Northern Trust (NTRS) 10-Q 25 Oct 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	30-Sep-13
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3
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	238,983,842

Consolidated_Balance_Sheet

Consolidated Balance Sheet (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Assets
Cash and Due from Banks	$2,690.70	$3,752.70
Federal Funds Sold and Securities Purchased under Agreements to Resell	534.6	60.8
Interest-Bearing Deposits with Banks	17,383.90	18,803.50
Federal Reserve Deposits and Other Interest-Bearing	8,452.80	7,619.70
Securities
Available for Sale	27,501.60	28,643.50
Held to Maturity (Fair value of $3,053.2 and $2,394.8)	3,053.60	2,382
Trading Account	9.6	8
Total Securities	30,564.80	31,033.50
Loans and Leases
Commercial	12,481.20	12,897.20
Personal	16,583.60	16,607.30
Total Loans and Leases	29,064.80	29,504.50
Allowance for Credit Losses Assigned to Loans and Leases	-287.2	-297.9
Buildings and Equipment	444.3	469.9
Client Security Settlement Receivables	1,630.20	2,049.10
Goodwill	537.7	537.8
Other Assets	4,952.90	3,930.20
Total Assets	95,969.50	97,463.80
Deposits
Demand and Other Noninterest-Bearing	15,990.10	20,519
Savings and Money Market	13,802.60	15,189.70
Savings Certificates and Other Time	1,939.60	2,466.10
Non U.S. Offices - Noninterest-Bearing	1,412.20	3,512.80
- Interest-Bearing	45,017.20	39,720.20
Total Deposits	78,161.70	81,407.80
Federal Funds Purchased	1,053.60	780.2
Securities Sold Under Agreements to Repurchase	469	699.8
Other Borrowings	1,804.70	367.4
Senior Notes	1,996.50	2,405.80
Long-Term Debt	993.5	1,421.60
Floating Rate Capital Debt	277.1	277
Other Liabilities	3,396.30	2,577.20
Total Liabilities	88,152.40	89,936.80
Stockholders' Equity
Common Stock, $1.66 2/3 Par Value; Authorized 560,000,000 shares; Outstanding shares of 238,983,842 and 238,914,988	408.6	408.6
Additional Paid-In Capital	1,031.80	1,012.70
Retained Earnings	7,040.10	6,702.70
Accumulated Other Comprehensive Loss	-337.9	-283
Treasury Stock (6,187,682 and 6,256,536 shares, at cost)	-325.5	-314
Total Stockholders' Equity	7,817.10	7,527
Total Liabilities and Stockholders' Equity	$95,969.50	$97,463.80

Consolidated_Balance_Sheet_Par

Consolidated Balance Sheet (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, except Share data, unless otherwise specified
Held to Maturity, Fair value	$3,053.20	$2,394.80
Total Loans and Leases, unearned income	$273	$297.90
Common Stock, Par Value	$1.67	$1.67
Common Stock, Authorized	560,000,000	560,000,000
Common Stock, Outstanding	238,983,842	238,914,988
Treasury Stock, shares	6,187,682	6,256,536

Consolidated_Statement_Of_Inco

Consolidated Statement Of Income (USD $)	3 Months Ended				9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Noninterest Income
Trust, Investment and Other Servicing Fees	$648		$601.90		$1,936		$1,782.90
Foreign Exchange Trading Income	62.8		44		193.6		165.3
Treasury Management Fees	17.6		16.3		51.5		51
Security Commissions and Trading Income	16.8		17.9		53.4		53.6
Other Operating Income	67.2		46.6		128.3		119.2
Investment Security Gains (Losses), net	-2.2	[1]	0.2	[1]	-1.9	[1]	-1.7	[1]
Total Noninterest Income	810.2		726.9		2,360.90		2,170.30
Net Interest Income
Interest Income	291.1		323.1		853.1		985.6
Interest Expense	54.1		77.5		169.9		229.5
Net Interest Income	237		245.6		683.2		756.1
Provision for Credit Losses	5		10		15		20
Net Interest Income after Provision for Credit Losses	232		235.6		668.2		736.1
Noninterest Expense
Compensation	324.6		315.7		971.8		951.1
Employee Benefits	63.5		61.3		191		194.3
Outside Services	145.9		126.6		412		388.5
Equipment and Software	95.5		86		279		276.2
Occupancy	43.3		43.8		130		128.2
Other Operating Expense	67.9		63		215.5		199
Total Noninterest Expense	740.7		696.4		2,199.30		2,137.30
Income before Income Taxes	301.5		266.1		829.8		769.1
Provision for Income Taxes	95		87.3		268.2		249.5
Net Income	206.5		178.8		561.6		519.6
Net Income Applicable to Common Stock	$206.50		$178.80		$561.60		$519.60
Per Common Share
Net Income - Basic	$0.85		$0.73		$2.31		$2.13
Net Income - Diluted	$0.84		$0.73		$2.29		$2.12
Average Number of Common Shares Outstanding - Basic	239,930,074		240,237,014		239,614,868		240,740,803
Average Number of Common Shares Outstanding - Diluted	241,330,652		240,697,062		240,857,697		241,205,186
[1]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ 0.4 $ - $ (2.7 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - (0.6 ) - (0.6 ) Other Security Gains (Losses), net (2.2 ) 0.4 (1.9 ) 1.6 Investment Security Gains (Losses), net $ (2.2 ) $ 0.2 $ (1.9 ) $ (1.7 )

Consolidated_Statement_Of_Comp

Consolidated Statement Of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net Income	$206.50	$178.80	$561.60	$519.60
Other Comprehensive Income (Net of Tax and Reclassifications)
Net Unrealized Gains (Losses) on Securities Available for Sale	5.8	35.6	-74.4	67.3
Net Unrealized Gains (Losses) on Cash Flow Hedges	4.3	3.1	1.2	5.5
Foreign Currency Translation Adjustments	-4.4	14.2	-2.8	24.1
Pension and Other Postretirement Benefit Adjustments	7.1	5.3	21.1	32.8
Other Comprehensive Income	12.8	58.2	-54.9	129.7
Comprehensive Income	$219.30	$237	$506.70	$649.30

Consolidated_Statement_Of_Inco1

Consolidated Statement Of Income (Parenthetical) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Changes in Other-Than-Temporary-Impairment (OTTI) Losses		[1]	$0.40	[1]		[1]	($2.70)	[1]
Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI		[2]	-0.6	[2]		[2]	-0.6	[2]
Other Security Gains (Losses), net	-2.2		0.4		-1.9		1.6
Investment Security Gains (Losses), net	($2.20)	[3]	$0.20	[3]	($1.90)	[3]	($1.70)	[3]
[1]	For initial other-than-temporary impairments in the respective 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 other-than-temporary impairments in the respective period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in 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 $ - $ 0.4 $ - $ (2.7 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - (0.6 ) - (0.6 ) Other Security Gains (Losses), net (2.2 ) 0.4 (1.9 ) 1.6 Investment Security Gains (Losses), net $ (2.2 ) $ 0.2 $ (1.9 ) $ (1.7 )

Consolidated_Statement_Of_Chan

Consolidated Statement Of Changes In Stockholders' Equity (USD $)	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
In Millions
Beginning Balance at Dec. 31, 2011		$408.60	$977.50	$6,302.30	($345.60)	($225.50)
Net Unrealized Gains (Losses) on Securities Available for Sale	67.3				67.3
Stock Options and Awards			-32.2			52.3
Net Income	519.6			519.6
Net Unrealized Gains (Losses) on Cash Flow Hedges	5.5				5.5
Stock Options and Awards - Amortization			58.2
Dividends Declared - Common Stock				-214
Stock Purchased						-100
Foreign Currency Translation Adjustments	24.1				24.1
Stock Options and Awards - Tax Benefits			1.3
Pension and Other Postretirement Benefit Adjustments	32.8				32.8
Ending Balance at Sep. 30, 2012	7,532.20	408.6	1,004.80	6,607.90	-215.9	-273.2
Beginning Balance at Dec. 31, 2012	7,527	408.6	1,012.70	6,702.70	-283	-314
Net Unrealized Gains (Losses) on Securities Available for Sale	-74.4				-74.4
Stock Options and Awards			-41.2			175.6
Net Income	561.6			561.6
Net Unrealized Gains (Losses) on Cash Flow Hedges	1.2				1.2
Stock Options and Awards - Amortization			57.4
Dividends Declared - Common Stock				-224.2
Stock Purchased						-187.1
Foreign Currency Translation Adjustments	-2.8				-2.8
Stock Options and Awards - Tax Benefits			2.9
Pension and Other Postretirement Benefit Adjustments	21.1				21.1
Ending Balance at Sep. 30, 2013	$7,817.10	$408.60	$1,031.80	$7,040.10	($337.90)	($325.50)

Consolidated_Statement_Of_Cash

Consolidated Statement Of Cash Flows (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012
Cash Flows from Operating Activities:
Net Income	$561.60		$519.60
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Investment Security (Gains) Losses, net	1.9	[1]	1.7	[1]
Amortization and Accretion of Securities and Unearned Income	25.8		-37.7
Provision for Credit Losses	15		20
Depreciation on Buildings and Equipment	69.5		66.2
(Gains) Losses on Sale of Buildings and Equipment	-32.6
Pension Plan Contribution	-16.4		-12.3
Change in Receivables	-27.9		-35.6
Change in Interest Payable	-14.7		1.2
Change in Collateral With Derivative Counterparties, net	111		-85.4
Other Operating Activities, net	99.7		204.3
Net Cash Provided by (Used in) Operating Activities	960.9		791.2
Cash Flows from Investing Activities:
Net Change in Federal Funds Sold and Securities Purchased under Agreements to Resell	-223.8		-246.5
Change in Interest-Bearing Deposits with Banks	1,150.70		-2,651.40
Net Change in Federal Reserve Deposits and Other Interest-Bearing Assets	-833.2		7,218.10
Purchases of Securities - Held to Maturity	-5,068.80		-1,210.70
Proceeds from Maturity and Redemption of Securities - Held to Maturity	4,515.80		968.2
Purchases of Securities - Available for Sale	-5,333.40		-16,885.10
Proceeds from Sale, Maturity and Redemption of Securities - Available for Sale	6,359.20		19,155.80
Change in Loans and Leases	387.6		-500.5
Purchases of Buildings and Equipment, net	-43.8		-27.9
Proceeds from Sale of Buildings	38.8
Purchases and Development of Computer Software	-203.8		-149.4
Change in Client Settlement Receivables	416.7		-299.9
Other Investing Activities, net	-652		-37.8
Net Cash Provided by (Used in) Investing Activities	510		5,332.90
Cash Flows from Financing Activities:
Change in Deposits	-2,845.70		-5,745.90
Change in Federal Funds Purchased	273.4		-241.7
Change in Securities Sold under Agreements to Repurchase	-230.8		-816.2
Change in Short-Term Other Borrowings	1,382.40		-367.4
Proceeds from Senior Notes and Long-Term Debt			500
Repayments of Senior Notes and Long-Term Debt	-803.2		-722.7
Treasury Stock Purchased	-186.8		-99.9
Net Proceeds from Stock Options	134.1		78.3
Cash Dividends Paid on Common Stock	-146.3		-209.5
Other Financing Activities, net	-2.9		573.5
Net Cash Provided by (Used in) Financing Activities	-2,425.80		-7,051.50
Effect of Foreign Currency Exchange Rates on Cash	-107.1		8.8
Increase (Decrease) in Cash and Due from Banks	-1,062		-918.6
Cash and Due from Banks at Beginning of Year	3,752.70		4,315.30
Cash and Due from Banks at End of Period	2,690.70		3,396.70
Supplemental Disclosures of Cash Flow Information:
Interest Paid	185		228.3
Income Taxes Paid	184.2		164.4
Transfers from Loans to OREO	16.4		36.3
Computer Software
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Amortization of Intangibles	152.5		134.2
Other Intangible Assets
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:
Amortization of Intangibles	$15.50		$15
[1]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ 0.4 $ - $ (2.7 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - (0.6 ) - (0.6 ) Other Security Gains (Losses), net (2.2 ) 0.4 (1.9 ) 1.6 Investment Security Gains (Losses), net $ (2.2 ) $ 0.2 $ (1.9 ) $ (1.7 )

Basis_of_Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2013
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 periods ended September 30, 2013 and 2012, 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. For a description of Northern Trust’s significant accounting policies, refer to Note 1 of the Notes to Consolidated Financial Statements in the 2012 Annual Report to Shareholders.

