subsequent to its most recently recorded OTTI.
[2]	For initial OTTI in the applicable period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in other comprehensive income (OCI). For subsequent impairments of the same security, the balance includes additional changes in OCI for that security subsequent to its most recently recorded OTTI.

CreditRelated_Losses_Recognize

Credit-Related Losses Recognized in Earnings on Debt Securities Other-Than-Temporarily Impaired (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$5.20	$8.80
Plus: Losses on Newly Identified Impairments		1.8
Additional Losses on Previously Identified Impairments		2.1
Less: Current and Prior Period Losses on Securities Sold During the Period	0	0
Cumulative Credit-Related Losses on Securities Held - End of Period	$5.20	$12.70

Amounts_Outstanding_for_Loans_

Amounts Outstanding for Loans and Leases by Segment and Class (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	$32,630.20	$31,640.20
Allowance for Credit Losses Assigned to Loans and Leases	-259	-267
Net Loans and Leases	32,371.20	31,373.20
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and Institutional	9,030.20	8,381.90
Commercial Real Estate	3,500.90	3,333.30
Lease Financing, net	870.6	916.3
Non-U.S.	1,661.60	1,530.60
Other	428	191.5
Total Loans and Leases	15,491.30	14,353.60
Allowance for Credit Losses Assigned to Loans and Leases	-139.5	-143.8
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential Real Estate	9,440.20	9,782.60
Private Client	7,615.80	7,466.90
Other	82.9	37.1
Total Loans and Leases	17,138.90	17,286.60
Allowance for Credit Losses Assigned to Loans and Leases	($119.50)	($123.20)

Loans_and_Leases_Additional_In

Loans and Leases - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
			Loan	Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan, lower limit	65.00%
Collateral required on Residential real estate loan, upper limit	80.00%
Draw period for credit line products	10 years
Other U.S. loans and non-U.S. loans, short duration advances	$1,700,000,000		1,500,000,000
Demand deposits	77,600,000		92,100,000
Loans held for sale	0		2,500,000
Maximum limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures	250,000
Interest income that would have been recorded for nonperforming loans	2,100,000	2,500,000
Undrawn loan commitments and standby letters of credit	2,300,000		2,400,000
Total Loans and Leases	32,630,200,000		31,640,200,000
Unpaid principal balance	301,700,000		301,000,000
Foreclosed residential real estate properties with carrying value	5,700,000
Consumer loans with carrying value collateralized by residential real estate property	29,300,000
Lower Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days contractually past due for interest and principal and the loan is not well-secured and in the process of collection	90 days
Upper Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of days past due for disclosure purposes to be categorized as a current	29 days
First Lien
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held by Northern trust	89.00%		89.00%
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	1,700,000,000		1,800,000,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	85,900,000		82,700,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	63,000,000		68,600,000
Performing Financing Receivable | Lower Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for which troubled debt restructurings with modified terms is performing before ceasing to be classified as impaired loan	6 months
Troubled debt restructuring loans and leases receivable to Performing status	6 months
Residential Real Estate | Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans and Leases			4	0
Recorded Investment	700,000
Unpaid principal balance	$700,000

Loan_and_Lease_Segment_and_Cla

Loan and Lease Segment and Class Balances Segregated by Borrower Ratings into "One to Three", "Four to Five" and "Six to Nine" (Watch List) Categories (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$32,630.20	$31,640.20
1 to 3 Category
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	16,168	16,322.90
4 to 5 Category
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	15,830.10	14,680.40
6 to 9 Category (Watch List)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	632.1	636.9
Commercial
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	9,030.20	8,381.90
Commercial Real Estate	3,500.90	3,333.30
Lease Financing, net	870.6	916.3
Non-U.S.	1,661.60	1,530.60
Other	428	191.5
Total Loans and Leases	15,491.30	14,353.60
Commercial | 1 to 3 Category
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	5,628.30	5,340.90
Commercial Real Estate	1,452.50	1,371.70
Lease Financing, net	510.6	552.5
Non-U.S.	513.8	636.8
Other	353	108.1
Total Loans and Leases	8,458.20	8,010
Commercial | 4 to 5 Category
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	3,314.90	2,947.30
Commercial Real Estate	1,949.90	1,861.80
Lease Financing, net	324	360.3
Non-U.S.	1,147.60	892.9
Other	75	83.4
Total Loans and Leases	6,811.40	6,145.70
Commercial | 6 to 9 Category (Watch List)
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	87	93.7
Commercial Real Estate	98.5	99.8
Lease Financing, net	36	3.5
Non-U.S.	0.2	0.9
Total Loans and Leases	221.7	197.9
Personal
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	9,440.20	9,782.60
Private Client	7,615.80	7,466.90
Other	82.9	37.1
Total Loans and Leases	17,138.90	17,286.60
Personal | 1 to 3 Category
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	2,734.20	3,148
Private Client	4,907.10	5,143.80
Other	68.5	21.1
Total Loans and Leases	7,709.80	8,312.90
Personal | 4 to 5 Category
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	6,304.90	6,207
Private Client	2,699.40	2,311.70
Other	14.4	16
Total Loans and Leases	9,018.70	8,534.70
Personal | 6 to 9 Category (Watch List)
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	401.1	427.6
Private Client	9.3	11.4
Total Loans and Leases	$410.40	$439

Balances_and_Delinquency_Statu

Balances and Delinquency Status of Performing and Nonperforming Loans and Leases by Segment and Class as well as Total OREO and Nonperforming Asset Balances (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Financing Receivable, Recorded Investment [Line Items]
Other Real Estate Owned	$8.50	$16.60
Total Nonperforming Assets	228.1	232.3
Total Loans and Leases	32,630.20	31,640.20
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	15,491.30	14,353.60
Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	9,030.20	8,381.90
Commercial | Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	3,500.90	3,333.30
Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	870.6	916.3
Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	1,661.60	1,530.60
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	428	191.5
Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	17,138.90	17,286.60
Personal | Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	9,440.20	9,782.60
Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	7,615.80	7,466.90
Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	82.9	37.1
Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Current	32,280.30	31,242.60
30-59 Days Past Due	101.7	129.6
60-89 Days Past Due	19.2	29.6
90 Days or More Past Due	9.4	22.7
Total Loans and Leases	32,410.60	31,424.50
Performing Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Current	15,383.50	14,253.20
30-59 Days Past Due	31.4	24.1
60-89 Days Past Due	9.4	13.8
90 Days or More Past Due	6.2	10.4
Total Loans and Leases	15,430.50	14,301.50
Performing Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Current	8,980.40	8,340.50
30-59 Days Past Due	21.2	14.5
60-89 Days Past Due	4.6	4
90 Days or More Past Due	3.1	7.9
Total Loans and Leases	9,009.30	8,366.90
Performing Financing Receivable | Commercial | Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Current	3,442.90	3,274.30
30-59 Days Past Due	10.2	9.6
60-89 Days Past Due	4.8	9.8
90 Days or More Past Due	3.1	2.5
Total Loans and Leases	3,461	3,296.20
Performing Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Current	870.6	916.3
Total Loans and Leases	870.6	916.3
Performing Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Current	1,661.60	1,530.60
Total Loans and Leases	1,661.60	1,530.60
Performing Financing Receivable | Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Current	428	191.5
Total Loans and Leases	428	191.5
Performing Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Current	16,896.80	16,989.40
30-59 Days Past Due	70.3	105.5
60-89 Days Past Due	9.8	15.8
90 Days or More Past Due	3.2	12.3
Total Loans and Leases	16,980.10	17,123
Performing Financing Receivable | Personal | Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Current	9,240.20	9,556.30
30-59 Days Past Due	37.5	49.5
60-89 Days Past Due	2.6	9.9
90 Days or More Past Due	2.2	4.5
Total Loans and Leases	9,282.50	9,620.20
Performing Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Current	7,573.70	7,396
30-59 Days Past Due	32.8	56
60-89 Days Past Due	7.2	5.9
90 Days or More Past Due	1	7.8
Total Loans and Leases	7,614.70	7,465.70
Performing Financing Receivable | Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Current	82.9	37.1
Total Loans and Leases	82.9	37.1
Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	219.6	215.7
Nonperforming Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	60.8	52.1
Nonperforming Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	20.9	15
Nonperforming Financing Receivable | Commercial | Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	39.9	37.1
Nonperforming Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	158.8	163.6
Nonperforming Financing Receivable | Personal | Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	157.7	162.4
Nonperforming Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$1.10	$1.20

Impaired_Loans_by_Segment_and_

Impaired Loans by Segment and Class (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Recorded Investment	$245.20				$242.20
Unpaid principal balance	301.7				301
Specific allowance	8.4				6.6
Average recorded investment	246.9	[1]	294.1	[1]
Interest income recognized	0.6		1
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	85.3				78.9
Unpaid principal balance	96				93.5
Specific allowance	6.8				5.8
Average recorded investment	81.8	[1]	97	[1]
Interest income recognized	0.3		0.4
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	7.9				9
With a related specific reserve, recorded investment	12.5				6.5
With no related specific reserve, unpaid principal balance	9.9				12
With a related specific reserve, unpaid principal balance	9.9				6.6
Specific allowance	4				2.9
With no related specific reserve, average recorded investment	9.2	[1]	11.4	[1]
With a related specific reserve, average recorded investment	8.5	[1]	10	[1]
With a related specific reserve, interest income recognized	0		0
Commercial | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	49.1				47
With a related specific reserve, recorded investment	12.4				12.2
With no related specific reserve, unpaid principal balance	54.1				52.4
With a related specific reserve, unpaid principal balance	18.7				18.3
Specific allowance	2.8				2.9
With no related specific reserve, average recorded investment	48.5	[1]	44.8	[1]
With a related specific reserve, average recorded investment	12.2	[1]	26.4	[1]
With no related specific reserve, interest income recognized	0.2		0.3
With a related specific reserve, interest income recognized	0		0
Commercial | Lease Financing, net
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	3.4				4.2
With no related specific reserve, unpaid principal balance	3.4				4.2
With no related specific reserve, average recorded investment	3.4	[1]	4.4	[1]
With no related specific reserve, interest income recognized	0.1		0.1
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	159.9				163.3
Unpaid principal balance	205.7				207.5
Specific allowance	1.6				0.8
Average recorded investment	165.1	[1]	197.1	[1]
Interest income recognized	0.3		0.6
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	153.1				160.9
With a related specific reserve, recorded investment	5.8				1.4
With no related specific reserve, unpaid principal balance	198.7				204.8
With a related specific reserve, unpaid principal balance	6				1.4
Specific allowance	1.2				0.4
With no related specific reserve, average recorded investment	161.3	[1]	189.2	[1]
With a related specific reserve, average recorded investment	2.8	[1]	7.1	[1]
With no related specific reserve, interest income recognized	0.3		0.6
With a related specific reserve, interest income recognized	0		0
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	0.2				0.2
With a related specific reserve, recorded investment	0.8				0.8
With no related specific reserve, unpaid principal balance	0.2				0.5
With a related specific reserve, unpaid principal balance	0.8				0.8
Specific allowance	0.4				0.4
With no related specific reserve, average recorded investment	0.2	[1]	0.7	[1]
With a related specific reserve, average recorded investment	0.8	[1]	0.1	[1]
With a related specific reserve, interest income recognized	$0		$0
[1]	Average recorded investment in impaired loans is calculated as the average of the month-end impaired loan balances for the period.