Recent_Accounting_Pronouncemen

Recent Accounting Pronouncements	9 Months Ended
	Sep. 30, 2013
Recent Accounting Pronouncements	2. Recent Accounting Pronouncements – In July 2013, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2013-10, “Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes.” Since this ASU provides the option to use an additional benchmark interest rate on a prospective basis for qualifying new or redesignated hedging relationships, its adoption effective July 17, 2013 does not materially impact Northern Trust’s consolidated financial position or results of operations.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
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 nine months ended September 30, 2013 or the year ended December 31, 2012.
	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 September 30, 2013, Northern Trust’s available for sale securities portfolio included 783 Level 2 securities with an aggregate market value of $25.9 billion. Of those, 780 securities, with a market value of $25.7 billion, were valued by external pricing vendors. The remaining 3 securities, with an aggregate market value of $158.8 million, were valued consistent with prices of similar securities as there were no vended prices available for these securities. As of December 31, 2012, Northern Trust’s available for sale securities portfolio included 696 Level 2 securities with an aggregate market value of $26.8 billion. Of those, 689 securities, with a market value of $26.5 billion, were valued by external pricing vendors. The remaining 7 securities, with an aggregate market value of $307.1 million, were valued consistent with prices of similar securities as there were no vended prices available for these securities. Trading account securities, which totaled $9.6 million and $8.0 million as of September 30, 2013 and December 31, 2012, 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 consist of acquisition related contingent consideration liabilities. The fair values of these contingent consideration liabilities have been determined using an income-based (discounted cash flow) model that incorporates Northern Trust’s own assumptions about business growth rates and applicable discount rates, which represent unobservable inputs to the model. Significant increases (decreases) in projected growth rates in isolation would result in significantly higher (lower) fair value measurements, while significant increases (decreases) in the discount rate in isolation would result in significantly lower (higher) fair value measurements.
	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 Policy, Corporate Financial Management, and relevant business unit personnel) determine the valuation policies and procedures for Level 3 assets and liabilities. Each business and department represents a component of Northern Trust’s business units, and reports to management of their respective business units. 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 and liabilities as of September 30, 2013.
	Financial Instrument		Fair Value			Valuation		Unobservable Input	Range of Lives
						Technique			and Rates
	Auction Rate Securities		$	98.7 million		Discounted Cash		Remaining lives	2.7 – 8.6 years
						Flow		Discount rates	0.2% – 8.1%
	Contingent Consideration		$	54.0 million		Discounted Cash		Discount rate	10.50%
						Flow		Business growth rates	19% – 35%
	The following presents assets and liabilities measured at fair value on a recurring basis as of September 30, 2013 and December 31, 2012, segregated by fair value hierarchy level.
														Assets/Liabilities
	(In Millions)		Level 1			Level 2			Level 3		Netting			at Fair Value
	30-Sep-13
	Securities
	Available for Sale
	U.S. Government		$	1,524.50		$	—		$	—	$	—		$	1,524.50
	Obligations of States and Political Subdivisions			—			14.4			—		—			14.4
	Government Sponsored Agency			—			17,024.60			—		—			17,024.60
	Corporate Debt			—			3,491.00			—		—			3,491.00
	Covered Bonds			—			1,805.60			—		—			1,805.60
	Supranational, Sovereign and Non-U.S. Agency Bonds			—			721.1			—		—			721.1
	Residential Mortgage-Backed			—			53.5			—		—			53.5
	Other Asset-Backed			—			2,535.70			—		—			2,535.70
	Auction Rate			—			—			98.7		—			98.7
	Other			—			232.5			—		—			232.5
	Total Available for Sale			1,524.50			25,878.40			98.7		—			27,501.60
	Trading Account			—			9.6			—		—			9.6
	Total Available for Sale and Trading			1,524.50			25,888.00			98.7		—			27,511.20
	Other Assets
	Derivative Assets
	Foreign Exchange Contracts			—			3,179.10			—		—			3,179.10
	Interest Rate Contracts			—			242.8			—		—			242.8
	Total Derivative Assets			—			3,421.90			—		(2,018.3	)		1,403.60
	Other Liabilities
	Derivative Liabilities
	Foreign Exchange Contracts			—			3,202.00			—		—			3,202.00
	Interest Rate Contracts			—			173			—		—			173
	Credit Default Swaps			—			—			—		—			—
	Total Derivative Liabilities			—			3,375.00			—		(2,213.7	)		1,161.30
	Contingent Consideration		$	—		$	—		$	54	$	—		$	54
	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 September 30, 2013, derivative assets and liabilities shown above also include reductions of $300.5 million and $495.9 million, 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
	31-Dec-12
	Securities
	Available for Sale
	U.S. Government		$	1,784.60		$	—		$	—	$	—		$	1,784.60
	Obligations of States and Political Subdivisions			—			14.1			—		—			14.1
	Government Sponsored Agency			—			18,638.80			—		—			18,638.80
	Corporate Debt			—			2,618.40			—		—			2,618.40
	Covered Bonds			—			1,748.00			—		—			1,748.00
	Supranational Bonds			—			1,060.70			—		—			1,060.70
	Residential Mortgage-Backed			—			92			—		—			92
	Other Asset-Backed			—			2,283.90			—		—			2,283.90
	Auction Rate			—			—			97.8		—			97.8
Other			—			305.2			—		—			305.2
Total Available for Sale			1,784.60			26,761.10			97.8		—			28,643.50
Trading Account			—			8			—		—			8
Total Available for Sale and Trading			1,784.60			26,769.10			97.8		—			28,651.50
Other Assets
Derivative Assets
Foreign Exchange Contracts			—			1,756.60			—		—			1,756.60
Interest Rate Contracts			—			310.3			—		—			310.3
Total Derivative Assets			—			2,066.90			—		(1,101.1	)		965.8
Other Liabilities
Derivative Liabilities
Foreign Exchange Contracts			—			1,772.70			—		—			1,772.70
Interest Rate Contracts			—			249.3			—		—			249.3
Credit Default Swaps			—			1			—		—			1
Total Derivative Liabilities			—			2,023.00			—		(1,407.5	)		615.5
Contingent Consideration		$	—		$	—		$	50.1	$	—		$	50.1
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, 2012, derivative assets and liabilities shown above also include reductions of $118.6 million and $425.0 million, 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 and nine months ended September 30, 2013 and 2012.
Level 3 Assets (In Millions)		Auction Rate Securities
Three Months Ended September 30,		2013			2012
Fair Value at July 1		$	99.1		$	105.2
Total Gains and (Losses):
Included in Earnings (1)			—			0.3
Included in Other Comprehensive Income (2)			0.3			(1.1	)
Purchases, Issues, Sales, and Settlements
Sales			—			(0.1	)
Settlements			(0.7	)		(4.8	)
Fair Value at September 30		$	98.7		$	99.5
Level 3 Assets (In Millions)		Auction Rate Securities
Nine Months Ended September 30,		2013			2012
Fair Value at January 1		$	97.8		$	178.3
Total Gains and (Losses):
Included in Earnings (1)			0.1			(22.0	)
Included in Other Comprehensive Income (2)			3			4.9
Purchases, Issues, Sales, and Settlements
Sales			—			(54.7	)
Settlements			(2.2	)		(7.0	)
Fair Value at September 30		$	98.7		$	99.5
-1	Realized gains for the three month period ended September 30, 2012 of $0.3 million represent gains from redemptions by issuers. Realized gains for the nine month period ended September 30, 2013 of $0.1 million represent gains from redemptions by issuers. Realized losses for the nine month period ended September 30, 2012 of $22.0 million include $20.8 million of losses from sales of securities and $1.6 million of impairment losses, partially offset by $0.4 million of gains from redemptions by issuers. Gains on redemptions are recorded in interest income and sales and impairment losses are recorded in investment security gains (losses), within the consolidated statement of income.
-2	Unrealized losses related to auction rate securities are included in net unrealized gains (losses) on securities available for sale within the consolidated statement of comprehensive income.
Level 3 Liabilities (In Millions)		Contingent Consideration
Three Months Ended September 30,		2013			2012
Fair Value at July 1		$	52.7		$	57.2
Total (Gains) and Losses:
Included in Earnings (1)			1.3			0.1
Included in Other Comprehensive Income (2)						(0.2	)
Purchases, Issues, Sales, and Settlements
Settlements			—			(8.2	)
Fair Value at September 30		$	54		$	48.9
Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held September 30 (1)		$	1.3		$	1.3
Level 3 Liabilities (In Millions)		Contingent Consideration
Nine Months Ended September 30,		2013			2012
Fair Value at January 1		$	50.1		$	56.8
Total (Gains) and Losses:
Included in Earnings (1)			3.9			0.8
Included in Other Comprehensive Income (2)			—			(0.5	)
Purchases, Issues, Sales, and Settlements
Settlements			—			(8.2	)
Fair Value at September 30		$	54		$	48.9
Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held at September 30 (1)		$	3.9		$	3.5
-1	Gains (losses) are recorded in other operating income (expense) within the consolidated statement of income.
-2	Unrealized foreign currency related losses are included in foreign currency translation adjustments within the consolidated statement of comprehensive income.
During the three and nine months ended September 30, 2013 and 2012, 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 separately disclose these subsequent fair value measurements and to classify them under the fair value hierarchy.
Assets measured at fair value on a nonrecurring basis at September 30, 2013 and 2012, 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 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.
Collateral-based impaired loans that have been adjusted to fair value totaled $38.1 million and $27.7 million at September 30, 2013 and 2012, respectively, and the level of specific allowances on these loans was decreased by $0.6 million during the three and nine months ended September 30, 2013. During the three and nine months ended September 30, 2012, Northern Trust increased by $5.7 million and reduced by $8.5 million, respectively, the level of specific allowances on these loans.
Other Real Estate Owned (OREO) classified as level 3 totaled $1.6 million at September 30, 2013 and $0.7 million was charged through other operating expense during the nine months ended September 30, 2013 to reduce the fair values of these OREO properties. There was no net amount charged through other operating expense during the three months ended September 30, 2013. OREO classified as level 3 totaled $1.0 million at September 30, 2012 and charges of $0.2 million and $0.4 million were recorded through other operating expense during the three and nine months ended September 30, 2012, respectively, to reduce the fair values of those OREO properties.
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 September 30, 2013.
Financial Instrument		Fair Value		Valuation Technique	Unobservable Input		Range of Discounts
							Applied
Loans		$38.1 million		Market Approach	Discount to reflect		15% – 40%
					realizable value
OREO		$1.6 million		Market Approach	Discount to reflect		15% – 40%
					realizable value
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 on Northern Trust’s consolidated balance sheet 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 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 and other interest-bearing assets; 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). As required by GAAP, the fair values required to be disclosed for demand, noninterest-bearing, savings, and money market deposits must equal the amounts disclosed in the consolidated balance sheet, even though such deposits are typically priced at a premium in banking industry consolidations.
The following tables summarize the fair values of financial instruments.
(In Millions)		30-Sep-13
		Book			Total			Fair Value
		Value			Fair Value			Level 1		Level 2			Level 3
Assets
Cash and Due from Banks		$	2,690.70		$	2,690.70		$	2,690.70	$	—		$	—
Federal Funds Sold and Resell Agreements			534.6			534.6			—		534.6			—
Interest-Bearing Deposits with Banks			17,383.90			17,383.90			—		17,383.90			—
Federal Reserve Deposits and Other Interest-Bearing			8,452.80			8,452.80			—		8,452.80			—
Securities
Available for Sale (1)			27,501.60			27,501.60			1,524.50		25,878.40			98.7
Held to Maturity			3,053.60			3,053.20			—		3,053.20			—
Trading Account			9.6			9.6			—		9.6			—
Loans (excluding Leases)
Held for Investment			27,797.00			27,752.50			—		—			27,752.50
Held for Sale			2.8			2.8			—		—			2.8
Client Security Settlement Receivables			1,630.20			1,630.20			—		1,630.20			—
Other Assets
Federal Reserve and Federal Home Loan Bank Stock			194.7			194.7			—		194.7			—
Community Development Investments			217.8			230			—		230			—
Employee Benefit and Deferred Compensation			131			126.2			82.3		43.9			—
Liabilities
Deposits
Demand, Noninterest-Bearing, Savings and Money Market		$	31,204.90		$	31,204.90		$	31,204.90	$	—		$	—
Savings Certificates and Other Time			1,939.60			1,943.90			—		1,943.90			—
Non U.S. Offices Interest-Bearing			45,017.20			45,017.20			—		45,017.20			—
Federal Funds Purchased			1,053.60			1,053.60			—		1,053.60			—
Securities Sold under Agreements to Repurchase			469			469			—		469			—
Other Borrowings			1,804.70			1,804.70			—		1,804.70			—
Senior Notes			1,996.50			2,002.30			—		2,002.30			—
Long Term Debt (excluding Leases)
Subordinated Debt			820.5			841.1			—		841.1			—
Federal Home Loan Bank Borrowings			135			138.6			—		138.6			—
Floating Rate Capital Debt			277.1			229.3			—		229.3			—
Other Liabilities
Standby Letters of Credit			55.6			55.6			—		—			55.6
Contingent Consideration			54			54			—		—			54
Loan Commitments			38.1			38.1			—		—			38.1
Derivative Instruments
Asset/Liability Management
Foreign Exchange Contracts
Assets		$	17		$	17		$	—	$	17		$	—
Liabilities			36.7			36.7			—		36.7			—
Interest Rate Contracts
Assets			107.9			107.9			—		107.9			—
Liabilities			44.4			44.4			—		44.4			—
Credit Default Swaps
Liabilities			—			—			—		—			—
Client-Related and Trading
Foreign Exchange Contracts
Assets			3,162.10			3,162.10			—		3,162.10			—
Liabilities			3,165.30			3,165.30			—		3,165.30			—
Interest Rate Contracts
Assets			134.9			134.9			—		134.9			—
Liabilities			128.6			128.6			—		128.6			—
-1	Refer to the table located on page 37 for the disaggregation of available for sale securities.
(In Millions)		31-Dec-12
		Book			Total			Fair Value
		Value			Fair Value			Level 1		Level 2			Level 3
Assets
Cash and Due from Banks		$	3,752.70		$	3,752.70		$	3,752.70	$	—		$	—
Federal Funds Sold and Resell Agreements			60.8			60.8			—		60.8			—
Interest-Bearing Deposits with Banks			18,803.50			18,803.50			—		18,803.50			—
Federal Reserve Deposits and Other Interest-Bearing			7,619.70			7,619.70			—		7,619.70			—
Securities
Available for Sale (1)			28,643.50			28,643.50			1,784.60		26,761.10			97.8
Held to Maturity			2,382.00			2,394.80			—		2,394.80			—
Trading Account			8			8			—		8			—
Loans (excluding Leases)
Held for Investment			28,165.40			28,220.20			—		—			28,220.20
Held for Sale			11.7			11.7			—		—			11.7
Client Security Settlement Receivables			2,049.10			2,049.10			—		2,049.10			—
Other Assets
Federal Reserve and Federal Home Loan Bank Stock			197.6			197.6			—		197.6			—
Community Development Investments			253.2			275.1			—		275.1			—
Employee Benefit and Deferred Compensation			121.3			126.1			86.7		39.4			—
Liabilities
Deposits
Demand, Noninterest-Bearing, Savings and Money Market		$	39,221.50		$	39,221.50		$	39,221.50	$	—		$	—
Savings Certificates and Other Time			2,466.10			2,746.70			—		2,746.70			—
Non U.S. Offices Interest-Bearing			39,720.20			39,720.20			—		39,720.20			—
Federal Funds Purchased			780.2			780.2			—		780.2			—
Securities Sold under Agreements to Repurchase			699.8			699.8			—		699.8			—
Other Borrowings			367.4			367.4			—		367.4			—
Senior Notes			2,405.80			2,513.40			—		2,513.40			—
Long Term Debt (excluding Leases)
Subordinated Debt			1,045.40			1,065.30			—		1,065.30			—
Federal Home Loan Bank Borrowings			335			345.4			—		345.4			—
Floating Rate Capital Debt			277			228			—		228			—
Other Liabilities
Standby Letters of Credit			60.5			60.5			—		—			60.5
Contingent Consideration			50.1			50.1			—		—			50.1
Loan Commitments			38.9			38.9			—		—			38.9
Derivative Instruments
Asset/Liability Management
Foreign Exchange Contracts
Assets		$	21.3		$	21.3		$	—	$	21.3		$	—
Liabilities			42.3			42.3			—		42.3			—
Interest Rate Contracts
Assets			129.7			129.7			—		129.7			—
Liabilities			75.3			75.3			—		75.3			—
Credit Default Swaps
Liabilities			1			1			—		1			—
Client-Related and Trading
Foreign Exchange Contracts
Assets			1,735.30			1,735.30			—		1,735.30			—
Liabilities			1,730.40			1,730.40			—		1,730.40			—
Interest Rate Contracts
Assets			180.6			180.6			—		180.6			—
Liabilities			174			174			—		174			—
-1	Refer to the table located on page 38 for the disaggregation of available for sale securities.

Securities

Securities	9 Months Ended
	Sep. 30, 2013
Securities	4. Securities – The following tables provide the amortized cost and fair values of securities at September 30, 2013 and December 31, 2012.
	Securities Available for Sale		September 30, 2013
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	U.S. Government		$	1,498.70		$	25.8		$	—		$	1,524.50
	Obligations of States and Political Subdivisions			13.9			0.5			—			14.4
	Government Sponsored Agency			16,979.10			81.6			36.1			17,024.60
	Corporate Debt			3,537.00			8.5			54.5			3,491.00
	Covered Bonds			1,774.20			33.7			2.3			1,805.60
	Supranational, Sovereign and Non-U.S. Agency Bonds			718.5			4.1			1.5			721.1
	Residential Mortgage-Backed			58.1			0.1			4.7			53.5
	Other Asset-Backed			2,535.70			1.2			1.2			2,535.70
	Auction Rate			98.1			2.1			1.5			98.7
	Other			232			0.5			—			232.5
	Total		$	27,445.30		$	158.1		$	101.8		$	27,501.60
	Securities Held to Maturity		September 30, 2013
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	Obligations of States and Political Subdivisions		$	248.8		$	11.3		$	—		$	260.1
	Government Sponsored Agency			41.3			1.2			—			42.5
	Non-U.S. Government Debt			219.3			—			—			219.3
	Certificates of Deposit			1,497.40			—			0.2			1,497.20
	Supranational and Sovereign Bonds			989.5			0.8			1.9			988.4
	Other			57.3			—			11.6			45.7
	Total		$	3,053.60		$	13.3		$	13.7		$	3,053.20
	Securities Available for Sale		December 31, 2012
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	U.S. Government		$	1,747.90		$	36.7		$	—		$	1,784.60
	Obligations of States and Political Subdivisions			13.9			0.2			—			14.1
	Government Sponsored Agency			18,520.60			122.2			4			18,638.80
	Corporate Debt			2,602.40			18.1			2.1			2,618.40
	Covered Bonds			1,697.10			51			0.1			1,748.00
	Supranational Bonds			1,053.90			7			0.2			1,060.70
	Residential Mortgage-Backed			102.4			0.4			10.8			92
	Other Asset-Backed			2,280.00			4.3			0.4			2,283.90
	Auction Rate			99.6			2.1			3.9			97.8
	Other			304.4			0.8			—			305.2
	Total		$	28,422.20		$	242.8		$	21.5		$	28,643.50
	Securities Held to Maturity		December 31, 2012
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	Obligations of States and Political Subdivisions		$	329.3		$	17.2		$	—		$	346.5
	Government Sponsored Agency			112.9			3.8			—			116.7
	Non-U.S. Government Debt			205			—			—			205
	Certificates of Deposit			1,667.60			0.2			0.6			1,667.20
	Other			67.2			0.3			8.1			59.4
	Total		$	2,382.00		$	21.5		$	8.7		$	2,394.80
	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 September 30, 2013.
	(In Millions)		Amortized			Fair
			Cost			Value
	Available for Sale
	Due in One Year or Less		$	9,630.10		$	9,645.70
	Due After One Year Through Five Years			14,551.90			14,617.70
	Due After Five Years Through Ten Years			2,264.20			2,242.10
	Due After Ten Years			999.1			996.1
	Total			27,445.30			27,501.60
	Held to Maturity
	Due in One Year or Less			1,844.30			1,846.20
	Due After One Year Through Five Years			1,134.80			1,138.70
	Due After Five Years Through Ten Years			37.7			40
	Due After Ten Years			36.8			28.3
	Total		$	3,053.60		$	3,053.20
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 losses of $2.2 million were recognized in the three months ended September 30, 2013, representing realized losses from the sale of securities. Net investment security gains of $0.2 million were recognized in the three months ended September 30, 2012, representing net realized gains from the sale of securities, partially offset by other-than-temporary impairment (OTTI) losses of $148.6 thousand. For the quarter ended September 30, 2013, gross proceeds of $316.4 million were received from the sales of securities and gross realized gains and losses totaled $0.1 million and $2.3 million, respectively. For the quarter ended September 30, 2012, gross proceeds from the sale of securities totaled $218.2 million and gross realized gains and losses totaled $356.0 thousand and $1.0 thousand, respectively.
Net investment security losses of $1.9 million were recognized for the nine months ended September 30, 2013, representing net realized losses from the sale of securities. Net investment security losses of $1.7 million for the nine months ended September 30, 2012 included OTTI losses of $3.3 million. For the nine months ended September 30, 2013, gross proceeds of $398.1 million were received from the sales of securities and gross realized gains and losses totaled $0.4 million and $2.3 million, respectively. For the nine months ended September 30, 2012, gross proceeds from the sale of securities totaled $2.7 billion and gross realized gains and losses totaled $23.5 million and $21.9 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 September 30, 2013 and December 31, 2012.
Securities with Unrealized Losses		Less than 12 Months						12 Months or Longer						Total
as of September 30, 2013
		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
(In Millions)		Value			Losses			Value			Losses			Value			Losses
Government Sponsored Agency		$	3,776.10		$	32.3		$	302.7		$	3.8		$	4,078.80		$	36.1
Corporate Debt			1,911.20			54.4			25			0.1			1,936.20			54.5
Covered Bonds			207.4			2.3			—			—			207.4			2.3
Supranational, Sovereign and Non-US Agency Bonds			549.6			3.4			—			—			549.6			3.4
Residential Mortgage-Backed			—			—			45.5			4.7			45.5			4.7
Other Asset-Backed			1,113.90			1.1			45.1			0.1			1,159.00			1.2
Certificates of Deposit			735.7			0.2			—			—			735.7			0.2
Auction Rate			27.8			0.4			16.5			1.1			44.3			1.5
Other			37.9			3.4			29			8.2			66.9			11.6
Total		$	8,359.60		$	97.5		$	463.8		$	18		$	8,823.40		$	115.5
Securities with Unrealized Losses		Less than 12 Months						12 Months or Longer						Total
as of December 31, 2012
		Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
(In Millions)		Value			Losses			Value			Losses			Value			Losses
Government Sponsored Agency		$	482.2		$	1		$	1,171.80		$	3		$	1,654.00		$	4
Corporate Debt			441.5			2			50			0.1			491.5			2.1
Covered Bonds			20.1			0.1			—			—			20.1			0.1
Supranational Bonds			113.8			0.2			—			—			113.8			0.2
Residential Mortgage-Backed			—			—			84.7			10.8			84.7			10.8
Other Asset-Backed			146.1			0.1			40			0.3			186.1			0.4
Certificates of Deposit			1,178.80			0.6			—			—			1,178.80			0.6
Auction Rate			2.7			0.3			41			3.6			43.7			3.9
Other			9.3			1.9			43.8			6.2			53.1			8.1
Total		$	2,394.50		$	6.2		$	1,431.30		$	24		$	3,825.80		$	30.2
As of September 30, 2013, 428 securities with a combined fair value of $8.8 billion were in an unrealized loss position, with their unrealized losses totaling $115.5 million. Unrealized losses on residential mortgage-backed securities totaling $4.7 million reflect the impact of wider credit and liquidity spreads on the valuations of 6 residential mortgage-backed securities since purchase, with $45.5 million having been in an unrealized loss position for more than 12 months. Residential mortgage-backed securities at September 30, 2013 had a total amortized cost and fair value of $58.1 million and $53.5 million, respectively. Securities classified as “other asset-backed” had average lives less than 5 years, and 100% were rated triple-A.
Unrealized losses of $36.1 million related to government sponsored agency securities are primarily attributable to changes in market rates since their purchase. The majority of the $11.6 million of unrealized losses in securities classified as “other” at September 30, 2013 relate to securities which Northern Trust purchases for compliance with the Community Reinvestment Act (CRA). Unrealized losses on these CRA related other securities are
attributable to their purchase at 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 $1.5 million related to auction rate securities primarily reflect reduced market liquidity as a majority of auctions continue to fail preventing holders from liquidating their investments at par. Unrealized losses of $54.5 million within corporate debt securities primarily reflect widened credit spreads and higher market rates since purchase; 51% of the corporate debt portfolio is backed by guarantees provided by U.S. and non-U.S. governmental entities. The remaining unrealized losses on Northern Trust’s securities portfolio as of September 30, 2013 are attributable to changes in overall market interest rates, increased credit spreads, or reduced market liquidity.
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 following describes Northern Trust’s process for identifying credit impairment within non-agency residential mortgage-backed securities, the security type for which Northern Trust has previously recognized the majority of its OTTI. To determine if an unrealized loss on a non-agency residential mortgage-backed security is other-than-temporary, economic models are used to perform cash flow analyses by developing multiple scenarios in order to create reasonable forecasts of the security’s future performance using available data including servicers’ loan charge off patterns, prepayment speeds, annualized default rates, each security’s current delinquency pipeline, the delinquency pipeline’s growth rate, the roll rate from delinquency to default, loan loss severities and historical performance of like collateral, along with Northern Trust’s outlook for the housing market and the overall economy. If the present value of future cash flows projected as a result of this analysis is less than the current amortized cost of the security, a credit-related OTTI loss is recorded to earnings equal to the difference between the two amounts.
Impairments of non-agency residential mortgage-backed securities are influenced by a number of factors, including but not limited to, U.S. economic and housing market performance, security credit enhancement level, insurance coverage, year of origination, and type of collateral. The factors used in estimating losses on non-agency residential mortgage-backed securities vary by year of origination and type of collateral.
As of September 30, 2013, loss estimates for subprime, Alt-A, prime and 2nd lien collateral portfolios were developed using default roll rates, determined primarily by the stage of delinquency of the underlying instrument, that generally assumed ultimate default rates approximating 5% to 30% for current loans; 30% for loans 30 to 60 days delinquent; 80% for loans 60 to 90 days delinquent; 90% for loans delinquent greater than 90 days; and 100% for OREO properties and loans that are in foreclosure. September 30, 2013 amortized cost, weighted average ultimate default rates, and impairment severity rates for the non-agency residential mortgage-backed securities portfolio, by security type, are provided in the following table.
								Loss Severity Rates
Security Type		Amortized			Weighted Average			Low			High			Weighted
		Cost			Ultimate Default Rates									Average
Prime		$	7.4			18.9	%		32.9	%		45	%		41.1	%
Alt-A			12.4			43.1			66.2			66.2			66.2
Subprime			26.7			48.8			72.5			82.1			77.6
2nd Lien			11.6			33.1			98.9			99			99
Total Non-Agency Residential Mortgage-Backed Securities		$	58.1			40.1	%		32.9	%		99	%		74.8	%
Northern Trust’s processes for identifying credit impairment within auction rate securities are largely consistent with the processes utilized for non-agency residential mortgage-backed securities and include analyses of expected loss severities and default rates adjusted for the type of underlying loan and the presence of government guarantees, as applicable. There were no OTTI losses recognized during the three or nine months ended September 30, 2013. During the three months ended September 30, 2012, OTTI losses totaling $148.6 thousand related to non-agency residential mortgage-backed securities were recognized. During the nine months ended September 30, 2012, OTTI losses totaling $3.3 million were recognized, of which $1.7 million related to non-agency residential mortgage-backed securities and $1.6 million related to auction rate 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 and nine months ended September 30, 2013 and 2012.
		Three Months Ended						Nine Months Ended
		September  30,						September  30,
(In Millions)		2013			2012			2013			2012
Changes in OTTI Losses*		$	—		$	0.4		$	—		$	(2.7	)
Noncredit-related Losses Recorded in / (Reclassified from) OCI**			—			(0.6	)		—			(0.6	)
Net Impairment Losses Recognized in Earnings		$	—		$	(0.2	)	$	—		$	(3.3	)
*	For initial other-than-temporary impairments in the respective 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 other-than-temporary impairments in the respective period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in 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.
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
(In Millions)		2013			2012			2013			2012
Cumulative Credit-Related Losses on Securities Held – Beginning of Period		$	8.8		$	43.2		$	42.3		$	68.2
Plus: Losses on Newly Identified Impairments			—			—			—			1.6
Additional Losses on Previously Identified Impairments			—			0.2			—			1.7
Less: Current and Prior Period Losses on Securities Sold During the Period			—			—			(33.5	)		(28.1	)
Cumulative Credit-Related Losses on Securities Held – End of Period		$	8.8		$	43.4		$	8.8		$	43.4
The table below provides information regarding debt securities held as of September 30, 2013 and December 31, 2012, for which an OTTI loss has been recognized in the current period or previously.
(In Millions)		September 30,			December 31,
		2013			2012
Fair Value		$	38.9		$	51.5
Amortized Cost Basis			44.3			59
Noncredit-related Losses Recognized in OCI			(5.4	)		(7.5	)
Tax Effect			2			2.8
Amount Recorded in OCI		$	(3.4	)	$	(4.7	)