Number_of_Loans_and_Leases_Mod

Number of Loans and Leases Modified in TDRs and Total Recorded Investments and Unpaid Principal Balances (Detail) (Class of Financing Receivable, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
	Contract	Contract
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	59	36
Recorded Investment	$7.90	$4.20
Unpaid Principal Balance	11	4.5
Commercial
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	2	1
Recorded Investment	0.8	0.7
Unpaid Principal Balance	0.8	0.7
Commercial | Commercial and Institutional
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	1
Recorded Investment	0.1
Unpaid Principal Balance	0.1
Commercial | Commercial Real Estate Portfolio Segment
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	1	1
Recorded Investment	0.7	0.7
Unpaid Principal Balance	0.7	0.7
Personal
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	57	35
Recorded Investment	7.1	3.5
Unpaid Principal Balance	10.2	3.8
Personal | Residential Real Estate
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	57	34
Recorded Investment	7.1	3.5
Unpaid Principal Balance	$10.20	$3.80
Personal | Private Client
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases		1

Changes_in_Allowance_for_Credi

Changes in Allowance for Credit Losses by Segment (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Period	$295.90	$307.90
Charge-Offs	-7.5	-11.5
Recoveries	2.9	10
Net (Charge-Offs) Recoveries	-4.6	-1.5
Provision for Credit Losses	-4.5	3
Effect of Foreign Exchange Rates	-0.1
Balance at End of Period	286.7	309.4
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Period	169.7	168
Charge-Offs	-2.7	-4.2
Recoveries	1.7	8.5
Net (Charge-Offs) Recoveries	-1	4.3
Provision for Credit Losses	-4.3	0.8
Effect of Foreign Exchange Rates	-0.1
Balance at End of Period	164.3	173.1
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Period	126.2	139.9
Charge-Offs	-4.8	-7.3
Recoveries	1.2	1.5
Net (Charge-Offs) Recoveries	-3.6	-5.8
Provision for Credit Losses	-0.2	2.2
Balance at End of Period	$122.40	$136.30

Allowances_for_Credit_Losses_a

Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$245.20	$242.20
Evaluated for Inherent Impairment	32,385	31,398
Total Loans and Leases	32,630.20	31,640.20
Specifically Evaluated for Impairment	8.4	6.6
Evaluated for Inherent Impairment	250.6	260.4
Allowance Assigned to Loans and Leases	259	267
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	27.7	28.9
Total Allowance for Credit Losses	286.7	295.9	309.4	307.9
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	85.3	78.9
Evaluated for Inherent Impairment	15,406	14,274.70
Total Loans and Leases	15,491.30	14,353.60
Specifically Evaluated for Impairment	6.8	5.8
Evaluated for Inherent Impairment	132.7	138
Allowance Assigned to Loans and Leases	139.5	143.8
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	24.8	25.9
Total Allowance for Credit Losses	164.3	169.7	173.1	168
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	159.9	163.3
Evaluated for Inherent Impairment	16,979	17,123.30
Total Loans and Leases	17,138.90	17,286.60
Specifically Evaluated for Impairment	1.6	0.8
Evaluated for Inherent Impairment	117.9	122.4
Allowance Assigned to Loans and Leases	119.5	123.2
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	2.9	3
Total Allowance for Credit Losses	$122.40	$126.20	$136.30	$139.90

Pledged_Assets_Additional_Info

Pledged Assets - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$33,200,000,000		$32,300,000,000
Fair value of accepted collateral	1,000,000,000		1,000,000,000
Amount of securities repledged or sold collateral	0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,500,000,000	1,200,000,000
US Government Corporations and Agencies Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	23,900,000,000		23,100,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	113,000,000		122,900,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	9,200,000,000		9,100,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	5,600,000,000		5,900,000,000
Available for Sale Investment Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$611,400,000		$884,800,000

Changes_by_Reporting_Segment_i

Changes by Reporting Segment in Carrying Amounts of Goodwill, including Effect of Foreign Exchange Rates on Non-U.S.-Dollar-Denominated Balances (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Goodwill [Line Items]
Total Goodwill	$527.30	$533.20
Corporate & Institutional Services
Goodwill [Line Items]
Total Goodwill	456	461.8
Wealth Management
Goodwill [Line Items]
Total Goodwill	$71.30	$71.40

Other_Intangible_Assets_Subjec

Other Intangible Assets Subject to Amortization (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$182.30	$189.50
Less: Accumulated Amortization	129.7	129.5
Net Book Value	$52.60	$60