Loans_and_Leases

Loans and Leases	9 Months Ended
	Sep. 30, 2013
Loans and Leases	5. Loans and Leases – Amounts outstanding for loans and leases, by segment and class, are shown below.
	(In Millions)		September 30,			December 31,
			2013			2012
	Commercial
	Commercial and Institutional		$	7,237.00		$	7,468.50
	Commercial Real Estate			2,821.80			2,859.80
	Lease Financing, net			982.1			1,035.00
	Non-U.S.			1,155.20			1,192.30
	Other			285.1			341.6
	Total Commercial			12,481.20			12,897.20
	Personal
	Residential Real Estate			10,359.50			10,375.20
	Private Client			6,139.10			6,130.10
	Other			85			102
	Total Personal			16,583.60			16,607.30
	Total Loans and Leases			29,064.80			29,504.50
	Allowance for Credit Losses Assigned to Loans and Leases			(287.2	)		(297.9	)
	Net Loans and Leases		$	28,777.60		$	29,206.60
	Residential real estate loans consist of conventional home 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 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 September 30, 2013 and December 31, 2012, equity credit lines totaled $2.0 billion and $2.3 billion, respectively, and equity credit lines for which the first liens were held by Northern Trust represented 87% and 86%, respectively, of the total equity credit lines as 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 $1.5 billion at both September 30, 2013 and December 31, 2012. Demand deposits reclassified as loan balances totaled $54.7 million and $224.7 million at September 30, 2013 and December 31, 2012, respectively. Loans classified as held for sale totaled $2.8 million and $11.7 million at September 30, 2013 and December 31, 2012, respectively.
	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, 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, return on assets, asset size, and capital levels;
		•		Commercial Real Estate: debt service coverage and leasing status for income-producing properties; loan-to-value and loan-to-cost ratios, leasing status, and guarantor support for loans associated with construction and development properties;
		•		Lease Financing and Commercial-Other: leverage and profit margin levels;
		•		Non-U.S.: entity type, liquidity, size, and leverage;
		•		Residential Real Estate: payment history, credit bureau scores, and cash flow-to-debt and net worth ratios;
		•		Private Client: cash flow-to-debt and net worth ratios, leverage, and profit margin levels; 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 September 30, 2013 and December 31, 2012 are provided below, segregated by borrower ratings into “1 to 3”, “4 to 5” and “6 to 9” (watch list), categories.
			September 30, 2013										December 31, 2012
	(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		$	4,384.70		$	2,679.00		$	173.3	$	7,237.00	$	4,291.80	$	3,040.60	$	136.1	$	7,468.50
	Commercial Real Estate			992.9			1,652.20			176.7		2,821.80		888.6		1,710.90		260.3		2,859.80
	Lease Financing, net			674.6			303			4.5		982.1		647.1		382.3		5.6		1,035.00
	Non-U.S.			523.4			631.1			0.7		1,155.20		542.7		646.6		3		1,192.30
	Other			107.5			177.6			—		285.1		167.2		174.4		—		341.6
	Total Commercial			6,683.10			5,442.90			355.2		12,481.20		6,537.40		5,954.80		405		12,897.20
	Personal
	Residential Real Estate			3,289.70			6,535.40			534.4		10,359.50		3,003.30		6,868.20		503.7		10,375.20
	Private Client			3,855.50			2,261.70			21.9		6,139.10		3,741.30		2,365.40		23.4		6,130.10
	Other			32.1			52.9			—		85		50		52		—		102
	Total Personal			7,177.30			8,850.00			556.3		16,583.60		6,794.60		9,285.60		527.1		16,607.30
	Total Loans and Leases		$	13,860.40		$	14,292.90		$	911.5	$	29,064.80	$	13,332.00	$	15,240.40	$	932.1	$	29,504.50
	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. At the time a loan is determined to be nonperforming, interest accrued but not collected is reversed against interest income of the current period and the loan is classified as nonperforming. 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 other real estate owned and nonperforming asset balances, as of September 30, 2013 and December 31, 2012.
	September 30, 2013
						30-59 Days			60-89 Days		90 Days or		Total				Total Loans
	(In Millions)		Current			Past Due			Past Due		More Past Due		Performing		Nonperforming		and Leases
	Commercial
	Commercial and Institutional		$	7,194.10		$	12.2		$	2.8	$	3.8	$	7,212.90	$	24.1	$	7,237.00
	Commercial Real Estate			2,745.90			10.7			2.7		8.2		2,767.50		54.3		2,821.80
	Lease Financing, net			982.1			—			—		—		982.1		—		982.1
	Non-U.S.			1,155.20			—			—		—		1,155.20		—		1,155.20
Other			285.1			—			—		—		285.1		—		285.1
Total Commercial			12,362.40			22.9			5.5		12		12,402.80		78.4		12,481.20
Personal
Residential Real Estate			10,135.90			11.7			12.5		9.6		10,169.70		189.8		10,359.50
Private Client			6,086.70			39.4			7.9		3.2		6,137.20		1.9		6,139.10
Other			85			—			—		—		85		—		85
Total Personal			16,307.60			51.1			20.4		12.8		16,391.90		191.7		16,583.60
Total Loans and Leases		$	28,670.00		$	74		$	25.9	$	24.8	$	28,794.70	$	270.1	$	29,064.80
											Other Real Estate Owned				13.9
											Total Nonperforming Assets			$	284
December 31, 2012
					30-59 Days			60-89 Days		90 Days or		Total				Total Loans
(In Millions)		Current			Past Due			Past Due		More Past Due		Performing		Nonperforming		and Leases
Commercial
Commercial and Institutional		$	7,433.40		$	6.4		$	5.5	$	1.6	$	7,446.90	$	21.6	$	7,468.50
Commercial Real Estate			2,782.00			6.9			13.1		1.4		2,803.40		56.4		2,859.80
Lease Financing, net			1,035.00			—			—		—		1,035.00		—		1,035.00
Non-U.S.			1,192.30			—			—		—		1,192.30		—		1,192.30
Other			341.6			—			—		—		341.6		—		341.6
Total Commercial			12,784.30			13.3			18.6		3		12,819.20		78		12,897.20
Personal
Residential Real Estate			10,096.30			68.1			25.7		10.5		10,200.60		174.6		10,375.20
Private Client			6,091.30			14.8			16.3		5.5		6,127.90		2.2		6,130.10
Other			102			—			—		—		102		—		102
Total Personal			16,289.60			82.9			42		16		16,430.50		176.8		16,607.30
Total Loans and Leases		$	29,073.90		$	96.2		$	60.6	$	19	$	29,249.70	$	254.8	$	29,504.50
											Other Real Estate Owned				20.3
											Total Nonperforming Assets			$	275.1
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. 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.
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.
The following tables provide information related to impaired loans by segment and class.
		As of September 30, 2013								As of December 31, 2012
					Unpaid							Unpaid
		Recorded			Principal			Specific		Recorded		Principal		Specific
(In Millions)		Investment			Balance			Allowance		Investment		Balance		Allowance
With No Related Specific Allowance
Commercial and Institutional		$	11.3		$	14.5		$	—	$	15.1	$	19.3	$	—
Commercial Real Estate			48.6			59.2			—		64.9		76.9		—
Lease Financing, net			4.5			4.5			—		4.6		4.6		—
Residential Real Estate			177.2			220.4			—		131.3		165.7		—
Private Client			13.3			13.3			—		0.8		1		—
With a Related Specific Allowance
Commercial and Institutional			14			16.2			5		8.1		10.2		2.8
Commercial Real Estate			31.1			32.6			8.7		32.3		33.8		8.2
Residential Real Estate			10			10.7			3.3		11.8		13		6.1
Private Client			0.8			0.8			0.8		0.9		0.9		0.9
Total
Commercial			109.5			127			13.7		125		144.8		11
Personal			201.3			245.2			4.1		144.8		180.6		7
Total		$	310.8		$	372.2		$	17.8	$	269.8	$	325.4	$	18
		Three Months Ended September 30,										Nine Months Ended September 30,
		2013						2012				2013				2012
		Average			Interest			Average		Interest		Average		Interest		Average		Interest
		Recorded			Income			Recorded		Income		Recorded		Income		Recorded		Income
(In Millions)		Investment			Recognized			Investment		Recognized		Investment		Recognized		Investment		Recognized
With No Related Specific
Commercial and Institutional		$	10.7		$	—		$	27	$	0.1	$	11.3	$	0.1	$	25.2	$	0.2
Commercial Real Estate			42			0.2			54.7		—		40.6		0.6		49.6		0.2
Lease Financing, net			4.5			0.1			5.2		—		4.6		0.2		3.5		—
Residential Real Estate			168.1			0.3			120.7		0.2		155.2		1.4		114		0.8
Private Client			13.3			0.2			2.2		—		12		0.5		1.8		—
With a Related Specific Allowance
Commercial and Institutional			14.1			—			5.2		—		12.5		—		6.5		—
Commercial Real Estate			31.2			—			22.1		—		31.6		—		20.2		—
Residential Real Estate			10			—			15.3		—		9.4		—		15.3		—
Private Client			0.8			—			0.9		—		1.1		—		1.1		—
Total
Commercial			102.5			0.3			114.2		0.1		100.6		0.9		105		0.4
Personal			192.2			0.5			139.1		0.2		177.7		1.9		132.2		0.8
Total		$	294.7		$	0.8		$	253.3	$	0.3	$	278.3	$	2.8	$	237.2	$	1.2
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 $2.8 million and $2.9 million, respectively, for the three months ended September 30, 2013 and 2012, and $8.0 million and $8.7 million, respectively, for the nine months ended September 30, 2013 and 2012.
There were $4.3 million and $2.1 million of aggregate unfunded loan commitments and standby letters of credit at September 30, 2012 and December 31, 2012, respectively, issued to borrowers whose loans were classified as nonperforming or impaired.
Troubled Debt Restructurings. Included within impaired loans were $65.8 million and $49.8 million of nonperforming TDRs, and $100.1 million and $74.7 million of performing TDRs as of September 30, 2013 and December 31, 2012, respectively. All TDRs are reported as impaired loans in the calendar year of their restructuring. In subsequent years, a TDR may cease being reported as impaired if the loan was modified at a market rate and has performed according to the modified terms for at least six months. A loan that has been modified at a below market rate will return to performing status if it satisfies the six month performance requirement; however, it will remain reported as impaired.
The following tables provide, by segment and class, the number of loans and leases modified in TDRs during the three and nine month periods ended September 30, 2013 and 2012, and the recorded investments and unpaid principal balances as of September 30, 2013 and 2012.
		Three Months Ended								Nine Months Ended
($ In Millions)		30-Sep-13								30-Sep-13
		Number of			Recorded			Unpaid Principal		Number of		Recorded		Unpaid Principal
		Loans and Leases			Investment			Balance		Loans and Leases		Investment		Balance
Commercial
Commercial and Institutional			—		$	—		$	—		7	$	0.5	$	1
Commercial Real Estate			7			22.8			25		11		27.4		31.9
Total Commercial			7			22.8			25		18		27.9		32.9
Personal
Residential Real Estate			38			6.7			11.4		122		51.2		59.8
Private Client			—			—			—		6		12.8		12.8
Total Personal			38			6.7			11.4		128		64		72.6
Total Loans and Leases			45		$	29.5		$	36.4		146	$	91.9	$	105.5
Note: Period end balances reflect all paydowns and charge-offs during the period.
		Three Months Ended								Nine Months Ended
($ In Millions)		30-Sep-12								30-Sep-12
		Number of			Recorded			Unpaid Principal		Number of		Recorded		Unpaid Principal
		Loans and Leases			Investment			Balance		Loans and Leases		Investment		Balance
Commercial
Commercial and Institutional			—		$	—		$	—		2	$	0.5	$	1.1
Commercial Real Estate			1			2.7			2.7		7		24.6		26.2
Lease Financing, net			–			–			–		1		5.2		5.2
Total Commercial			1			2.7			2.7		10		30.3		32.5
Personal
Residential Real Estate			23			2			2.6		84		10.1		13.5
Private Client			–			–			–		1		0.8		0.8
Total Personal			23			2			2.6		85		10.9		14.3
Total Loans and Leases			24		$	4.7		$	5.3		95	$	41.2	$	46.8
Note: Period end balances reflect all paydowns and charge-offs during the period.
TDR modifications primarily 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 and nine month periods ended September 30, 2013, TDR modifications of loans within the commercial and institutional, commercial real estate, and private client classes were primarily deferrals of principal, extensions of term, and other modifications. During the three and nine months ended September 30, 2013, TDR modifications of loans within the residential real estate class were primarily deferrals of principal, interest rate concessions, extensions of term, and other modifications.
During the three and nine months ended September 30, 2012, TDR modifications of loans within the commercial and institutional, commercial real estate, lease financing, and private client classes were primarily extensions of term and deferral of principal; modifications of residential real estate loans were primarily interest rate concessions and deferrals of principal.
There were no loans or leases modified as TDRs in the 12 months ended June 30, 2013 which subsequently became nonperforming during the three or nine months ended September 30, 2013.
There were no loans or leases modified as TDRs in the 12 months ended June 30, 2012 which subsequently became nonperforming during the three months ended September 30, 2012. There were 2 residential real estate loans modified as TDRs in the 12 months ended June 30, 2012 which subsequently became nonperforming during the nine months ended September 30, 2012. The total recorded investment and unpaid principal balance for these loans were each $89.4 thousand as of September 30, 2012.
All loans and leases modified in troubled debt restructurings are evaluated for impairment. The nature and extent of impairment of TDRs, including those which have experienced a subsequent default, is considered in the determination of an appropriate level of allowance for credit losses.

Allowance_for_Credit_Losses

Allowance for Credit Losses	9 Months Ended
	Sep. 30, 2013
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, unfunded 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 Allowance 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 unfunded 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 and nine month periods ended September 30, 2013 and 2012.
		Three Months Ended September 30,
		2013									2012
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Balance at Beginning of Period	$	184.1		$	136.6		$	320.7		$	196.3		$	133.6		$	329.9
	Charge-Offs		(1.7	)		(9.9	)		(11.6	)		(7.2	)		(9.1	)		(16.3	)
	Recoveries		1.7			1.6			3.3			2.4			2			4.4
	Net (Charge-Offs) Recoveries		—			(8.3	)		(8.3	)		(4.8	)		(7.1	)		(11.9	)
	Provision for Credit Losses		(4.6	)		9.6			5			6.9			3.1			10
	Effect of Foreign Exchange Rates		0.1			—			0.1			—			—			—
	Balance at End of Period	$	179.6		$	137.9		$	317.5		$	198.4		$	129.6		$	328
		Nine Months Ended September 30,
		2013									2012
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Balance at Beginning of Period	$	194.2		$	133.4		$	327.6		$	211		$	117.9		$	328.9
	Charge-Offs		(6.9	)		(32.9	)		(39.8	)		(15.4	)		(31.5	)		(46.9	)
	Recoveries		8.1			6.6			14.7			13.6			12.4			26
	Net (Charge-Offs) Recoveries		1.2			(26.3	)		(25.1	)		(1.8	)		(19.1	)		(20.9	)
	Provision for Credit Losses		(15.8	)		30.8			15			(10.8	)		30.8			20
	Effect of Foreign Exchange Rates		—			—			—			—			—			—
	Balance at End of Period	$	179.6		$	137.9		$	317.5		$	198.4		$	129.6		$	328
	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 September 30, 2013 and December 31, 2012.
		September 30, 2013									December 31, 2012
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Loans and Leases
	Specifically Evaluated for Impairment	$	109.7		$	201.3		$	311		$	125		$	144.8		$	269.8
	Evaluated for Inherent Impairment		12,371.50			16,382.30			28,753.80			12,772.20			16,462.50			29,234.70
	Total Loans and Leases		12,481.20			16,583.60			29,064.80			12,897.20			16,607.30			29,504.50
	Allowance for Loans and Leases
	Specifically Evaluated for Impairment		13.7			4.1			17.8			11			7			18
	Evaluated for Inherent Impairment		138.4			131			269.4			155.1			124.8			279.9
	Allowance Assigned to Loans and Leases		152.1			135.1			287.2			166.1			131.8			297.9
	Allowance for Unfunded Exposures
	Commitments and Standby Letters of Credit		27.5			2.8			30.3			28.1			1.6			29.7
	Total Allowance for Credit Losses	$	179.6		$	137.9		$	317.5		$	194.2		$	133.4		$	327.6

Pledged_Assets

Pledged Assets	9 Months Ended
	Sep. 30, 2013
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, Federal Home Loan Bank borrowings, and for other purposes, including support for securities settlement, primarily related to client activities, and for potential Federal Reserve Bank discount window borrowings. At September 30, 2013, securities and loans totaling $28.3 billion ($19.4 billion of government sponsored agency and other securities, $256.3 million of obligations of states and political subdivisions, and $8.7 billion of loans) were pledged. This compares to $27.2 billion ($19.0 billion of government sponsored agency and other securities, $334.0 million of obligations of states and political subdivisions, and $7.9 billion of loans) at December 31, 2012. Collateral required for these purposes totaled $4.5 billion and $3.4 billion at September 30, 2013 and December 31, 2012, respectively. Included in the total pledged assets at September 30, 2013 and December 31, 2012 were available for sale securities with a total fair value of $460.4 million and $690.1 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 as of September 30, 2013 and December 31, 2012 was $500.0 million and $35.4 million, respectively. There was no repledged or sold collateral at September 30, 2013 or December 31, 2012. Deposits maintained to meet Federal Reserve Bank reserve requirements averaged $0.9 billion for both the three and nine months ended September 30, 2013, and $1.1 billion for both the three and nine months ended September 30, 2012.

Goodwill_and_Other_Intangibles

Goodwill and Other Intangibles	9 Months Ended
	Sep. 30, 2013
Goodwill and Other Intangibles	8. Goodwill and Other Intangibles – The carrying amounts of goodwill at September 30, 2013 and December 31, 2012 were as follows:
	(In Millions)	September 30,		December 31,
		2013		2012
	Corporate and Institutional Services	$	466.2	$	466.3
	Wealth Management		71.5		71.5
	Total Goodwill	$	537.7	$	537.8
	Note: Amounts include the effect of foreign exchange rates on non-U.S. dollar denominated goodwill.
	Other intangible assets are included within other assets in the consolidated balance sheet. The gross carrying amount and accumulated amortization of other intangible assets subject to amortization at September 30, 2013 and December 31, 2012 were as follows:
	(In Millions)	September 30,		December 31,
		2013		2012
	Gross Carrying Amount	$	251.3	$	252.1
	Less: Accumulated Amortization		163.5		148.1
	Net Book Value	$	87.8	$	104
	Note: Amounts include the effect of foreign exchange rates on non-U.S. dollar denominated intangible assets.
	Other intangible assets consist primarily of the value of acquired client relationships. Amortization expense related to other intangible assets totaled $5.2 million and $5.2 million for the three months ended September 30, 2013 and 2012, respectively, and $15.5 million and $15.0 million for the nine months ended September 30, 2013 and 2012, respectively. Amortization for the remainder of 2013 and for the years 2014, 2015, 2016, and 2017 is estimated to be $5.6 million, $19.4 million, $11.8 million, $9.2 million and $9.2 million, respectively.

Business_Units

Business Units	9 Months Ended
	Sep. 30, 2013
Business Units	9. Business Units – The following tables show the earnings contributions of Northern Trust’s business units for the three and nine month periods ended September 30, 2013 and 2012.
	Three Months Ended		Corporate &						Wealth						Treasury and						Total
	September 30,		Institutional Services						Management						Other						Consolidated
	($ In Millions)		2013			2012			2013			2012			2013			2012			2013			2012
	Noninterest Income
	Trust, Investment and Other Servicing Fees		$	359.8		$	334.4		$	288.2		$	267.5		$	—		$	—		$	648		$	601.9
	Foreign Exchange Trading Income			61.8			41.3			1			2.7			—			—			62.8			44
	Other Noninterest Income			44.3			50.2			54.3			23.3			0.8			7.5			99.4			81
	Net Interest Income (FTE)*			70.1			68			136.3			157.4			38.4			31.5			244.8			256.9
	Revenue*			536			493.9			479.8			450.9			39.2			39			1,055.00			983.8
	Provision for Credit Losses			0.4			(1.6	)		4.6			11.6			—			—			5			10
	Noninterest Expense			412			394.5			297.3			285.2			31.4			16.7			740.7			696.4
	Income (loss) before Income Taxes*			123.6			101			177.9			154.1			7.8			22.3			309.3			277.4
	Provision (Benefit) for Taxes*			38.8			32.7			67			58.1			(3.0	)		7.8			102.8			98.6
	Net Income		$	84.8		$	68.3		$	110.9		$	96		$	10.8		$	14.5		$	206.5		$	178.8
	Percentage of Consolidated Net Income			41	%		38	%		54	%		54	%		5	%		8	%		100	%		100	%
	Average Assets		$	53,653.50		$	50,638.60		$	22,923.60		$	23,530.70		$	18,635.40		$	18,540.60		$	95,212.50		$	92,709.90
	*	Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $7.8 million for 2013 and $11.3 million for 2012.
	Nine Months Ended		Corporate &						Wealth						Treasury and						Total Consolidated
	September 30,		Institutional Services						Management						Other
	($ In Millions)		2013			2012			2013			2012			2013			2012			2013			2012
	Noninterest Income
	Trust, Investment and Other Servicing Fees		$	1,072.70		$	989.8		$	863.3		$	793.1		$	—		$	—		$	1,936.00		$	1,782.90
	Foreign Exchange Trading Income			189.8			155.3			3.8			10			—			—			193.6			165.3
	Other Noninterest Income			130.1			145.9			95.8			70.1			5.4			6.1			231.3			222.1
	Net Interest Income (FTE)*			200.2			217			425.3			476.9			81			93.6			706.5			787.5
	Revenue*			1,592.80			1,508.00			1,388.20			1,350.10			86.4			99.7			3,067.40			2,957.80
	Provision for Credit Losses			(1.1	)		(1.6	)		16.1			21.6			—			—			15			20
	Noninterest Expense			1,206.70			1,190.00			900.9			878.4			91.7			68.9			2,199.30			2,137.30
	Income (loss) before Income Taxes*			387.2			319.6			471.2			450.1			(5.3	)		30.8			853.1			800.5
	Provision (Benefit) for Taxes*			123			102.1			177.8			170.2			(9.3	)		8.6			291.5			280.9
	Net Income		$	264.2		$	217.5		$	293.4		$	279.9		$	4		$	22.2		$	561.6		$	519.6
	Percentage of Consolidated Net Income			47	%		42	%		52	%		54	%		1	%		4	%		100	%		100	%
	Average Assets		$	52,323.80		$	49,642.80		$	22,863.10		$	23,527.60		$	18,036.60		$	20,243.20		$	93,223.50		$	93,413.60
	*	Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $23.3 million for 2013 and $31.4 million for 2012.
	Further discussion of business unit results is provided within the “Business Unit Reporting” section of Management’s Discussion and Analysis of Financial Condition and Results of Operations.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss)	10. Accumulated Other Comprehensive Income (Loss) – The following tables summarize the components of accumulated other comprehensive income (loss) at September 30, 2013 and 2012, and changes during the three and nine month periods then ended.
	(In Millions)	Balance at				Net Change				Balance at
		December 31,								September 30,
		2012								2013
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	101			$	(74.4	)		$	26.6
	Net Unrealized Gains (Losses) on Cash Flow Hedges		(1.4	)			1.2				(0.2	)
	Net Foreign Currency Adjustments		10.5				(2.8	)			7.7
	Net Pension and Other Postretirement Benefit Adjustments		(393.1	)			21.1				(372.0	)
	Total	$	(283.0	)		$	(54.9	)		$	(337.9	)
	(In Millions)	Balance at				Net Change				Balance at
		December 31,								September 30,
		2011								2012
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	39.8			$	67.3			$	107.1
	Net Unrealized Gains (Losses) on Cash Flow Hedges		(7.0	)			5.5				(1.5	)
	Net Foreign Currency Adjustments		(9.5	)			24.1				14.6
	Net Pension and Other Postretirement Benefit Adjustments		(368.9	)			32.8				(336.1	)
	Total	$	(345.6	)		$	129.7			$	(215.9	)
		Three Months Ended September 30,
		2013											2012
	(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 Losses on Securities OTTI	$	—			$	—			$	—		$	4.9		$	(1.8	)	$	3.1
	Other Unrealized Gains (Losses) on Securities Available for Sale		7.3				(2.8	)			4.5			57.3			(21.6	)		35.7
	Reclassification Adjustment for (Gains) Losses Included in Net Income		2.1				(0.8	)			1.3			(5.1	)		1.9			(3.2	)
	Net Change		9.4				(3.6	)			5.8			57.1			(21.5	)		35.6
	Unrealized Gains (Losses) on Cash Flow Hedges
	Unrealized Gains (Losses) on Cash Flow Hedges		5.3				(1.7	)			3.6			2			(0.5	)		1.5
	Reclassification Adjustment for (Gains) Losses Included in Net Income		1.1				(0.4	)			0.7			2.6			(1.0	)		1.6
	Net Change		6.4				(2.1	)			4.3			4.6			(1.5	)		3.1
	Foreign Currency Adjustments
	Foreign Currency Translation Adjustments		55				(4.0	)			51			42.1			—			42.1
	Net Investment Hedge Gains (Losses)		(88.8	)			33.4				(55.4	)		(44.8	)		16.9			(27.9	)
	Reclassification Adjustment for (Gains) Losses Included in Net Income		—				—				—			—			—			—
	Net Change		(33.8	)			29.4				(4.4	)		(2.7	)		16.9			14.2
	Pension and Other Postretirement Benefit Adjustments
	Net Actuarial Gain (Loss)		—				—				—			—			—			—
	Reclassification Adjustment for (Gains) Losses Included in Net Income		11.3				(4.2	)			7.1			8.5			(3.2	)		5.3
	Net Change	$	11.3			$	(4.2	)		$	7.1		$	8.5		$	(3.2	)	$	5.3
		Nine Months Ended September 30,
		2013											2012
	(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 Losses on Securities OTTI	$	2.1			$	(0.8	)		$	1.3		$	15		$	(5.6	)	$	9.4
	Other Unrealized Gains (Losses) on Securities Available for Sale		(123.2	)			46.3				(76.9	)		105.9			(39.7	)		66.2
	Reclassification Adjustment for (Gains) Losses Included in Net Income		1.9				(0.7	)			1.2			(13.2	)		4.9			(8.3	)
	Net Change		(119.2	)			44.8				(74.4	)		107.7			(40.4	)		67.3
	Unrealized Gains (Losses) on Cash Flow Hedges
	Unrealized Gains (Losses) on Cash Flow Hedges		(4.0	)			1.6				(2.4	)		4.7			(1.5	)		3.2
	Reclassification Adjustment for (Gains) Losses Included in Net Income		5.7				(2.1	)			3.6			3.7			(1.4	)		2.3
	Net Change		1.7				(0.5	)			1.2			8.4			(2.9	)		5.5
	Foreign Currency Adjustments
	Foreign Currency Translation Adjustments		5.3				(2.3	)			3			31.3			—			31.3
	Net Investment Hedge Gains (Losses)		(9.3	)			3.5				(5.8	)		(17.4	)		10.2			(7.2	)
	Reclassification Adjustment for (Gains) Losses Included in Net Income		—				—				—			—			—			—
	Net Change		(4.0	)			1.2				(2.8	)		13.9			10.2			24.1
	Pension and Other Postretirement Benefit Adjustments
	Net Actuarial Gain (Loss)		—				—				—			26.7			(10.1	)		16.6
	Reclassification Adjustment for (Gains) Losses Included in Net Income		33.7				(12.6	)			21.1			25.6			(9.4	)		16.2
	Net Change	$	33.7			$	(12.6	)		$	21.1		$	52.3		$	(19.5	)	$	32.8
	The following table provides the location and before-tax amounts of reclassifications out of accumulated other comprehensive income (loss) during the three and nine months ended September 30, 2013.
				Amount of Reclassification
				Adjustments Recognized
		Location of		in Income
		Reclassification Adjustments		Three Months Ended				Nine Months Ended
(In Millions)	Recognized in Income		30-Sep-13
Securities Available for Sale
Realized (Gains) Losses on Securities Available for Sale	Investment Security Gains (Losses), net		$	2.1			$	1.9
Realized (Gains) Losses on Cash Flow Hedges
Foreign Exchange Contracts	Other Operating Income/ Expense			1.1				5.7
Pension and Other Postretirement Adjustments
Amortization of Net Actuarial (Gain) Loss	Employee Benefits			11.9				35.8
Amortization of Prior Service Cost	Employee Benefits			(0.6	)			(2.1	)
Gross Reclassification Adjustment			$	11.3			$	33.7

Net_Income_Per_Common_Share_Co

Net Income Per Common Share Computations	9 Months Ended
	Sep. 30, 2013
Net Income Per Common Share Computations	11. Net Income Per Common Share Computations – The computations of net income per common share are presented in the following table.
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
	($ In Millions Except Per Common Share Information)	2013		2012		2013		2012
	Basic Net Income Per Common Share
	Average Number of Common Shares Outstanding		239,930,074		240,237,014		239,614,868		240,740,803
	Net Income Applicable to Common Stock	$	206.5	$	178.8	$	561.6	$	519.6
	Less: Earnings Allocated to Participating Securities		3.5		2.8		9.1		7.6
	Earnings Allocated to Common Shares Outstanding		203		176		552.5		512
	Basic Net Income Per Common Share		0.85		0.73		2.31		2.13
	Diluted Net Income Per Common Share
	Average Number of Common Shares Outstanding		239,930,074		240,237,014		239,614,868		240,740,803
	Plus: Dilutive Effect of Share-based Compensation		1,400,578		460,048		1,242,829		464,383
	Average Common and Potential Common Shares		241,330,652		240,697,062		240,857,697		241,205,186
	Earnings Allocated to Common and Potential Common Shares	$	203	$	176	$	552.5	$	512
	Diluted Net Income Per Common Share		0.84		0.73		2.29		2.12
	Note: Common stock equivalents totaling 3,371,680 and 3,783,018 for the three and nine months ended September 30, 2013, respectively, and 11,890,286 and 12,412,541 for the three and nine months ended September 30, 2012, 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	9 Months Ended
	Sep. 30, 2013
Net Interest Income	12. Net Interest Income – The components of net interest income were as follows:
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
	(In Millions)	2013		2012		2013		2012
	Interest Income
	Loans and Leases	$	185.4	$	206.9	$	558.5	$	630.4
	Securities – Taxable		60.9		63.2		167.6		191
	– Non-Taxable		2.7		4.1		9		13.9
	Interest-Bearing Deposits with Banks		36.3		44.8		105.8		138.8
	Federal Reserve Deposits and Other		5.8		4.1		12.2		11.5
	Total Interest Income		291.1		323.1		853.1		985.6
	Interest Expense
	Deposits		25.8		43.8		79.9		122.9
	Federal Funds Purchased		0.4		0.1		1		1.1
	Securities Sold Under Agreements to Repurchase		0.1		0.1		0.3		0.3
	Other Borrowings		0.8		1.2		2.5		3.3
	Senior Notes		18.4		18.6		56.9		52.4
	Long-Term Debt		8		13		27.5		47.3
	Floating Rate Capital Debt		0.6		0.7		1.8		2.2
	Total Interest Expense		54.1		77.5		169.9		229.5
	Net Interest Income	$	237	$	245.6	$	683.2	$	756.1

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Taxes	13. Income Taxes – Income tax expense for the three and nine months ended September 30, 2013 of $95.0 million and $268.2 million was recorded, representing an effective tax rate of 31.5% and 32.3%, respectively. The prior year three and nine month provisions for income tax were $87.3 million and $249.5 million, representing effective tax rates of 32.8% and 32.4%, respectively.