Goodwill_and_Other_Intangibles2

Goodwill and Other Intangibles - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Goodwill and Intangible Assets Disclosure [Line Items]
Estimated future amortization expense for 2015	$6.30
Estimated future amortization expense for 2016	8.4
Estimated future amortization expense for 2017	8.3
Estimated future amortization expense for 2018	7.7
Estimated future amortization expense for 2019	7.5
Other Intangible Assets
Goodwill and Intangible Assets Disclosure [Line Items]
Amortization expense related to other intangible assets	$4.60	$4.90

Earnings_Contribution_of_North

Earnings Contribution of Northern Trust's Reporting Segments (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$727.50		$679.50
Foreign Exchange Trading Income	71.6		50.1
Other Noninterest Income	74.8		65.2
Net Interest Income	266.8	[1]	254.4	[1]
Revenue	1,140.70	[1]	1,049.20	[1]
Provision for Credit Losses	-4.5		3
Noninterest Expense	789		768
Income before Income Taxes	356.2	[1]	278.2	[1]
Provision for Income Taxes	125.5	[1]	96.8	[1]
Net Income	230.7		181.4
Percentage of Consolidated Net Income	100.00%		100.00%
Average Assets	107,513.20		100,243.50
Corporate & Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	407.3		379.2
Foreign Exchange Trading Income	67.5		48.1
Other Noninterest Income	41.9		44.4
Net Interest Income	96	[1]	73.7	[1]
Revenue	612.7	[1]	545.4	[1]
Provision for Credit Losses	-2.2		1.2
Noninterest Expense	434.9		423.5
Income before Income Taxes	180	[1]	120.7	[1]
Provision for Income Taxes	57.3	[1]	34.8	[1]
Net Income	122.7		85.9
Percentage of Consolidated Net Income	53.00%		47.00%
Average Assets	39,762.40		57,252.40
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	320.2		300.3
Foreign Exchange Trading Income	4.1		2
Other Noninterest Income	28.8		22.7
Net Interest Income	138.3	[1]	135	[1]
Revenue	491.4	[1]	460	[1]
Provision for Credit Losses	-2.3		1.8
Noninterest Expense	321.9		318.8
Income before Income Taxes	171.8	[1]	139.4	[1]
Provision for Income Taxes	64.6	[1]	52.6	[1]
Net Income	107.2		86.8
Percentage of Consolidated Net Income	47.00%		48.00%
Average Assets	24,308.80		22,528.70
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	4.1		-1.9
Net Interest Income	32.5	[1]	45.7	[1]
Revenue	36.6	[1]	43.8	[1]
Noninterest Expense	32.2		25.7
Income before Income Taxes	4.4	[1]	18.1	[1]
Provision for Income Taxes	3.6	[1]	9.4	[1]
Net Income	0.8		8.7
Percentage of Consolidated Net Income			5.00%
Average Assets	$43,442		$20,462.40
[1]	Stated on an FTE basis. Total consolidated includes FTE adjustments of $6.2 million for 2015 and $8.7 million for 2014.

Earnings_Contribution_of_North1

Earnings Contribution of Northern Trust's Reporting Segments (Parenthetical) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$6.20	$8.70

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended	12 Months Ended
	Jan. 21, 2015	Mar. 31, 2015	Dec. 31, 2014
Stockholders Equity Note [Line Items]
Preferred stock, shares authorized		10,000,000	10,000,000
Preferred stock, without par value		$0	$0
Preferred stock		$388,500,000	$388,500,000
Stock repurchase, shares		1,600,000
Average price paid per share for common stock repurchased		$68.93
Stock repurchase, aggregate cost		107,200,000
Upper Limit
Stockholders Equity Note [Line Items]
Stock repurchase authorized value		15,000,000
Between April 2015 and June 2016 | Upper Limit
Stockholders Equity Note [Line Items]
Stock Repurchase Authorized Amount		675,000,000
Series C Preferred Stock
Stockholders Equity Note [Line Items]
Stock issued during period		16,000,000	16,000,000
Per share ownership interest, percentage		0.10%	0.10%
Preferred stock, without par value		$0	$0
Preferred stock, liquidation preference		25,000	25,000
Depository shares, liquidation preference per share		$25	$25
Preferred stock		$388,500,000	$388,500,000
Preferred stock, annual dividend rate		5.85%
Preferred Stock, Dividends Per Share, Declared	$365.63
Dividends Payable, Date to be Paid	1-Apr-15
Dividends Payable, Date of Record	15-Mar-15

Components_of_Accumulated_Othe

Components of Accumulated Other Comprehensive Income (Loss) (AOCI) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Ending Balance	($266.40)	($236.10)
Net Change	53.3	8.2
Beginning Balance	-319.7	-244.3
Net Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Ending Balance	81.7	10.5
Net Change	54.1	4.5
Beginning Balance	27.6	6
Net Unrealized Gains (Losses) on Cash Flow Hedges Designations
Accumulated Other Comprehensive Income (Loss) [Line Items]
Ending Balance	-5.8	4.5
Net Change	-1.1	1.6
Beginning Balance	-4.7	2.9
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Ending Balance	-7.6	5.8
Net Change	-5.9	-1.3
Beginning Balance	-1.7	7.1
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Ending Balance	-334.7	-256.9
Net Change	6.2	3.4
Beginning Balance	($340.90)	($260.30)