Pension_and_Other_Postretireme

Pension and Other Postretirement Plans	9 Months Ended
	Sep. 30, 2013
Pension and Other Postretirement Plans	14. Pension and Other Postretirement Plans – 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 other postretirement plan for the three and nine months ended September 30, 2013 and 2012.
	Net Periodic Pension Expense	Three Months Ended						Nine Months Ended
	U.S. Plan	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Service Cost	$	7.5		$	8.9		$	22.7		$	26.5
	Interest Cost		10.5			10.3			31.6			31.1
	Expected Return on Plan Assets		(23.1	)		(21.7	)		(69.2	)		(65.3	)
	Amortization
	Net Actuarial Loss		10.3			8.6			30.9			25.8
	Prior Service Cost		(0.1	)		(0.1	)		(0.3	)		(0.3	)
	Net Periodic Pension Expense	$	5.1		$	6		$	15.7		$	17.8
	Net Periodic Pension Expense	Three Months Ended						Nine Months Ended
	Non U.S. Plans	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Interest Cost	$	1.6		$	1.6		$	4.9		$	4.7
	Expected Return on Plan Assets		(1.5	)		(1.7	)		(4.6	)		(4.9	)
	Net Actuarial Loss Amortization		0.2			0.2			0.7			0.6
	Net Periodic Pension Expense	$	0.3		$	0.1		$	1		$	0.4
	Net Periodic Pension Expense	Three Months Ended						Nine Months Ended
	Supplemental Plan	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Service Cost	$	0.4		$	0.8		$	1.2		$	2.2
	Interest Cost		1.1			1.2			3.3			3.4
	Amortization
	Net Actuarial Loss		1.7			1.5			5.1			4.5
	Prior Service Cost		0.2			0.2			0.4			0.4
	Net Periodic Pension Expense	$	3.4		$	3.7		$	10		$	10.5
	Net Periodic Postretirement Benefit	Three Months Ended						Nine Months Ended
	Other Postretirement Plan	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Service Cost	$	—		$	0.1		$	0.1		$	0.2
	Interest Cost		0.3			0.3			0.9			1
	Amortization
	Net Actuarial Gain		(0.3	)		(0.7	)		(0.9	)		(1.6	)
	Prior Service Cost		(0.7	)		(1.2	)		(2.2	)		(3.8	)
	Net Periodic Postretirement Benefit	$	(0.7	)	$	(1.5	)	$	(2.1	)	$	(4.2	)

ShareBased_Compensation_Plans

Share-Based Compensation Plans	9 Months Ended
	Sep. 30, 2013
Share-Based Compensation Plans	15. 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, stock units, and performance stock units.
	Total compensation expense for share-based payment arrangements and the associated tax impacts were as follows for the three and nine months ended September 30, 2013 and 2012.
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
	(In Millions)	2013		2012		2013		2012
	Stock and Stock Unit Awards	$	12.1	$	10.2	$	36.4	$	33.9
	Stock Options		3.7		5.7		14.9		22.3
	Performance Stock Units		2.1		0.7		5.4		1.8
	Total Share-Based Compensation Expense		17.9		16.6		56.7		58
	Tax Benefits Recognized	$	6.8	$	6.2	$	21.3	$	21.8

Variable_Interest_Entities

Variable Interest Entities	9 Months Ended
	Sep. 30, 2013
Variable Interest Entities	16. Variable Interest Entities – Variable Interest Entities (VIEs) are defined within GAAP as entities which 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 and 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% of the asset’s cost via an equity ownership in a trust with the remaining 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 September 30, 2013 and December 31, 2012, the carrying amounts of these investments, which are included in loans and leases in the consolidated balance sheet, were $669.4 million and $673.6 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 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, and Northern Trust typically invests as a limited partner/investor member in the form of equity contributions. The economic performance of the community development projects, which are deemed to be VIEs, is driven by the performance of their underlying investment projects as well as the VIEs’ 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 project or to affect the VIEs’ 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 amounts of its investments, including any unfunded commitments. As of September 30, 2013 and December 31, 2012, the carrying amounts of these investments in community development projects deemed to be VIEs, which are included in other assets in the consolidated balance sheet, were $213.3 million and $248.2 million, respectively. As of September 30, 2013 and December 31, 2012, liabilities related to unfunded commitments on investments in community development projects, which are included in other liabilities in the consolidated balance sheet, were $22.4 million and $33.1 million, respectively. Northern Trust’s funding requirements are limited to its invested capital and any additional unfunded commitments for future equity contributions. Northern Trust has no other liquidity arrangements or obligations to purchase assets of the community development projects that would expose it to a loss.
	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 on its consolidated balance sheet 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	9 Months Ended
	Sep. 30, 2013
Contingent Liabilities	17. Contingent Liabilities – Standby Letters of Credit and Indemnifications. 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. Certain standby letters of credit have been secured with cash deposits or participated to others and in certain cases Northern Trust is able to recover the amounts paid through recourse against these cash deposits or other participants. Standby letters of credit outstanding were $4.3 billion at September 30, 2013 and $4.6 billion at December 31, 2012.
	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 by the Northern Trust Senior Credit Committee. In connection with these activities, Northern Trust has issued indemnifications against certain losses resulting from the bankruptcy of borrowers of securities. Borrowers are required to fully collateralize 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 subject to indemnification was $84.7 billion at September 30, 2013 and $69.7 billion at December 31, 2012. Because of the credit quality of the borrowers and the requirement to fully collateralize securities borrowed, management believes that the exposure to credit loss from this activity is not significant and no liability was recorded at September 30, 2013 or December 31, 2012 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”. Thus, references to the upper end of the range of reasonably possible loss for cases in which the Corporation is able to estimate a range of reasonably possible loss mean the upper end of the range of loss for cases for which the Corporation believes the risk of loss is more than remote.
	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, or (vi) 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 September 30, 2013, the Corporation has estimated the upper end of the range of reasonably possible losses for these matters to be approximately $117 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. The following is a description of the nature of certain of these matters.
	As previously disclosed, a number of participants in our securities lending program, which is associated with the Corporation’s asset servicing business, have commenced putative class actions in which they claim, among other things, that we 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). Based on our review of these matters, we believe we operated our securities lending program prudently and appropriately. At this stage of these proceedings, however, it is not possible for management to assess the probability of a material adverse outcome or reasonably estimate the amount of any potential loss.
	Visa Membership. Northern Trust, as a member of Visa U.S.A. Inc. (Visa U.S.A.) and in connection with the 2007 initial public offering of Visa Inc. (Visa), received shares of restricted stock in Visa, a portion of which was redeemed pursuant to a mandatory redemption. The proceeds of the redemption totaled $167.9 million and were recorded as a gain in 2008. The remaining Visa shares held by Northern Trust are recorded at their original cost basis of zero and as of September 30, 2013 had restrictions as to their sale or transfer.
	Northern Trust is obligated to indemnify Visa for losses resulting from certain indemnified litigation involving Visa 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 charges and corresponding liabilities of $150 million 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 Visa related indemnification liability and of the future realization of the value of outstanding shares of Visa common stock held by Northern Trust as a member bank of Visa U.S.A. Reductions of Northern Trust’s indemnification liability totaling $23.1 million, $33.0 million, and $17.8 million were recorded in 2011, 2010, and 2009, respectively, which combined with a $76.1 million reduction recorded in 2008, fully eliminated the recorded indemnification liability as of December 31, 2011.
	On October 19, 2012, Visa signed a settlement agreement with plaintiff representatives for binding settlement of the indemnified litigation relating to interchange fees. While the final settlement and ultimate resolution of outstanding Visa related litigation and the timing for removal of selling restrictions on shares owned by Northern Trust are highly uncertain, based upon the settlement terms announced by Visa, Northern Trust anticipates that the value of its remaining shares of Visa stock will be adequate to offset any remaining indemnification obligations related to Visa litigation.
	Contingent Purchase Consideration. In connection with acquisitions consummated in 2011, contingent consideration was recorded relating to certain performance-related purchase price adjustments. The fair value of the contingent consideration at September 30, 2013 and December 31, 2012 was $54.0 million and $50.1 million, respectively.

Derivative_Financial_Instrumen

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2013
Derivative Financial Instruments	18. 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, interest rate contracts, and credit default swap 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 and the failure of Northern Trust to pay based on the contractual terms of the agreement, and is generally limited to the unrealized fair value gains and losses on these instruments, respectively. 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. This 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 Annex and other similar agreements are currently in place with a number of 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 sheet 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 statement 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 sheet were each reduced by $1.7 billion as of September 30, 2013 and by $982.5 million as of December 31, 2012, as a result of master netting arrangements and similar agreements in place. Derivative assets and liabilities recorded at September 30, 2013 also reflect reductions of $300.5 million and $495.9 million, 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 $118.6 million and $425.0 million, respectively, at December 31, 2012. Additional cash collateral received from and deposited with derivative counterparties totaling $152.8 million and $50.7 million, respectively, as of September 30, 2013, and $1.6 million and $73.3 million, respectively, as of December 31, 2012, were not offset against derivative assets and liabilities on the consolidated balance sheet as the amounts exceeded the net derivative positions with those counterparties. Effective in the second quarter of 2013, Northern Trust centrally clears interest rate derivative instruments that are addressed under Title VII of the Dodd-Frank Wall Street Reform and Consumer Protection Act. Securities posted as collateral for these transactions totaled $12.9 million, are not offset against derivative assets and liabilities on the consolidated balance sheet, 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 the 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 $104.3 million and $178.9 million at September 30, 2013 and December 31, 2012, respectively. Cash collateral amounts deposited with derivative counterparties on those dates included $39.3 million and $155.4 million, respectively, posted against these liabilities, resulting in a net maximum amount of termination payments that could have been required at September 30, 2013 and December 31, 2012 of $65.0 million and $23.5 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 on behalf of its clients and 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, and swaptions, and provide for the transfer or reduction of interest rate risk 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 option contracts for risk management purposes.
	Credit default swap contracts are agreements to transfer credit default risk from one party to another in exchange for a fee. Northern Trust enters into credit default swaps with outside counterparties where the counterparty agrees to assume the underlying credit exposure of a specific Northern Trust commercial loan or loan commitment.
	Client-Related and Trading Derivative Instruments. In excess of 96% of Northern Trust’s derivatives outstanding at September 30, 2013 and December 31, 2012, 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 sheet. 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.
		30-Sep-13												31-Dec-12
		Notional				Fair Value								Notional				Fair Value
	(In Millions)	Value				Asset				Liability				Value				Asset			Liability
	Foreign Exchange Contracts	$	252,806.20			$	3,162.10			$	3,165.30			$	213,246.70			$	1,735.30		$	1,730.40
	Interest Rate Contracts		5,039.20				134.9				128.6				4,946.60				180.6			174
	Total	$	257,845.40			$	3,297.00			$	3,293.90			$	218,193.30			$	1,915.90		$	1,904.40
	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 statement of income for the three and nine months ended September 30, 2013 and 2012.
				Amount of Derivative Gain/(Loss) Recognized in
				Income
		Location of Derivative		Three Months Ended								Nine Months Ended
		Gain/(Loss) Recognized in		September 30,								September 30,
	(In Millions)	Income		2013				2012				2013				2012
	Foreign Exchange Contracts	Foreign Exchange Trading Income		$	62.8			$	44			$	193.6			$	165.3
	Interest Rate Contracts	Security Commissions and Trading Income			4.4				3.7				11.4				8.9
	Total			$	67.2			$	47.7			$	205			$	174.2
	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 designated as hedges and used by Northern Trust to manage risk, their notional and fair values, and the respective risks addressed.
						30-Sep-13												31-Dec-12
		Derivative		Risk		Notional				Fair Value								Notional			Fair Value
	(In Millions)	Instrument		Classification		Value				Asset				Liability				Value			Asset		Liability
	Fair Value Hedges
	Available for Sale Investment Securities	Interest Rate		Interest		$	3,646.90			$	23.7			$	44.4			$	3,617.00		$	3.4	$	75.1
		Swap Contracts		Rate
	Senior Notes and Long- Term Subordinated Debt	Interest Rate		Interest			500				84.2				—				900			126.3		0.2
		Swap Contracts		Rate
	Cash Flow Hedges
	Forecasted Foreign Currency Denominated Transactions	Foreign Exchange		Foreign			390.9				8				9				669			8.7		11.5
		Contracts		Currency
	Net Investment Hedges
	Net Investments in Non-U.S. Affiliates	Foreign Exchange		Foreign			1,661.30				7.1				26.8				1,451.40			2.3		27.8
		Contracts		Currency
	Total					$	6,199.10			$	123			$	80.2			$	6,637.40		$	140.7	$	114.6
	In addition to the above, Sterling denominated debt, totaling $249.9 million and $242.3 million at September 30, 2013 and December 31, 2012, respectively, was designated as a hedge of the foreign exchange risk associated with the net investment in certain non-U.S. affiliates.
	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 statement of income related to fair value hedges for the three and nine months ended September 30, 2013 and 2012.
				Location of		Amount of Derivative
				Derivative		Gain/(Loss) Recognized in Income
		Derivative		Gain/(Loss)		Three Months Ended								Nine Months Ended
				Recognized		September 30,								September 30,
	(In Millions)	Instrument		in Income		2013				2012				2013				2012
	Available for Sale Investment Securities	Interest Rate		Interest		$	(16.4	)		$	(23.3	)		$	21.6			$	(93.5	)
		Swap Contracts		Income
	Senior Notes and Long- Term Subordinated Debt	Interest Rate		Interest			(3.6	)			8.9				(16.1	)			171.8
		Swap Contracts		Expense
	Total					$	(20.0	)		$	(14.4	)		$	5.5			$	78.3
	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 as the gain or loss. For fair value hedges that do not qualify for the “shortcut” method of accounting, Northern Trust utilizes regression analysis, a “long-haul” method of accounting, in assessing whether the hedging relationships are highly effective at inception and on an ongoing basis. There were $0.3 million of losses and $0.2 million of gains recorded within the fair values of hedged items for such “long-haul” hedges during the three months ended September 30, 2013 and 2012, respectively. There were $0.9 million of losses and $0.5 million of gains recorded within the fair values of the hedged items for the nine months ended September 30, 2013 and 2012, respectively. There were losses of $0.3 million and $0.8 million from ineffectiveness recorded during the three and nine months ended September 30, 2013, and a loss of $0.1 million and a gain of $0.4 million from ineffectiveness recorded during the three and nine month ended September 30, 2012 for available for sale investment securities, senior notes, and subordinated debt. Ineffectiveness resulting from fair value hedges is recorded in either interest income or interest expense.
	Derivatives are also designated as cash flow hedges in order to minimize the variability in cash flows of forecasted transactions caused by movements in 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. When the hedged forecasted transaction impacts earnings, balances in AOCI are reclassified to earnings. For cash flow hedges of 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. 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 and nine months ended September 30, 2013 and 2012. As of September 30, 2013, 23 months is the maximum length of time over which the exposure to variability in future cash flows of forecasted foreign currency denominated transactions is being hedged.
	The following tables provide cash flow hedge derivative gains and losses recognized in AOCI and the amounts reclassified to earnings during the three and nine months ended September 30, 2013 and 2012.
	(In Millions)	Foreign Exchange								Interest Rate Swap
		Contracts (Before Tax)								Contracts (Before Tax)
	Three Months Ended September 30,	2013				2012				2013				2012
	Net Gain/(Loss) Recognized in AOCI	$	5.3			$	2			$	—			$	—
	Net Gain/(Loss) Reclassified from AOCI to Earnings
	Other Operating Income		—				(1.0	)			—				—
Interest Income		—				—				—				—
Other Operating Expense		(1.1	)			(1.6	)			—				—
Total	$	(1.1	)		$	(2.6	)		$	—			$	—
(In Millions)	Foreign Exchange								Interest Rate Swap
	Contracts (Before Tax)								Contracts (Before Tax)
Nine Months Ended September 30,	2013				2012				2013				2012
Net Gain/(Loss) Recognized in AOCI	$	(4.0	)		$	4.7			$	—			$	—
Net Gain/(Loss) Reclassified from AOCI to Earnings
Other Operating Income		(1.8	)			(3.3	)			—				—
Interest Income		—				—				—				(0.2	)
Other Operating Expense		(3.9	)			(0.2	)			—				—
Total	$	(5.7	)		$	(3.5	)		$	—			$	(0.2	)
During the three and nine months ended September 30, 2013 and 2012, 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 $1.0 million will be reclassified into earnings within the next twelve months relating to cash flow hedges.
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 and nine months ended September 30, 2013 and 2012. Amounts recorded in AOCI are reclassified to earnings 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 and nine months ended September 30, 2013 and 2012.
	Hedging Gain/(Loss)
	Recognized in OCI (Before Tax)
	Three Months Ended September 30,								Nine Months Ended September 30,
(In Millions)	2013				2012				2013				2012
Foreign Exchange Contracts	$	(73.8	)		$	(35.4	)		$	(9.5	)		$	(8.1	)
Sterling Denominated Subordinated Debt		(15.0	)			(9.4	)			0.2				(9.3	)
Total	$	(88.8	)		$	(44.8	)		$	(9.3	)		$	(17.4	)
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. Credit default swaps are entered into to manage the credit risk associated with certain loans and loan commitments. The following table identifies the types of risk management derivative instruments not formally designated as hedges and their notional amounts and fair values.
	30-Sep-13												31-Dec-12
	Notional				Fair Value								Notional				Fair Value
(In Millions)	Value				Asset				Liability				Value				Asset			Liability
Credit Default Swap Contracts	$	—			$	—			$	—			$	42.5			$	—		$	1
Foreign Exchange Contracts		144				1.9				0.9				1,189.80				10.3			3
Total	$	144			$	1.9			$	0.9			$	1,232.30			$	10.3		$	4
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 statement of income for the three and nine months ended September 30, 2013 and 2012.
	Location of		Amount of Derivative Gain/(Loss)
			Recognized in Income
	Derivative Gain/		Three Months Ended								Nine Months Ended
	(Loss) Recognized		September 30,								September 30,
(In Millions)	in Income		2013				2012				2013				2012
Credit Default Swap Contracts	Other Operating Income		$	—			$	(0.6	)		$	(0.1	)		$	(2.5	)
Foreign Exchange Contracts	Other Operating Income			6.7				5				(4.3	)			3.8
Total			$	6.7			$	4.4			$	(4.4	)		$	1.3