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Loss) Changes During Period (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss)	$53.30	$8.20
Net Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	86.7	7.3
Tax Effect	-32.6	-2.8
Other Comprehensive Income (Loss)	54.1	4.5
Net Unrealized Gains (Losses) on Securities Available for Sale | Noncredit-Related Unrealized Losses on Securities OTTI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax		1.5
Tax Effect		-0.6
Other Comprehensive Income (Loss)		0.9
Net Unrealized Gains (Losses) on Securities Available for Sale | Other Unrealized Gains (Losses) on Securities Available-for-Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	86.8	5.7
Tax Effect	-32.7	-2.2
Other Comprehensive Income (Loss)	54.1	3.5
Net Unrealized Gains (Losses) on Securities Available for Sale | Reclassification Adjustment for (Gains) Losses Included in Net Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-0.1	0.1
Tax Effect	0.1
Other Comprehensive Income (Loss)		0.1
Net Unrealized Gains (Losses) on Cash Flow Hedges Designations
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-3.7	2.6
Tax Effect	2.6	-1
Other Comprehensive Income (Loss)	-1.1	1.6
Net Unrealized Gains (Losses) on Cash Flow Hedges Designations | Reclassification Adjustment for (Gains) Losses Included in Net Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	1	0.2
Tax Effect	-0.4	-0.1
Other Comprehensive Income (Loss)	0.6	0.1
Net Unrealized Gains (Losses) on Cash Flow Hedges Designations | Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-4.7	2.4
Tax Effect	3	-0.9
Other Comprehensive Income (Loss)	-1.7	1.5
Net Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	25.5	-4.7
Tax Effect	-31.4	3.4
Other Comprehensive Income (Loss)	-5.9	-1.3
Net Foreign Currency Adjustments | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-90.1	6.5
Tax Effect	12.2	-0.9
Other Comprehensive Income (Loss)	-77.9	5.6
Net Foreign Currency Adjustments | Long-Term Intra-Entity Foreign Currency Transaction Losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-1	-0.6
Tax Effect	0.3	0.2
Other Comprehensive Income (Loss)	-0.7	-0.4
Net Foreign Currency Adjustments | Net Investment Hedge Gains (Losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	116.6	-10.6
Tax Effect	-43.9	4.1
Other Comprehensive Income (Loss)	72.7	-6.5
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	9.5	6.4
Tax Effect	-3.3	-3
Other Comprehensive Income (Loss)	6.2	3.4
Net Pension and Other Postretirement Benefit Adjustments | Reclassification Adjustment for (Gains) Losses Included in Net Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	9.5	6.4
Tax Effect	-3.3	-3
Other Comprehensive Income (Loss)	$6.20	$3.40

Reclassification_Adjustments_o

Reclassification Adjustments out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Available for Sale Investment Securities | Realized (Gains) Losses on Securities Available for Sale | Investment Security Gains (Losses), net
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	($0.10)
CASH FLOW HEDGES | Foreign Exchange Contracts | Other Operating Income/Expense
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	1
Pension and Other Postretirement Benefit Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	9.5
Pension and Other Postretirement Benefit Adjustments | Employee Benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of Net Actuarial (Gains) Losses	9.6
Amortization of Prior Service Cost	($0.10)

Computations_of_Net_Income_Per

Computations of Net Income Per Common Share (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	233,381,168	237,208,151
Net Income	$230.70	$181.40
Less: Dividends on Preferred Stock	5.9
Net Income Applicable to Common Stock	224.8	181.4
Less: Earnings Allocated to Participating Securities	3.7	2.9
Earnings Allocated to Common Shares Outstanding	221.1	178.5
Basic Net Income Per Common Share	$0.95	$0.75
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	233,381,168	237,208,151
Plus: Dilutive Effect of Share-based Compensation	1,907,527	1,842,563
Average Common and Potential Common Shares	235,288,695	239,050,714
Earnings Allocated to Common and Potential Common Shares	$221.10	$178.50
Diluted Net Income Per Common Share	$0.94	$0.75

Computations_of_Net_Income_Per1

Computations of Net Income Per Common Share (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Earnings Per Share, Basic and Diluted, by Common Class [Line Items]
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	1,504,850	1,997,337

Components_of_Net_Interest_Inc

Components of Net Interest Income (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Interest Income
Loans and Leases	$183.90		$182.90
Securities - Taxable	76.6		69.5
Securities - Non-Taxable	1.3		2.2
Interest-Bearing Due from and Deposits with Banks	26.6	[1]	32.1	[1]
Federal Reserve Deposits and Other	10.4		8.7
Total Interest Income	298.8		295.4
Interest Expense
Deposits	16.7		20.1
Federal Funds Purchased	0.2		0.3
Securities Sold Under Agreements to Repurchase	0.1		0.1
Other Borrowings	1.2		0.7
Senior Notes	11.6		17.5
Long-Term Debt	7.8		10.4
Floating Rate Capital Debt	0.6		0.6
Total Interest Expense	38.2		49.7
Net Interest Income	$260.60		$245.70
[1]	Interest income for the three months ended March 31, 2015, was earned on cash and due from banks of $2.2 billion and interest-bearing deposits with banks of $15.4 billion as of March 31, 2015.