Offsetting_of_Assets_and_Liabi

Offsetting of Assets and Liabilities	9 Months Ended
	Sep. 30, 2013
Offsetting of Assets and Liabilities	19. 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 sheet as of September 30, 2013 and December 31, 2012.
	30-Sep-13
	(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)		$	2,880.10	$	1,593.70	$	1,286.40	$	—	$	1,286.40
	Interest Rate Swaps OTC			237.6		46.8		190.8		—		190.8
	Interest Rate Swaps Exchange Cleared			5.2		4.3		0.9		—		0.9
	Interest Rate Options OTC			—		—		—		—		—
	Cross Product Netting Adjustment			—		73		—		—		—
	Cross Product Collateral Adjustment			—		300.5		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,122.90		2,018.30		1,104.60		—		1,104.60
	Total Derivatives Not Subject to a Master Netting Arrangement			299		—		299		—		299
	Total Derivatives			3,421.90		2,018.30		1,403.60		—		1,403.60
	Securities Purchased under Agreements to Resell (2)		$	500	$	—	$	500	$	500	$	—
	-1	Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $3,549.3 million as of September 30, 2013.
	-2	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheet. Federal funds sold totaled $34.6 million as of September 30, 2013.
	-3	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of September 30, 2013.
	31-Dec-12
	(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		$	1,756.60	$	877.1	$	879.5	$	—	$	879.5
	Interest Rate Swaps OTC			310.3		68.3		242		—		242
	Cross Product Netting Adjustment			—		37.1		—		—		—
	Cross Product Collateral Adjustment			—		118.6		—		—		—
	Total			2,066.90		1,101.10		965.8		—		965.8
	Securities Purchased under Agreements to Resell (2)		$	35.4	$	—	$	35.4	$	35.4	$	—
	-1	Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $2,964.4 million as of December 31, 2012.
	-2	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheet. Federal funds sold totaled $25.4 million as of December 31, 2012.
	-3	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of December 31, 2012.
	The following tables provide information regarding the offsetting of derivative liabilities and securities sold under agreements to repurchase within the consolidated balance sheet as of September 30, 2013 and December 31, 2012.
	30-Sep-13
	(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		$	2,550.30	$	1,593.70	$	956.6	$	—	$	956.6
	Interest Rate Swaps OTC			168.7		46.8		121.9		—		121.9
	Interest Rate Swaps Exchange Cleared			4.3		4.3		—		—		—
	Interest Rate Options OTC			—		—		—		—		—
	Credit Default Swaps OTC			—		—		—		—		—
	Cross Product Netting Adjustment			—		73		—		—		—
	Cross Product Collateral Adjustment			—		495.9		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			2,723.30		2,213.70		509.6		—		509.6
	Total Derivatives Not Subject to a Master Netting Arrangement			651.7		—		651.7		—		651.7
	Total Derivatives			3,375.00		2,213.70		1,161.30		—		1,161.30
	Securities Sold under Agreements to Repurchase		$	469	$	—	$	469	$	469	$	—
	-1	Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,235.0 million as of September 30, 2013.
	-2	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of September 30, 2013.
	31-Dec-12
	(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		$	1,772.70	$	877.1	$	895.6	$	—	$	895.6
	Interest Rate Swaps OTC			249.3		68.3		181		—		181
	Credit Default Swaps OTC			1		—		1		—		1
	Cross Product Netting Adjustment			—		37.1		—		—		—
	Cross Product Collateral Adjustment			—		425		—		—		—
	Total			2,023.00		1,407.50		615.5		—		615.5
Securities Sold under Agreements to Repurchase		$	699.8	$	—	$	699.8	$	699.8	$	—
-1	Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $1,961.7 million as of December 31, 2012.
-2	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of December 31, 2012.
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. Effective in the second quarter of 2013, 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)	9 Months Ended
	Sep. 30, 2013
Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level	The following presents assets and liabilities measured at fair value on a recurring basis as of September 30, 2013 and December 31, 2012, segregated by fair value hierarchy level.
														Assets/Liabilities
	(In Millions)		Level 1			Level 2			Level 3		Netting			at Fair Value
	September 30, 2013
	Securities
	Available for Sale
	U.S. Government		$	1,524.50		$	—		$	—	$	—		$	1,524.50
	Obligations of States and Political Subdivisions			—			14.4			—		—			14.4
	Government Sponsored Agency			—			17,024.60			—		—			17,024.60
	Corporate Debt			—			3,491.00			—		—			3,491.00
	Covered Bonds			—			1,805.60			—		—			1,805.60
	Supranational, Sovereign and Non-U.S. Agency Bonds			—			721.1			—		—			721.1
	Residential Mortgage-Backed			—			53.5			—		—			53.5
	Other Asset-Backed			—			2,535.70			—		—			2,535.70
	Auction Rate			—			—			98.7		—			98.7
	Other			—			232.5			—		—			232.5
	Total Available for Sale			1,524.50			25,878.40			98.7		—			27,501.60
	Trading Account			—			9.6			—		—			9.6
	Total Available for Sale and Trading			1,524.50			25,888.00			98.7		—			27,511.20
	Other Assets
	Derivative Assets
	Foreign Exchange Contracts			—			3,179.10			—		—			3,179.10
	Interest Rate Contracts			—			242.8			—		—			242.8
	Total Derivative Assets			—			3,421.90			—		(2,018.3	)		1,403.60
	Other Liabilities
	Derivative Liabilities
	Foreign Exchange Contracts			—			3,202.00			—		—			3,202.00
	Interest Rate Contracts			—			173			—		—			173
	Credit Default Swaps			—			—			—		—			—
	Total Derivative Liabilities			—			3,375.00			—		(2,213.7	)		1,161.30
	Contingent Consideration		$	—		$	—		$	54	$	—		$	54
	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 September 30, 2013, derivative assets and liabilities shown above also include reductions of $300.5 million and $495.9 million, 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, 2012
	Securities
	Available for Sale
	U.S. Government		$	1,784.60		$	—		$	—	$	—		$	1,784.60
	Obligations of States and Political Subdivisions			—			14.1			—		—			14.1
	Government Sponsored Agency			—			18,638.80			—		—			18,638.80
	Corporate Debt			—			2,618.40			—		—			2,618.40
	Covered Bonds			—			1,748.00			—		—			1,748.00
	Supranational Bonds			—			1,060.70			—		—			1,060.70
	Residential Mortgage-Backed			—			92			—		—			92
	Other Asset-Backed			—			2,283.90			—		—			2,283.90
	Auction Rate			—			—			97.8		—			97.8
	Other			—			305.2			—		—			305.2
	Total Available for Sale			1,784.60			26,761.10			97.8		—			28,643.50
	Trading Account			—			8			—		—			8
	Total Available for Sale and Trading			1,784.60			26,769.10			97.8		—			28,651.50
	Other Assets
	Derivative Assets
	Foreign Exchange Contracts			—			1,756.60			—		—			1,756.60
	Interest Rate Contracts			—			310.3			—		—			310.3
	Total Derivative Assets			—			2,066.90			—		(1,101.1	)		965.8
	Other Liabilities
	Derivative Liabilities
	Foreign Exchange Contracts			—			1,772.70			—		—			1,772.70
	Interest Rate Contracts			—			249.3			—		—			249.3
	Credit Default Swaps			—			1			—		—			1
	Total Derivative Liabilities			—			2,023.00			—		(1,407.5	)		615.5
	Contingent Consideration		$	—		$	—		$	50.1	$	—		$	50.1
	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, 2012, derivative assets and liabilities shown above also include reductions of $118.6 million and $425.0 million, 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 and nine months ended September 30, 2013 and 2012.
	Level 3 Assets (In Millions)		Auction Rate Securities
	Three Months Ended September 30,		2013			2012
	Fair Value at July 1		$	99.1		$	105.2
	Total Gains and (Losses):
	Included in Earnings (1)			—			0.3
	Included in Other Comprehensive Income (2)			0.3			(1.1	)
	Purchases, Issues, Sales, and Settlements
	Sales			—			(0.1	)
	Settlements			(0.7	)		(4.8	)
	Fair Value at September 30		$	98.7		$	99.5
	Level 3 Assets (In Millions)		Auction Rate Securities
	Nine Months Ended September 30,		2013			2012
	Fair Value at January 1		$	97.8		$	178.3
	Total Gains and (Losses):
	Included in Earnings (1)			0.1			(22.0	)
	Included in Other Comprehensive Income (2)			3			4.9
	Purchases, Issues, Sales, and Settlements
	Sales			—			(54.7	)
	Settlements			(2.2	)		(7.0	)
	Fair Value at September 30		$	98.7		$	99.5
	-1	Realized gains for the three month period ended September 30, 2012 of $0.3 million represent gains from redemptions by issuers. Realized gains for the nine month period ended September 30, 2013 of $0.1 million represent gains from redemptions by issuers. Realized losses for the nine month period ended September 30, 2012 of $22.0 million include $20.8 million of losses from sales of securities and $1.6 million of impairment losses, partially offset by $0.4 million of gains from redemptions by issuers. Gains on redemptions are recorded in interest income and sales and impairment losses are recorded in investment security gains (losses), within the consolidated statement of income.
	-2	Unrealized losses related to auction rate securities are included in net unrealized gains (losses) on securities available for sale within the consolidated statement of comprehensive income.
Changes in Level 3 Liabilities
	Level 3 Liabilities (In Millions)		Contingent Consideration
	Three Months Ended September 30,		2013			2012
	Fair Value at July 1		$	52.7		$	57.2
	Total (Gains) and Losses:
	Included in Earnings (1)			1.3			0.1
	Included in Other Comprehensive Income (2)						(0.2	)
	Purchases, Issues, Sales, and Settlements
	Settlements			—			(8.2	)
	Fair Value at September 30		$	54		$	48.9
	Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held September 30 (1)		$	1.3		$	1.3
	Level 3 Liabilities (In Millions)		Contingent Consideration
	Nine Months Ended September 30,		2013			2012
	Fair Value at January 1		$	50.1		$	56.8
	Total (Gains) and Losses:
	Included in Earnings (1)			3.9			0.8
	Included in Other Comprehensive Income (2)			—			(0.5	)
	Purchases, Issues, Sales, and Settlements
	Settlements			—			(8.2	)
	Fair Value at September 30		$	54		$	48.9
	Unrealized (Gains) Losses Included in Earnings Related to Financial Instruments Held at September 30 (1)		$	3.9		$	3.5
	-1	Gains (losses) are recorded in other operating income (expense) within the consolidated statement of income.
	-2	Unrealized foreign currency related losses are included in foreign currency translation adjustments within the consolidated statement of comprehensive income.
Book and Fair Values of Financial Instruments	The following tables summarize the fair values of financial instruments.
	(In Millions)		September 30, 2013
			Book			Total			Fair Value
			Value			Fair Value			Level 1		Level 2			Level 3
	Assets
	Cash and Due from Banks		$	2,690.70		$	2,690.70		$	2,690.70	$	—		$	—
	Federal Funds Sold and Resell Agreements			534.6			534.6			—		534.6			—
	Interest-Bearing Deposits with Banks			17,383.90			17,383.90			—		17,383.90			—
	Federal Reserve Deposits and Other Interest-Bearing			8,452.80			8,452.80			—		8,452.80			—
	Securities
	Available for Sale (1)			27,501.60			27,501.60			1,524.50		25,878.40			98.7
	Held to Maturity			3,053.60			3,053.20			—		3,053.20			—
	Trading Account			9.6			9.6			—		9.6			—
	Loans (excluding Leases)
	Held for Investment			27,797.00			27,752.50			—		—			27,752.50
	Held for Sale			2.8			2.8			—		—			2.8
	Client Security Settlement Receivables			1,630.20			1,630.20			—		1,630.20			—
	Other Assets
	Federal Reserve and Federal Home Loan Bank Stock			194.7			194.7			—		194.7			—
	Community Development Investments			217.8			230			—		230			—
	Employee Benefit and Deferred Compensation			131			126.2			82.3		43.9			—
	Liabilities
	Deposits
	Demand, Noninterest-Bearing, Savings and Money Market		$	31,204.90		$	31,204.90		$	31,204.90	$	—		$	—
	Savings Certificates and Other Time			1,939.60			1,943.90			—		1,943.90			—
	Non U.S. Offices Interest-Bearing			45,017.20			45,017.20			—		45,017.20			—
	Federal Funds Purchased			1,053.60			1,053.60			—		1,053.60			—
	Securities Sold under Agreements to Repurchase			469			469			—		469			—
	Other Borrowings			1,804.70			1,804.70			—		1,804.70			—
	Senior Notes			1,996.50			2,002.30			—		2,002.30			—
	Long Term Debt (excluding Leases)
	Subordinated Debt			820.5			841.1			—		841.1			—
	Federal Home Loan Bank Borrowings			135			138.6			—		138.6			—
	Floating Rate Capital Debt			277.1			229.3			—		229.3			—
	Other Liabilities
	Standby Letters of Credit			55.6			55.6			—		—			55.6
	Contingent Consideration			54			54			—		—			54
	Loan Commitments			38.1			38.1			—		—			38.1
	Derivative Instruments
	Asset/Liability Management
	Foreign Exchange Contracts
	Assets		$	17		$	17		$	—	$	17		$	—
	Liabilities			36.7			36.7			—		36.7			—
	Interest Rate Contracts
	Assets			107.9			107.9			—		107.9			—
	Liabilities			44.4			44.4			—		44.4			—
	Credit Default Swaps
	Liabilities			—			—			—		—			—
	Client-Related and Trading
	Foreign Exchange Contracts
	Assets			3,162.10			3,162.10			—		3,162.10			—
	Liabilities			3,165.30			3,165.30			—		3,165.30			—
	Interest Rate Contracts
	Assets			134.9			134.9			—		134.9			—
	Liabilities			128.6			128.6			—		128.6			—
	-1	Refer to the table located on page 37 for the disaggregation of available for sale securities.
	(In Millions)		December 31, 2012
			Book			Total			Fair Value
			Value			Fair Value			Level 1		Level 2			Level 3
	Assets
	Cash and Due from Banks		$	3,752.70		$	3,752.70		$	3,752.70	$	—		$	—
	Federal Funds Sold and Resell Agreements			60.8			60.8			—		60.8			—
	Interest-Bearing Deposits with Banks			18,803.50			18,803.50			—		18,803.50			—
	Federal Reserve Deposits and Other Interest-Bearing			7,619.70			7,619.70			—		7,619.70			—
Securities
Available for Sale (1)			28,643.50			28,643.50			1,784.60		26,761.10			97.8
Held to Maturity			2,382.00			2,394.80			—		2,394.80			—
Trading Account			8			8			—		8			—
Loans (excluding Leases)
Held for Investment			28,165.40			28,220.20			—		—			28,220.20
Held for Sale			11.7			11.7			—		—			11.7
Client Security Settlement Receivables			2,049.10			2,049.10			—		2,049.10			—
Other Assets
Federal Reserve and Federal Home Loan Bank Stock			197.6			197.6			—		197.6			—
Community Development Investments			253.2			275.1			—		275.1			—
Employee Benefit and Deferred Compensation			121.3			126.1			86.7		39.4			—
Liabilities
Deposits
Demand, Noninterest-Bearing, Savings and Money Market		$	39,221.50		$	39,221.50		$	39,221.50	$	—		$	—
Savings Certificates and Other Time			2,466.10			2,746.70			—		2,746.70			—
Non U.S. Offices Interest-Bearing			39,720.20			39,720.20			—		39,720.20			—
Federal Funds Purchased			780.2			780.2			—		780.2			—
Securities Sold under Agreements to Repurchase			699.8			699.8			—		699.8			—
Other Borrowings			367.4			367.4			—		367.4			—
Senior Notes			2,405.80			2,513.40			—		2,513.40			—
Long Term Debt (excluding Leases)
Subordinated Debt			1,045.40			1,065.30			—		1,065.30			—
Federal Home Loan Bank Borrowings			335			345.4			—		345.4			—
Floating Rate Capital Debt			277			228			—		228			—
Other Liabilities
Standby Letters of Credit			60.5			60.5			—		—			60.5
Contingent Consideration			50.1			50.1			—		—			50.1
Loan Commitments			38.9			38.9			—		—			38.9
Derivative Instruments
Asset/Liability Management
Foreign Exchange Contracts
Assets		$	21.3		$	21.3		$	—	$	21.3		$	—
Liabilities			42.3			42.3			—		42.3			—
Interest Rate Contracts
Assets			129.7			129.7			—		129.7			—
Liabilities			75.3			75.3			—		75.3			—
Credit Default Swaps
Liabilities			1			1			—		1			—
Client-Related and Trading
Foreign Exchange Contracts
Assets			1,735.30			1,735.30			—		1,735.30			—
Liabilities			1,730.40			1,730.40			—		1,730.40			—
Interest Rate Contracts
Assets			180.6			180.6			—		180.6			—
Liabilities			174			174			—		174			—
-1	Refer to the table located on page 38 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 and liabilities as of September 30, 2013.
	Financial Instrument		Fair Value			Valuation		Unobservable Input	Range of Lives
						Technique			and Rates
	Auction Rate Securities		$	98.7 million		Discounted Cash		Remaining lives	2.7 – 8.6 years
						Flow		Discount rates	0.2% – 8.1%
	Contingent Consideration		$	54.0 million		Discounted Cash		Discount rate	10.50%
						Flow		Business growth rates	19% – 35%
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 September 30, 2013.
	Financial Instrument		Fair Value		Valuation Technique	Unobservable Input		Range of Discounts
								Applied
	Loans		$38.1 million		Market Approach	Discount to reflect		15% – 40%
						realizable value
	OREO		$1.6 million		Market Approach	Discount to reflect		15% – 40%
						realizable value