Components_of_Net_Interest_Inc1

Components of Net Interest Income (Parenthetical) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Net Interest Income [Line Items]
Cash and Due from Banks	$4,074.80	$3,050.60	$3,676.30	$3,162.40
Interest-Bearing Deposits with Banks	15,432.30	14,928.30
Bank Time Deposits
Net Interest Income [Line Items]
Cash and Due from Banks	$2,200

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Taxes [Line Items]
Income tax expense	$119.30	$88.10
Effective tax rate	34.10%	32.70%

Components_of_Companys_Net_Per

Components of Company's Net Periodic Benefit Cost (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
U.S. Plan
Defined Benefit plan Net periodic benefit cost
Service Cost	$9.50	$8.20
Interest Cost	11.2	11.1
Expected Return on Plan Assets	-24.1	-24.4
Amortization
Net Actuarial Loss (Gain)	7.4	5.4
Prior Service Cost	-0.1	-0.1
Net Periodic Expense	3.9	0.2
Non U.S. Plans
Defined Benefit plan Net periodic benefit cost
Interest Cost	1.4	1.7
Expected Return on Plan Assets	-1.5	-1.7
Amortization
Net Actuarial Loss (Gain)	0.4	-0.4
Net Periodic Expense	0.3	-0.4
Supplemental Plan
Defined Benefit plan Net periodic benefit cost
Service Cost	0.9	0.8
Interest Cost	1.3	1.2
Amortization
Net Actuarial Loss (Gain)	1.8	1.5
Prior Service Cost		0.1
Net Periodic Expense	4	3.6
Postretirement Health Care Plan
Defined Benefit plan Net periodic benefit cost
Interest Cost	0.3	0.4
Amortization
Net Actuarial Loss (Gain)		-0.1
Net Periodic Expense	$0.30	$0.30

ShareBased_Compensation_Plans_1

Share-Based Compensation Plans - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	$22.90	$23.10
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options granted	453,862
Stock granted, total grant-date fair value	8.5
Share-Based Compensation Expense	6	6.4
Stock Options | Retirement Eligible Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	4.3	3
Restricted Stock Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock granted, total grant-date fair value	62.2
Number of stock units granted	886,452
Share-Based Compensation Expense	13.5	14.1
Restricted Stock Unit Awards | Fully Vested Restricted Stock Unit Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	1.5	1.4
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock granted, total grant-date fair value	19.1
Number of stock units granted	272,319
Share-Based Compensation Expense	$3.40	$2.60

Compensation_Expense_for_Share

Compensation Expense for Share-Based Payment Arrangements and Associated Tax Impacts (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	$22.90	$23.10
Tax Benefits Recognized	8.6	8.7
Restricted Stock Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	13.5	14.1
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	6	6.4
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	$3.40	$2.60

Variable_Interest_Entities_Add

Variable Interest Entities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Tax Credit Structures
Variable Interest Entity [Line Items]
Carrying amounts	$198,200,000		$208,900,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	0
Liabilities related to undrawn commitments	13,900,000		15,600,000
Affordable housing tax credits and other tax benefits	13,200,000	14,800,000
Leveraged Leasing Transactions
Variable Interest Entity [Line Items]
Carrying amounts	516,400,000		547,600,000
Exposure to loss from liquidity arrangements and obligations to purchase assets	$0
Leveraged Leasing Transactions | Lower Limit
Variable Interest Entity [Line Items]
Funding of asset's cost via an equity interest	20.00%	20.00%
Funding of asset's cost via third party non recourse debt holders	70.00%	70.00%
Leveraged Leasing Transactions | Upper Limit
Variable Interest Entity [Line Items]
Funding of asset's cost via an equity interest	30.00%	30.00%
Funding of asset's cost via third party non recourse debt holders	80.00%	80.00%

Contingent_Liabilities_Additio

Contingent Liabilities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Mar. 31, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Line Items]
Commitments and Letters of Credit		$35,600,000,000	$35,100,000,000
Legally binding commitments		477,200,000	481,400,000
Standby letters of credit outstanding		4,400,000,000	4,500,000,000
Minimum securities lending related collateral percentage		100.00%
Possible loss in future periods in excess of accrual		125,000,000
Legal Settlement Charge	19,200,000
Remaining Visa shares held by Northern Trust, original cost basis		0
Indemnification Agreement
Commitments and Contingencies Disclosure [Line Items]
Collateralized securities loaned, subject to indemnification		105,500,000,000	98,100,000,000
Credit loss liability		$0	$0

Derivative_Financial_Instrumen2

Derivative Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Derivative [Line Items]
Derivative assets recorded on the consolidated balance sheet, reduced as a result of master netting agreements	$2,400,000,000		$1,900,000,000
Derivative liabilities recorded on the consolidated balance sheet, reduced as a result of master netting agreements	2,400,000,000		1,900,000,000
Reduction in derivative assets due to cash collateral received from derivative counterparties	440,800,000		315,800,000
Reduction in derivative liabilities due to cash collateral deposited with derivative counterparties	1,100,000,000		1,200,000,000
This amount represents additional cash collateral received from derivative counterparties not offset against derivative assets	29,400,000		19,600,000
This amount represents additional cash collateral deposited with derivative counterparties not offset against derivative liabilities	124,300,000		153,200,000
Securities posted as collateral	27,300,000		27,400,000
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position	245,400,000		299,500,000
Collateral deposited with derivative counterparties for derivatives instruments with credit-risk-related contingent features that are in a liability position	201,700,000		272,900,000
Termination payments that could have been required for derivative instruments with credit-risk-related contingent features	43,700,000		26,600,000
Ineffectiveness or changes in the fair value of hedged items recognized in earnings for fair value hedges	0	0
Ineffectiveness recognized in earnings for cash flow hedges	0	0
Maximum length of time hedged cash flow hedges	23 months
Ineffectiveness from net investment hedges	0	0
Foreign Currency Denominate
Derivative [Line Items]
Estimated net gain (loss) to be reclassified into net income within the next twelve months relating to cash flow hedges	11,100,000	11,100,000
Available for Sale Investment Securities
Derivative [Line Items]
Estimated net gain (loss) to be reclassified into net income within the next twelve months relating to cash flow hedges	4,100,000	4,100,000
Net Investment Hedges
Derivative [Line Items]
Sterling denominated debt designated as hedges of foreign exchange risk			$243,900,000
Client Related and Trading
Derivative [Line Items]
Percentage of derivatives outstanding related to client-related and trading activities	97.00%		97.00%

Notional_and_Fair_Value_Amount

Notional and Fair Value Amounts of Client-related and Trading Derivative Financial Instruments (Detail) (Client Related and Trading, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Derivative [Line Items]
Notional Value	$276,139.10	$262,922.50
Fair Value Asset	4,854.30	4,255
Fair Value Liability	4,805.70	4,173.30
Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	270,476.90	257,568.70
Fair Value Asset	4,731.20	4,149.50
Fair Value Liability	4,686.40	4,072
Interest Rate Contracts
Derivative [Line Items]
Notional Value	5,662.20	5,353.80
Fair Value Asset	123.1	105.5
Fair Value Liability	$119.30	$101.30

Gains_and_Losses_Recorded_in_C

Gains and Losses Recorded in Consolidated Statements of Income Related to Client Related and Trading Derivative Financial Instruments (Detail) (Client Related and Trading, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$76.50	$52
Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	71.6	50.1
Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$4.90	$1.90

Types_and_Classifications_of_D

Types and Classifications of Derivative Instruments Designated as Hedges (Detail) (Asset And Liability Management, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Derivative [Line Items]
Notional Value	$7,015.40	$6,884.80
Fair Value Asset	215.4	251.7
Fair Value Liability	53.6	48.7
FAIR VALUE HEDGES | Senior Notes and Long- Term Subordinated Debt | Interest Rate Swaps
Derivative [Line Items]
Notional Value	1,250	1,250
Fair Value Asset	138.4	112.8
Fair Value Liability	2	2
FAIR VALUE HEDGES | Available for Sale Investment Securities | Interest Rate Swaps
Derivative [Line Items]
Notional Value	2,815.70	2,859.50
Fair Value Asset	6	12.7
Fair Value Liability	26.9	28.5
CASH FLOW HEDGES | Forecasted Foreign Currency Denominated Transactions | Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	340.5	344.9
Fair Value Asset	8.8	6
Fair Value Liability	24.4	14.8
CASH FLOW HEDGES | Available for Sale Investment Securities | Interest Rate Swaps
Derivative [Line Items]
Notional Value	10	10
CASH FLOW HEDGES | Available for Sale Investment Securities | Interest Rate Option Contracts
Derivative [Line Items]
Notional Value	725	625
Fair Value Asset	4.5	1.3
Net Investment Hedges | Net Investments in Non-U.S. Affiliates | Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	1,874.20	1,795.40
Fair Value Asset	57.7	118.9
Fair Value Liability	$0.30	$3.40

Gains_and_Losses_Recorded_in_C1

Gains and Losses Recorded in Consolidated Statements of Income Related to Fair Value Hedges (Detail) (Asset And Liability Management, FAIR VALUE HEDGES, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$12.50	$20.40
Available for Sale Investment Securities | Interest Rate Swaps | Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	-17.3	-12.1
Senior Notes and Long- Term Subordinated Debt | Interest Rate Swaps | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$29.80	$32.50

Cash_Flow_Hedge_Derivative_Gai

Cash Flow Hedge Derivative Gains and Losses Recognized in AOCI and the Amounts Reclassified to Earnings (Detail) (Asset And Liability Management, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Net Gain/(Loss) Recognized in AOCI	($7.90)	$2.40
Net Gain/(Loss) Reclassified from AOCI to Net Income	-2.1	0.6
Interest Rate Option Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Net Gain/(Loss) Recognized in AOCI	3.2
Net Gain/(Loss) Reclassified from AOCI to Net Income	1.1
Net Gain/(Loss) Reclassified from AOCI to Net Income | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Other Operating Income	-1.6	0.8
Other Operating Expense	-0.5	-0.2
Net Gain/(Loss) Reclassified from AOCI to Net Income | Interest Rate Option Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Interest Income	$1.10

Net_Investment_Hedge_Gains_and

Net Investment Hedge Gains and Losses Recognized in AOCI (Detail) (Net Investment Hedges, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative [Line Items]
Hedging Instrument Gain/(Loss) Recognized in AOCI (Before Tax)	$116.60	($10.60)
Foreign Exchange Contracts
Derivative [Line Items]
Hedging Instrument Gain/(Loss) Recognized in AOCI (Before Tax)	111.6	-9
Sterling Denominated Subordinated Debt
Derivative [Line Items]
Hedging Instrument Gain/(Loss) Recognized in AOCI (Before Tax)	$5	($1.60)

Types_of_Risk_Management_Deriv

Types of Risk Management Derivative Instruments Not Formally Designated as Hedges and Their Notional Amounts and Fair Values (Detail) (Nondesignated, Foreign Exchange Contracts, USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Nondesignated | Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	$257	$246.30
Fair Value Asset	0.9	0.8
Fair Value Liability	$3	$5.30