Securities_Tables

Securities (Tables)	9 Months Ended
	Sep. 30, 2013
Reconciliation of Amortized Cost to Fair Values of Securities Available for Sale	The following tables provide the amortized cost and fair values of securities at September 30, 2013 and December 31, 2012.
	Securities Available for Sale		September 30, 2013
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	U.S. Government		$	1,498.70		$	25.8		$	—		$	1,524.50
	Obligations of States and Political Subdivisions			13.9			0.5			—			14.4
	Government Sponsored Agency			16,979.10			81.6			36.1			17,024.60
	Corporate Debt			3,537.00			8.5			54.5			3,491.00
	Covered Bonds			1,774.20			33.7			2.3			1,805.60
	Supranational, Sovereign and Non-U.S. Agency Bonds			718.5			4.1			1.5			721.1
	Residential Mortgage-Backed			58.1			0.1			4.7			53.5
	Other Asset-Backed			2,535.70			1.2			1.2			2,535.70
	Auction Rate			98.1			2.1			1.5			98.7
	Other			232			0.5			—			232.5
	Total		$	27,445.30		$	158.1		$	101.8		$	27,501.60
	Securities Available for Sale		December 31, 2012
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	U.S. Government		$	1,747.90		$	36.7		$	—		$	1,784.60
	Obligations of States and Political Subdivisions			13.9			0.2			—			14.1
	Government Sponsored Agency			18,520.60			122.2			4			18,638.80
	Corporate Debt			2,602.40			18.1			2.1			2,618.40
	Covered Bonds			1,697.10			51			0.1			1,748.00
	Supranational Bonds			1,053.90			7			0.2			1,060.70
	Residential Mortgage-Backed			102.4			0.4			10.8			92
	Other Asset-Backed			2,280.00			4.3			0.4			2,283.90
	Auction Rate			99.6			2.1			3.9			97.8
	Other			304.4			0.8			—			305.2
	Total		$	28,422.20		$	242.8		$	21.5		$	28,643.50
Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity	Securities Held to Maturity		September 30, 2013
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	Obligations of States and Political Subdivisions		$	248.8		$	11.3		$	—		$	260.1
	Government Sponsored Agency			41.3			1.2			—			42.5
	Non-U.S. Government Debt			219.3			—			—			219.3
	Certificates of Deposit			1,497.40			—			0.2			1,497.20
	Supranational and Sovereign Bonds			989.5			0.8			1.9			988.4
	Other			57.3			—			11.6			45.7
	Total		$	3,053.60		$	13.3		$	13.7		$	3,053.20
	Securities Held to Maturity		December 31, 2012
			Amortized			Gross Unrealized						Fair
	(In Millions)		Cost			Gains			Losses			Value
	Obligations of States and Political Subdivisions		$	329.3		$	17.2		$	—		$	346.5
	Government Sponsored Agency			112.9			3.8			—			116.7
	Non-U.S. Government Debt			205			—			—			205
	Certificates of Deposit			1,667.60			0.2			0.6			1,667.20
	Other			67.2			0.3			8.1			59.4
	Total		$	2,382.00		$	21.5		$	8.7		$	2,394.80
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 September 30, 2013 and December 31, 2012.
	Securities with Unrealized Losses		Less than 12 Months						12 Months or Longer						Total
	as of September 30, 2013
			Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
	(In Millions)		Value			Losses			Value			Losses			Value			Losses
	Government Sponsored Agency		$	3,776.10		$	32.3		$	302.7		$	3.8		$	4,078.80		$	36.1
	Corporate Debt			1,911.20			54.4			25			0.1			1,936.20			54.5
	Covered Bonds			207.4			2.3			—			—			207.4			2.3
	Supranational, Sovereign and Non-US Agency Bonds			549.6			3.4			—			—			549.6			3.4
	Residential Mortgage-Backed			—			—			45.5			4.7			45.5			4.7
	Other Asset-Backed			1,113.90			1.1			45.1			0.1			1,159.00			1.2
	Certificates of Deposit			735.7			0.2			—			—			735.7			0.2
	Auction Rate			27.8			0.4			16.5			1.1			44.3			1.5
	Other			37.9			3.4			29			8.2			66.9			11.6
	Total		$	8,359.60		$	97.5		$	463.8		$	18		$	8,823.40		$	115.5
	Securities with Unrealized Losses		Less than 12 Months						12 Months or Longer						Total
	as of December 31, 2012
			Fair			Unrealized			Fair			Unrealized			Fair			Unrealized
	(In Millions)		Value			Losses			Value			Losses			Value			Losses
	Government Sponsored Agency		$	482.2		$	1		$	1,171.80		$	3		$	1,654.00		$	4
	Corporate Debt			441.5			2			50			0.1			491.5			2.1
	Covered Bonds			20.1			0.1			—			—			20.1			0.1
	Supranational Bonds			113.8			0.2			—			—			113.8			0.2
	Residential Mortgage-Backed			—			—			84.7			10.8			84.7			10.8
	Other Asset-Backed			146.1			0.1			40			0.3			186.1			0.4
	Certificates of Deposit			1,178.80			0.6			—			—			1,178.80			0.6
	Auction Rate			2.7			0.3			41			3.6			43.7			3.9
	Other			9.3			1.9			43.8			6.2			53.1			8.1
	Total		$	2,394.50		$	6.2		$	1,431.30		$	24		$	3,825.80		$	30.2
Book Values, Ultimate Default Rates, and Loss Severity Rates for Non-Agency Residential Mortgage-Backed Securities Portfolio, by Security Type	September 30, 2013 amortized cost, weighted average ultimate default rates, and impairment severity rates for the non-agency residential mortgage-backed securities portfolio, by security type, are provided in the following table.
									Loss Severity Rates
	Security Type		Amortized			Weighted Average			Low			High			Weighted
			Cost			Ultimate Default Rates									Average
	Prime		$	7.4			18.9	%		32.9	%		45	%		41.1	%
	Alt-A			12.4			43.1			66.2			66.2			66.2
	Subprime			26.7			48.8			72.5			82.1			77.6
	2nd Lien			11.6			33.1			98.9			99			99
	Total Non-Agency Residential Mortgage-Backed Securities		$	58.1			40.1	%		32.9	%		99	%		74.8	%
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 and nine months ended September 30, 2013 and 2012.
			Three Months Ended						Nine Months Ended
			September  30,						September  30,
	(In Millions)		2013			2012			2013			2012
	Changes in OTTI Losses*		$	—		$	0.4		$	—		$	(2.7	)
	Noncredit-related Losses Recorded in / (Reclassified from) OCI**			—			(0.6	)		—			(0.6	)
	Net Impairment Losses Recognized in Earnings		$	—		$	(0.2	)	$	—		$	(3.3	)
	*	For initial other-than-temporary impairments in the respective 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 other-than-temporary impairments in the respective period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in 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.
			Three Months Ended						Nine Months Ended
			September 30,						September 30,
	(In Millions)		2013			2012			2013			2012
	Cumulative Credit-Related Losses on Securities Held – Beginning of Period		$	8.8		$	43.2		$	42.3		$	68.2
	Plus: Losses on Newly Identified Impairments			—			—			—			1.6
	Additional Losses on Previously Identified Impairments			—			0.2			—			1.7
	Less: Current and Prior Period Losses on Securities Sold During the Period			—			—			(33.5	)		(28.1	)
	Cumulative Credit-Related Losses on Securities Held – End of Period		$	8.8		$	43.4		$	8.8		$	43.4
Debt Securities Held for which Other-Than-Temporary Impairment Loss has been Recognized in Current Period or Previously	The table below provides information regarding debt securities held as of September 30, 2013 and December 31, 2012, for which an OTTI loss has been recognized in the current period or previously.
	(In Millions)		September 30,			December 31,
			2013			2012
	Fair Value		$	38.9		$	51.5
	Amortized Cost Basis			44.3			59
	Noncredit-related Losses Recognized in OCI			(5.4	)		(7.5	)
	Tax Effect			2			2.8
	Amount Recorded in OCI		$	(3.4	)	$	(4.7	)
Available for Sale Investment Securities
Remaining Maturity of Securities	The following table provides the remaining maturity of securities as of September 30, 2013.
	(In Millions)		Amortized			Fair
			Cost			Value
	Available for Sale
	Due in One Year or Less		$	9,630.10		$	9,645.70
	Due After One Year Through Five Years			14,551.90			14,617.70
	Due After Five Years Through Ten Years			2,264.20			2,242.10
	Due After Ten Years			999.1			996.1
	Total			27,445.30			27,501.60
Held-to-maturity Securities
Remaining Maturity of Securities	Held to Maturity
	Due in One Year or Less			1,844.30			1,846.20
	Due After One Year Through Five Years			1,134.80			1,138.70
	Due After Five Years Through Ten Years			37.7			40
	Due After Ten Years			36.8			28.3
	Total		$	3,053.60		$	3,053.20
	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)	9 Months Ended
	Sep. 30, 2013
Amounts Outstanding for Loans and Leases by Segment and Class	Amounts outstanding for loans and leases, by segment and class, are shown below.
	(In Millions)	September 30,			December 31,
		2013			2012
	Commercial
	Commercial and Institutional	$	7,237.00		$	7,468.50
	Commercial Real Estate		2,821.80			2,859.80
	Lease Financing, net		982.1			1,035.00
	Non-U.S.		1,155.20			1,192.30
	Other		285.1			341.6
	Total Commercial		12,481.20			12,897.20
	Personal
	Residential Real Estate		10,359.50			10,375.20
	Private Client		6,139.10			6,130.10
	Other		85			102
	Total Personal		16,583.60			16,607.30
	Total Loans and Leases		29,064.80			29,504.50
	Allowance for Credit Losses Assigned to Loans and Leases		(287.2	)		(297.9	)
	Net Loans and Leases	$	28,777.60		$	29,206.60
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 September 30, 2013 and December 31, 2012 are provided below, segregated by borrower ratings into “1 to 3”, “4 to 5” and “6 to 9” (watch list), categories.
		September 30, 2013										December 31, 2012
	(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	$	4,384.70		$	2,679.00		$	173.3	$	7,237.00	$	4,291.80	$	3,040.60	$	136.1	$	7,468.50
	Commercial Real Estate		992.9			1,652.20			176.7		2,821.80		888.6		1,710.90		260.3		2,859.80
	Lease Financing, net		674.6			303			4.5		982.1		647.1		382.3		5.6		1,035.00
	Non-U.S.		523.4			631.1			0.7		1,155.20		542.7		646.6		3		1,192.30
	Other		107.5			177.6			—		285.1		167.2		174.4		—		341.6
	Total Commercial		6,683.10			5,442.90			355.2		12,481.20		6,537.40		5,954.80		405		12,897.20
	Personal
	Residential Real Estate		3,289.70			6,535.40			534.4		10,359.50		3,003.30		6,868.20		503.7		10,375.20
	Private Client		3,855.50			2,261.70			21.9		6,139.10		3,741.30		2,365.40		23.4		6,130.10
	Other		32.1			52.9			—		85		50		52		—		102
	Total Personal		7,177.30			8,850.00			556.3		16,583.60		6,794.60		9,285.60		527.1		16,607.30
	Total Loans and Leases	$	13,860.40		$	14,292.90		$	911.5	$	29,064.80	$	13,332.00	$	15,240.40	$	932.1	$	29,504.50
Balances and Delinquency Status of Performing and Nonperforming Loans and Leases by Segment and Class as well as Total Other Real Estate Owned 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 other real estate owned and nonperforming asset balances, as of September 30, 2013 and December 31, 2012.
	September 30, 2013
					30-59 Days			60-89 Days		90 Days or		Total				Total Loans
	(In Millions)	Current			Past Due			Past Due		More Past Due		Performing		Nonperforming		and Leases
	Commercial
	Commercial and Institutional	$	7,194.10		$	12.2		$	2.8	$	3.8	$	7,212.90	$	24.1	$	7,237.00
	Commercial Real Estate		2,745.90			10.7			2.7		8.2		2,767.50		54.3		2,821.80
	Lease Financing, net		982.1			—			—		—		982.1		—		982.1
	Non-U.S.		1,155.20			—			—		—		1,155.20		—		1,155.20
	Other		285.1			—			—		—		285.1		—		285.1
	Total Commercial		12,362.40			22.9			5.5		12		12,402.80		78.4		12,481.20
	Personal
	Residential Real Estate		10,135.90			11.7			12.5		9.6		10,169.70		189.8		10,359.50
	Private Client		6,086.70			39.4			7.9		3.2		6,137.20		1.9		6,139.10
	Other		85			—			—		—		85		—		85
	Total Personal		16,307.60			51.1			20.4		12.8		16,391.90		191.7		16,583.60
	Total Loans and Leases	$	28,670.00		$	74		$	25.9	$	24.8	$	28,794.70	$	270.1	$	29,064.80
											Other Real Estate Owned				13.9
											Total Nonperforming Assets			$	284
	December 31, 2012
					30-59 Days			60-89 Days		90 Days or		Total				Total Loans
	(In Millions)	Current			Past Due			Past Due		More Past Due		Performing		Nonperforming		and Leases
	Commercial
	Commercial and Institutional	$	7,433.40		$	6.4		$	5.5	$	1.6	$	7,446.90	$	21.6	$	7,468.50
	Commercial Real Estate		2,782.00			6.9			13.1		1.4		2,803.40		56.4		2,859.80
	Lease Financing, net		1,035.00			—			—		—		1,035.00		—		1,035.00
	Non-U.S.		1,192.30			—			—		—		1,192.30		—		1,192.30
	Other		341.6			—			—		—		341.6		—		341.6
	Total Commercial		12,784.30			13.3			18.6		3		12,819.20		78		12,897.20
	Personal
	Residential Real Estate		10,096.30			68.1			25.7		10.5		10,200.60		174.6		10,375.20
	Private Client		6,091.30			14.8			16.3		5.5		6,127.90		2.2		6,130.10
	Other		102			—			—		—		102		—		102
	Total Personal		16,289.60			82.9			42		16		16,430.50		176.8		16,607.30
	Total Loans and Leases	$	29,073.90		$	96.2		$	60.6	$	19	$	29,249.70	$	254.8	$	29,504.50
											Other Real Estate Owned				20.3
											Total Nonperforming Assets			$	275.1
Impaired Loans By Segment and Class	The following tables provide information related to impaired loans by segment and class.
		As of September 30, 2013								As of December 31, 2012
					Unpaid							Unpaid
		Recorded			Principal			Specific		Recorded		Principal		Specific
	(In Millions)	Investment			Balance			Allowance		Investment		Balance		Allowance
	With No Related Specific Allowance
	Commercial and Institutional	$	11.3		$	14.5		$	—	$	15.1	$	19.3	$	—
	Commercial Real Estate		48.6			59.2			—		64.9		76.9		—
	Lease Financing, net		4.5			4.5			—		4.6		4.6		—
	Residential Real Estate		177.2			220.4			—		131.3		165.7		—
	Private Client		13.3			13.3			—		0.8		1		—
	With a Related Specific Allowance
	Commercial and Institutional		14			16.2			5		8.1		10.2		2.8
	Commercial Real Estate		31.1			32.6			8.7		32.3		33.8		8.2
	Residential Real Estate		10			10.7			3.3		11.8		13		6.1
	Private Client		0.8			0.8			0.8		0.9		0.9		0.9
	Total
	Commercial		109.5			127			13.7		125		144.8		11
	Personal		201.3			245.2			4.1		144.8		180.6		7
	Total	$	310.8		$	372.2		$	17.8	$	269.8	$	325.4	$	18
		Three Months Ended September 30,										Nine Months Ended September 30,
		2013						2012				2013				2012
		Average			Interest			Average		Interest		Average		Interest		Average		Interest
		Recorded			Income			Recorded		Income		Recorded		Income		Recorded		Income
	(In Millions)	Investment			Recognized			Investment		Recognized		Investment		Recognized		Investment		Recognized
	With No Related Specific
	Commercial and Institutional	$	10.7		$	—		$	27	$	0.1	$	11.3	$	0.1	$	25.2	$	0.2
	Commercial Real Estate		42			0.2			54.7		—		40.6		0.6		49.6		0.2
	Lease Financing, net		4.5			0.1			5.2		—		4.6		0.2		3.5		—
	Residential Real Estate		168.1			0.3			120.7		0.2		155.2		1.4		114		0.8
	Private Client		13.3			0.2			2.2		—		12		0.5		1.8		—
	With a Related Specific Allowance
	Commercial and Institutional		14.1			—			5.2		—		12.5		—		6.5		—
	Commercial Real Estate		31.2			—			22.1		—		31.6		—		20.2		—
	Residential Real Estate		10			—			15.3		—		9.4		—		15.3		—
	Private Client		0.8			—			0.9		—		1.1		—		1.1		—
	Total
	Commercial		102.5			0.3			114.2		0.1		100.6		0.9		105		0.4
	Personal		192.2			0.5			139.1		0.2		177.7		1.9		132.2		0.8
	Total	$	294.7		$	0.8		$	253.3	$	0.3	$	278.3	$	2.8	$	237.2	$	1.2
	Note: 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 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 and nine month periods ended September 30, 2013 and 2012, and the recorded investments and unpaid principal balances as of September 30, 2013 and 2012.
		Three Months Ended								Nine Months Ended
	($ In Millions)	30-Sep-13								30-Sep-13
		Number of			Recorded			Unpaid Principal		Number of		Recorded		Unpaid Principal
		Loans and Leases			Investment			Balance		Loans and Leases		Investment		Balance
	Commercial
	Commercial and Institutional		—		$	—		$	—		7	$	0.5	$	1
	Commercial Real Estate		7			22.8			25		11		27.4		31.9
	Total Commercial		7			22.8			25		18		27.9		32.9
	Personal
	Residential Real Estate		38			6.7			11.4		122		51.2		59.8
	Private Client		—			—			—		6		12.8		12.8
	Total Personal		38			6.7			11.4		128		64		72.6
	Total Loans and Leases		45		$	29.5		$	36.4		146	$	91.9	$	105.5
	Note: Period end balances reflect all paydowns and charge-offs during the period.
		Three Months Ended								Nine Months Ended
	($ In Millions)	30-Sep-12								30-Sep-12
		Number of			Recorded			Unpaid Principal		Number of		Recorded		Unpaid Principal
		Loans and Leases			Investment			Balance		Loans and Leases		Investment		Balance
	Commercial
	Commercial and Institutional		—		$	—		$	—		2	$	0.5	$	1.1
	Commercial Real Estate		1			2.7			2.7		7		24.6		26.2
	Lease Financing, net		–			–			–		1		5.2		5.2
	Total Commercial		1			2.7			2.7		10		30.3		32.5
	Personal
	Residential Real Estate		23			2			2.6		84		10.1		13.5
	Private Client		–			–			–		1		0.8		0.8
	Total Personal		23			2			2.6		85		10.9		14.3
	Total Loans and Leases		24		$	4.7		$	5.3		95	$	41.2	$	46.8
	Note: Period end balances reflect all paydowns and charge-offs during the period.

Allowance_for_Credit_Losses_Ta

Allowance for Credit Losses (Tables)	9 Months Ended
	Sep. 30, 2013
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 and nine month periods ended September 30, 2013 and 2012.
		Three Months Ended September 30,
		2013									2012
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Balance at Beginning of Period	$	184.1		$	136.6		$	320.7		$	196.3		$	133.6		$	329.9
	Charge-Offs		(1.7	)		(9.9	)		(11.6	)		(7.2	)		(9.1	)		(16.3	)
	Recoveries		1.7			1.6			3.3			2.4			2			4.4
	Net (Charge-Offs) Recoveries		—			(8.3	)		(8.3	)		(4.8	)		(7.1	)		(11.9	)
	Provision for Credit Losses		(4.6	)		9.6			5			6.9			3.1			10
	Effect of Foreign Exchange Rates		0.1			—			0.1			—			—			—
	Balance at End of Period	$	179.6		$	137.9		$	317.5		$	198.4		$	129.6		$	328
		Nine Months Ended September 30,
		2013									2012
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Balance at Beginning of Period	$	194.2		$	133.4		$	327.6		$	211		$	117.9		$	328.9
	Charge-Offs		(6.9	)		(32.9	)		(39.8	)		(15.4	)		(31.5	)		(46.9	)
	Recoveries		8.1			6.6			14.7			13.6			12.4			26
	Net (Charge-Offs) Recoveries		1.2			(26.3	)		(25.1	)		(1.8	)		(19.1	)		(20.9	)
	Provision for Credit Losses		(15.8	)		30.8			15			(10.8	)		30.8			20
	Effect of Foreign Exchange Rates		—			—			—			—			—			—
	Balance at End of Period	$	179.6		$	137.9		$	317.5		$	198.4		$	129.6		$	328
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 September 30, 2013 and December 31, 2012.
		September 30, 2013									December 31, 2012
	(In Millions)	Commercial			Personal			Total			Commercial			Personal			Total
	Loans and Leases
	Specifically Evaluated for Impairment	$	109.7		$	201.3		$	311		$	125		$	144.8		$	269.8
	Evaluated for Inherent Impairment		12,371.50			16,382.30			28,753.80			12,772.20			16,462.50			29,234.70
	Total Loans and Leases		12,481.20			16,583.60			29,064.80			12,897.20			16,607.30			29,504.50
	Allowance for Loans and Leases
	Specifically Evaluated for Impairment		13.7			4.1			17.8			11			7			18
	Evaluated for Inherent Impairment		138.4			131			269.4			155.1			124.8			279.9
	Allowance Assigned to Loans and Leases		152.1			135.1			287.2			166.1			131.8			297.9
	Allowance for Unfunded Exposures
	Commitments and Standby Letters of Credit		27.5			2.8			30.3			28.1			1.6			29.7
	Total Allowance for Credit Losses	$	179.6		$	137.9		$	317.5		$	194.2		$	133.4		$	327.6

Goodwill_and_Other_Intangibles1

Goodwill and Other Intangibles (Tables)	9 Months Ended
	Sep. 30, 2013
Carrying Amounts of Goodwill	The carrying amounts of goodwill at September 30, 2013 and December 31, 2012 were as follows:
	(In Millions)	September 30,		December 31,
		2013		2012
	Corporate and Institutional Services	$	466.2	$	466.3
	Wealth Management		71.5		71.5
	Total Goodwill	$	537.7	$	537.8
	Note: Amounts include the effect of foreign exchange rates on non-U.S. dollar denominated goodwill.
Other Intangible Assets Subject to Amortization	The gross carrying amount and accumulated amortization of other intangible assets subject to amortization at September 30, 2013 and December 31, 2012 were as follows:
	(In Millions)	September 30,		December 31,
		2013		2012
	Gross Carrying Amount	$	251.3	$	252.1
	Less: Accumulated Amortization		163.5		148.1
	Net Book Value	$	87.8	$	104
	Note: Amounts include the effect of foreign exchange rates on non-U.S. dollar denominated intangible assets.