Gains_and_Losses_Recorded_in_C2

Gains and Losses Recorded in Consolidated Statements of Income related to Derivative Instruments Not Formally Designated as Hedges (Detail) (Nondesignated, Foreign Exchange Contracts, Others Operating Income, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Nondesignated | Foreign Exchange Contracts | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	($8.70)	($0.70)

Offsetting_of_Derivative_Asset

Offsetting of Derivative Assets and of Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheets (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	$1,000	[1]	$1,000	[1]
Net Amounts Presented	1,000	[1]	1,000	[1]
Gross Amounts Not Offset	1,000	[1]	1,000	[1]
Derivatives
Derivative [Line Items]
Gross Recognized Assets	5,070.60	[2]	4,507.50	[2]
Gross Amounts Offset	2,884.20	[2]	2,257.10	[2]
Net Amounts Presented	2,186.40	[2]	2,250.40	[2]
Net Amount	2,186.40	[2],[3]	2,250.40	[2],[3]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	3,977.10	[2]	3,675.10	[2]
Gross Amounts Offset	2,884.20	[2]	2,257.10	[2]
Net Amounts Presented	1,092.90	[2]	1,418	[2]
Net Amount	1,092.90	[2],[3]	1,418	[2],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	1,093.50	[2]	832.4	[2]
Net Amounts Presented	1,093.50	[2]	832.4	[2]
Net Amount	1,093.50	[2],[3]	832.4	[2],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	3,705.10	[2]	3,442.80	[2]
Gross Amounts Offset	2,360.50	[2]	1,889.80	[2]
Net Amounts Presented	1,344.60	[2]	1,553	[2]
Net Amount	1,344.60	[2],[3]	1,553	[2],[3]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	197.9	[2]	183.9	[2]
Gross Amounts Offset	31.8	[2]	32.1	[2]
Net Amounts Presented	166.1	[2]	151.8	[2]
Net Amount	166.1	[2],[3]	151.8	[2],[3]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	74.1	[2]	48.4	[2]
Gross Amounts Offset	25.7	[2]	13.1	[2]
Net Amounts Presented	48.4	[2]	35.3	[2]
Net Amount	48.4	[2],[3]	35.3	[2],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	25.4	[2]	6.3	[2]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$440.80	[2]	$315.80	[2]
[1]	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheets. Federal funds sold totaled $113.9 million and $62.7 million as of March 31, 2015, and December 31, 2014, respectively.
[2]	Derivative assets are reported in other assets in the consolidated balance sheets. Other assets (excluding derivative assets) totaled $3,177.3 million and $3,614.7 million as of March 31, 2015, and December 31, 2014, respectively.
[3]	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.

Offsetting_of_Derivative_Asset1

Offsetting of Derivative Assets and of Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheets (Parenthetical) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Other Assets (excluding Derivative Assets)	$3,177,300,000	$3,614,700,000
Federal Funds Sold	113,900,000	62,700,000
Cash collateral possessed that was not offset	$0	$0

Offsetting_of_Derivative_Liabi

Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified
Derivative [Line Items]
Net Amounts Presented	$613.20		$885.10
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	613.2		885.1
Net Amounts Presented	613.2		885.1
Gross Amounts Not Offset	613.2		885.1
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	4,862.30	[1]	4,227.30	[1]
Gross Amounts Offset	3,510.90	[1]	3,173.30	[1]
Net Amounts Presented	1,351.40	[1]	1,054	[1]
Net Amount	1,351.40	[1],[2]	1,054	[1],[2]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	4,043.30	[1]	3,562.80	[1]
Gross Amounts Offset	3,510.90	[1]	3,173.30	[1]
Net Amounts Presented	532.4	[1]	389.5	[1]
Net Amount	532.4	[1],[2]	389.5	[1],[2]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	819	[1]	664.5	[1]
Net Amounts Presented	819	[1]	664.5	[1]
Net Amount	819	[1],[2]	664.5	[1],[2]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	3,895.10	[1]	3,431	[1]
Gross Amounts Offset	2,360.50	[1]	1,889.80	[1]
Net Amounts Presented	1,534.60	[1]	1,541.20	[1]
Net Amount	1,534.60	[1],[2]	1,541.20	[1],[2]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	122.5	[1]	118.7	[1]
Gross Amounts Offset	31.8	[1]	32.1	[1]
Net Amounts Presented	90.7	[1]	86.6	[1]
Net Amount	90.7	[1],[2]	86.6	[1],[2]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	25.7	[1]	13.1	[1]
Gross Amounts Offset	25.7	[1]	13.1	[1]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Amounts Offset	25.4	[1]	6.3	[1]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Amounts Offset	$1,067.50	[1]	$1,232	[1]
[1]	Derivative liabilities are reported in other liabilities in the consolidated balance sheets. Other liabilities (excluding derivative liabilities) totaled $3,185.1 million and $2,794.1 million as of March 31, 2015, and December 31, 2014, respectively.
[2]	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheets that could have been used to offset the net amounts presented in the consolidated balance sheets as of March 31, 2015, and December 31, 2014.

Offsetting_of_Derivative_Liabi1

Offsetting of Derivative Liabilities and of Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Parenthetical) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Other liabilities	$3,185,100,000	$2,794,100,000
Cash collateral placed that was not offset	$0	$0