Business_Units_Tables

Business Units (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Contribution of Northern Trust's Business Units	The following tables show the earnings contributions of Northern Trust’s business units for the three and nine month periods ended September 30, 2013 and 2012.
	Three Months Ended		Corporate &						Wealth						Treasury and						Total
	September 30,		Institutional Services						Management						Other						Consolidated
	($ In Millions)		2013			2012			2013			2012			2013			2012			2013			2012
	Noninterest Income
	Trust, Investment and Other Servicing Fees		$	359.8		$	334.4		$	288.2		$	267.5		$	—		$	—		$	648		$	601.9
	Foreign Exchange Trading Income			61.8			41.3			1			2.7			—			—			62.8			44
	Other Noninterest Income			44.3			50.2			54.3			23.3			0.8			7.5			99.4			81
	Net Interest Income (FTE)*			70.1			68			136.3			157.4			38.4			31.5			244.8			256.9
	Revenue*			536			493.9			479.8			450.9			39.2			39			1,055.00			983.8
	Provision for Credit Losses			0.4			(1.6	)		4.6			11.6			—			—			5			10
	Noninterest Expense			412			394.5			297.3			285.2			31.4			16.7			740.7			696.4
	Income (loss) before Income Taxes*			123.6			101			177.9			154.1			7.8			22.3			309.3			277.4
	Provision (Benefit) for Taxes*			38.8			32.7			67			58.1			(3.0	)		7.8			102.8			98.6
	Net Income		$	84.8		$	68.3		$	110.9		$	96		$	10.8		$	14.5		$	206.5		$	178.8
	Percentage of Consolidated Net Income			41	%		38	%		54	%		54	%		5	%		8	%		100	%		100	%
	Average Assets		$	53,653.50		$	50,638.60		$	22,923.60		$	23,530.70		$	18,635.40		$	18,540.60		$	95,212.50		$	92,709.90
	*	Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $7.8 million for 2013 and $11.3 million for 2012.
	Nine Months Ended		Corporate &						Wealth						Treasury and						Total Consolidated
	September 30,		Institutional Services						Management						Other
	($ In Millions)		2013			2012			2013			2012			2013			2012			2013			2012
	Noninterest Income
	Trust, Investment and Other Servicing Fees		$	1,072.70		$	989.8		$	863.3		$	793.1		$	—		$	—		$	1,936.00		$	1,782.90
	Foreign Exchange Trading Income			189.8			155.3			3.8			10			—			—			193.6			165.3
	Other Noninterest Income			130.1			145.9			95.8			70.1			5.4			6.1			231.3			222.1
	Net Interest Income (FTE)*			200.2			217			425.3			476.9			81			93.6			706.5			787.5
	Revenue*			1,592.80			1,508.00			1,388.20			1,350.10			86.4			99.7			3,067.40			2,957.80
	Provision for Credit Losses			(1.1	)		(1.6	)		16.1			21.6			—			—			15			20
	Noninterest Expense			1,206.70			1,190.00			900.9			878.4			91.7			68.9			2,199.30			2,137.30
	Income (loss) before Income Taxes*			387.2			319.6			471.2			450.1			(5.3	)		30.8			853.1			800.5
	Provision (Benefit) for Taxes*			123			102.1			177.8			170.2			(9.3	)		8.6			291.5			280.9
	Net Income		$	264.2		$	217.5		$	293.4		$	279.9		$	4		$	22.2		$	561.6		$	519.6
	Percentage of Consolidated Net Income			47	%		42	%		52	%		54	%		1	%		4	%		100	%		100	%
	Average Assets		$	52,323.80		$	49,642.80		$	22,863.10		$	23,527.60		$	18,036.60		$	20,243.20		$	93,223.50		$	93,413.60
	*	Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $23.3 million for 2013 and $31.4 million for 2012.

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2013
Components of Accumulated Other Comprehensive Income (Loss)	The following tables summarize the components of accumulated other comprehensive income (loss) at September 30, 2013 and 2012, and changes during the three and nine month periods then ended.
	(In Millions)	Balance at				Net Change				Balance at
		December 31,								September 30,
		2012								2013
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	101			$	(74.4	)		$	26.6
	Net Unrealized Gains (Losses) on Cash Flow Hedges		(1.4	)			1.2				(0.2	)
	Net Foreign Currency Adjustments		10.5				(2.8	)			7.7
	Net Pension and Other Postretirement Benefit Adjustments		(393.1	)			21.1				(372.0	)
	Total	$	(283.0	)		$	(54.9	)		$	(337.9	)
	(In Millions)	Balance at				Net Change				Balance at
		December 31,								September 30,
		2011								2012
	Net Unrealized Gains (Losses) on Securities Available for Sale	$	39.8			$	67.3			$	107.1
	Net Unrealized Gains (Losses) on Cash Flow Hedges		(7.0	)			5.5				(1.5	)
	Net Foreign Currency Adjustments		(9.5	)			24.1				14.6
	Net Pension and Other Postretirement Benefit Adjustments		(368.9	)			32.8				(336.1	)
	Total	$	(345.6	)		$	129.7			$	(215.9	)
Components of Changes in Accumulated Other Comprehensive Income (Loss)
		Three Months Ended September 30,
		2013											2012
	(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 Losses on Securities OTTI	$	—			$	—			$	—		$	4.9		$	(1.8	)	$	3.1
	Other Unrealized Gains (Losses) on Securities Available for Sale		7.3				(2.8	)			4.5			57.3			(21.6	)		35.7
	Reclassification Adjustment for (Gains) Losses Included in Net Income		2.1				(0.8	)			1.3			(5.1	)		1.9			(3.2	)
	Net Change		9.4				(3.6	)			5.8			57.1			(21.5	)		35.6
	Unrealized Gains (Losses) on Cash Flow Hedges
	Unrealized Gains (Losses) on Cash Flow Hedges		5.3				(1.7	)			3.6			2			(0.5	)		1.5
	Reclassification Adjustment for (Gains) Losses Included in Net Income		1.1				(0.4	)			0.7			2.6			(1.0	)		1.6
	Net Change		6.4				(2.1	)			4.3			4.6			(1.5	)		3.1
	Foreign Currency Adjustments
	Foreign Currency Translation Adjustments		55				(4.0	)			51			42.1			—			42.1
	Net Investment Hedge Gains (Losses)		(88.8	)			33.4				(55.4	)		(44.8	)		16.9			(27.9	)
	Reclassification Adjustment for (Gains) Losses Included in Net Income		—				—				—			—			—			—
	Net Change		(33.8	)			29.4				(4.4	)		(2.7	)		16.9			14.2
	Pension and Other Postretirement Benefit Adjustments
	Net Actuarial Gain (Loss)		—				—				—			—			—			—
	Reclassification Adjustment for (Gains) Losses Included in Net Income		11.3				(4.2	)			7.1			8.5			(3.2	)		5.3
	Net Change	$	11.3			$	(4.2	)		$	7.1		$	8.5		$	(3.2	)	$	5.3
		Nine Months Ended September 30,
		2013											2012
	(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 Losses on Securities OTTI	$	2.1			$	(0.8	)		$	1.3		$	15		$	(5.6	)	$	9.4
	Other Unrealized Gains (Losses) on Securities Available for Sale		(123.2	)			46.3				(76.9	)		105.9			(39.7	)		66.2
	Reclassification Adjustment for (Gains) Losses Included in Net Income		1.9				(0.7	)			1.2			(13.2	)		4.9			(8.3	)
	Net Change		(119.2	)			44.8				(74.4	)		107.7			(40.4	)		67.3
	Unrealized Gains (Losses) on Cash Flow Hedges
	Unrealized Gains (Losses) on Cash Flow Hedges		(4.0	)			1.6				(2.4	)		4.7			(1.5	)		3.2
	Reclassification Adjustment for (Gains) Losses Included in Net Income		5.7				(2.1	)			3.6			3.7			(1.4	)		2.3
	Net Change		1.7				(0.5	)			1.2			8.4			(2.9	)		5.5
	Foreign Currency Adjustments
	Foreign Currency Translation Adjustments		5.3				(2.3	)			3			31.3			—			31.3
	Net Investment Hedge Gains (Losses)		(9.3	)			3.5				(5.8	)		(17.4	)		10.2			(7.2	)
	Reclassification Adjustment for (Gains) Losses Included in Net Income		—				—				—			—			—			—
	Net Change		(4.0	)			1.2				(2.8	)		13.9			10.2			24.1
	Pension and Other Postretirement Benefit Adjustments
	Net Actuarial Gain (Loss)		—				—				—			26.7			(10.1	)		16.6
	Reclassification Adjustment for (Gains) Losses Included in Net Income		33.7				(12.6	)			21.1			25.6			(9.4	)		16.2
	Net Change	$	33.7			$	(12.6	)		$	21.1		$	52.3		$	(19.5	)	$	32.8
Reclassification Out of Accumulated Other Comprehensive Income (Loss)	The following table provides the location and before-tax amounts of reclassifications out of accumulated other comprehensive income (loss) during the three and nine months ended September 30, 2013.
				Amount of Reclassification
				Adjustments Recognized
		Location of		in Income
		Reclassification Adjustments		Three Months Ended				Nine Months Ended
	(In Millions)	Recognized in Income		30-Sep-13
	Securities Available for Sale
	Realized (Gains) Losses on Securities Available for Sale	Investment Security Gains (Losses), net		$	2.1			$	1.9
	Realized (Gains) Losses on Cash Flow Hedges
	Foreign Exchange Contracts	Other Operating Income/ Expense			1.1				5.7
	Pension and Other Postretirement Adjustments
	Amortization of Net Actuarial (Gain) Loss	Employee Benefits			11.9				35.8
	Amortization of Prior Service Cost	Employee Benefits			(0.6	)			(2.1	)
	Gross Reclassification Adjustment			$	11.3			$	33.7

Net_Income_Per_Common_Share_Co1

Net Income Per Common Share Computations (Tables)	9 Months Ended
	Sep. 30, 2013
Computations of Net Income Per Common Share	The computations of net income per common share are presented in the following table.
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
	($ In Millions Except Per Common Share Information)	2013		2012		2013		2012
	Basic Net Income Per Common Share
	Average Number of Common Shares Outstanding		239,930,074		240,237,014		239,614,868		240,740,803
	Net Income Applicable to Common Stock	$	206.5	$	178.8	$	561.6	$	519.6
	Less: Earnings Allocated to Participating Securities		3.5		2.8		9.1		7.6
	Earnings Allocated to Common Shares Outstanding		203		176		552.5		512
	Basic Net Income Per Common Share		0.85		0.73		2.31		2.13
	Diluted Net Income Per Common Share
	Average Number of Common Shares Outstanding		239,930,074		240,237,014		239,614,868		240,740,803
	Plus: Dilutive Effect of Share-based Compensation		1,400,578		460,048		1,242,829		464,383
	Average Common and Potential Common Shares		241,330,652		240,697,062		240,857,697		241,205,186
	Earnings Allocated to Common and Potential Common Shares	$	203	$	176	$	552.5	$	512
	Diluted Net Income Per Common Share		0.84		0.73		2.29		2.12
	Note: Common stock equivalents totaling 3,371,680 and 3,783,018 for the three and nine months ended September 30, 2013, respectively, and 11,890,286 and 12,412,541 for the three and nine months ended September 30, 2012, 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)	9 Months Ended
	Sep. 30, 2013
Components of Net Interest Income	The components of net interest income were as follows:
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
	(In Millions)	2013		2012		2013		2012
	Interest Income
	Loans and Leases	$	185.4	$	206.9	$	558.5	$	630.4
	Securities – Taxable		60.9		63.2		167.6		191
	– Non-Taxable		2.7		4.1		9		13.9
	Interest-Bearing Deposits with Banks		36.3		44.8		105.8		138.8
	Federal Reserve Deposits and Other		5.8		4.1		12.2		11.5
	Total Interest Income		291.1		323.1		853.1		985.6
	Interest Expense
	Deposits		25.8		43.8		79.9		122.9
	Federal Funds Purchased		0.4		0.1		1		1.1
	Securities Sold Under Agreements to Repurchase		0.1		0.1		0.3		0.3
	Other Borrowings		0.8		1.2		2.5		3.3
	Senior Notes		18.4		18.6		56.9		52.4
	Long-Term Debt		8		13		27.5		47.3
	Floating Rate Capital Debt		0.6		0.7		1.8		2.2
	Total Interest Expense		54.1		77.5		169.9		229.5
	Net Interest Income	$	237	$	245.6	$	683.2	$	756.1

Pension_and_Other_Postretireme1

Pension and Other Postretirement Plans (Tables)	9 Months Ended
	Sep. 30, 2013
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 other postretirement plan for the three and nine months ended September 30, 2013 and 2012.
	Net Periodic Pension Expense	Three Months Ended						Nine Months Ended
	U.S. Plan	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Service Cost	$	7.5		$	8.9		$	22.7		$	26.5
	Interest Cost		10.5			10.3			31.6			31.1
	Expected Return on Plan Assets		(23.1	)		(21.7	)		(69.2	)		(65.3	)
	Amortization
	Net Actuarial Loss		10.3			8.6			30.9			25.8
	Prior Service Cost		(0.1	)		(0.1	)		(0.3	)		(0.3	)
	Net Periodic Pension Expense	$	5.1		$	6		$	15.7		$	17.8
	Net Periodic Pension Expense	Three Months Ended						Nine Months Ended
	Non U.S. Plans	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Interest Cost	$	1.6		$	1.6		$	4.9		$	4.7
	Expected Return on Plan Assets		(1.5	)		(1.7	)		(4.6	)		(4.9	)
	Net Actuarial Loss Amortization		0.2			0.2			0.7			0.6
	Net Periodic Pension Expense	$	0.3		$	0.1		$	1		$	0.4
	Net Periodic Pension Expense	Three Months Ended						Nine Months Ended
	Supplemental Plan	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Service Cost	$	0.4		$	0.8		$	1.2		$	2.2
	Interest Cost		1.1			1.2			3.3			3.4
	Amortization
	Net Actuarial Loss		1.7			1.5			5.1			4.5
	Prior Service Cost		0.2			0.2			0.4			0.4
	Net Periodic Pension Expense	$	3.4		$	3.7		$	10		$	10.5
	Net Periodic Postretirement Benefit	Three Months Ended						Nine Months Ended
	Other Postretirement Plan	September 30,						September 30,
	(In Millions)	2013			2012			2013			2012
	Service Cost	$	—		$	0.1		$	0.1		$	0.2
	Interest Cost		0.3			0.3			0.9			1
	Amortization
	Net Actuarial Gain		(0.3	)		(0.7	)		(0.9	)		(1.6	)
	Prior Service Cost		(0.7	)		(1.2	)		(2.2	)		(3.8	)
	Net Periodic Postretirement Benefit	$	(0.7	)	$	(1.5	)	$	(2.1	)	$	(4.2	)

ShareBased_Compensation_Plans_

Share-Based Compensation Plans (Tables)	9 Months Ended
	Sep. 30, 2013
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 and nine months ended September 30, 2013 and 2012.
		Three Months Ended				Nine Months Ended
		September 30,				September 30,
	(In Millions)	2013		2012		2013		2012
	Stock and Stock Unit Awards	$	12.1	$	10.2	$	36.4	$	33.9
	Stock Options		3.7		5.7		14.9		22.3
	Performance Stock Units		2.1		0.7		5.4		1.8
	Total Share-Based Compensation Expense		17.9		16.6		56.7		58
	Tax Benefits Recognized	$	6.8	$	6.2	$	21.3	$	21.8

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
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 sheet. 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.
		30-Sep-13												31-Dec-12
		Notional				Fair Value								Notional				Fair Value
	(In Millions)	Value				Asset				Liability				Value				Asset			Liability
	Foreign Exchange Contracts	$	252,806.20			$	3,162.10			$	3,165.30			$	213,246.70			$	1,735.30		$	1,730.40
	Interest Rate Contracts		5,039.20				134.9				128.6				4,946.60				180.6			174
	Total	$	257,845.40			$	3,297.00			$	3,293.90			$	218,193.30			$	1,915.90		$	1,904.40
Location and Amount of Gains and Losses Recorded in Consolidated Statement of Income	The following table shows the location and amount of gains and losses recorded in the consolidated statement of income for the three and nine months ended September 30, 2013 and 2012.
				Amount of Derivative Gain/(Loss) Recognized in
				Income
		Location of Derivative		Three Months Ended								Nine Months Ended
		Gain/(Loss) Recognized in		September 30,								September 30,
	(In Millions)	Income		2013				2012				2013				2012
	Foreign Exchange Contracts	Foreign Exchange Trading Income		$	62.8			$	44			$	193.6			$	165.3
	Interest Rate Contracts	Security Commissions and Trading Income			4.4				3.7				11.4				8.9
	Total			$	67.2			$	47.7			$	205			$	174.2
Types and Classifications of Derivative Instruments	The following table identifies the types and classifications of derivative instruments designated as hedges and used by Northern Trust to manage risk, their notional and fair values, and the respective risks addressed.
						30-Sep-13												31-Dec-12
		Derivative		Risk		Notional				Fair Value								Notional			Fair Value
	(In Millions)	Instrument		Classification		Value				Asset				Liability				Value			Asset		Liability
	Fair Value Hedges
	Available for Sale Investment Securities	Interest Rate		Interest		$	3,646.90			$	23.7			$	44.4			$	3,617.00		$	3.4	$	75.1
		Swap Contracts		Rate
	Senior Notes and Long- Term Subordinated Debt	Interest Rate		Interest			500				84.2				—				900			126.3		0.2
		Swap Contracts		Rate
	Cash Flow Hedges
	Forecasted Foreign Currency Denominated Transactions	Foreign Exchange		Foreign			390.9				8				9				669			8.7		11.5
		Contracts		Currency
	Net Investment Hedges
	Net Investments in Non-U.S. Affiliates	Foreign Exchange		Foreign			1,661.30				7.1				26.8				1,451.40			2.3		27.8
		Contracts		Currency
	Total					$	6,199.10			$	123			$	80.2			$	6,637.40		$	140.7	$	114.6
Gains/Losses of Derivative Financial Instruments	The following table shows the location and amount of derivative gains and losses recorded in the consolidated statement of income related to fair value hedges for the three and nine months ended September 30, 2013 and 2012.
				Location of		Amount of Derivative
				Derivative		Gain/(Loss) Recognized in Income
		Derivative		Gain/(Loss)		Three Months Ended								Nine Months Ended
				Recognized		September 30,								September 30,
	(In Millions)	Instrument		in Income		2013				2012				2013				2012
	Available for Sale Investment Securities	Interest Rate		Interest		$	(16.4	)		$	(23.3	)		$	21.6			$	(93.5	)
		Swap Contracts		Income
	Senior Notes and Long- Term Subordinated Debt	Interest Rate		Interest			(3.6	)			8.9				(16.1	)			171.8
		Swap Contracts		Expense
	Total					$	(20.0	)		$	(14.4	)		$	5.5			$	78.3
Cash Flow Hedge Derivative Gains and Losses Recognized in AOCI and the Amounts Reclassified to Earnings	The following tables provide cash flow hedge derivative gains and losses recognized in AOCI and the amounts reclassified to earnings during the three and nine months ended September 30, 2013 and 2012.
	(In Millions)	Foreign Exchange								Interest Rate Swap
		Contracts (Before Tax)								Contracts (Before Tax)
	Three Months Ended September 30,	2013				2012				2013				2012
	Net Gain/(Loss) Recognized in AOCI	$	5.3			$	2			$	—			$	—
	Net Gain/(Loss) Reclassified from AOCI to Earnings
	Other Operating Income		—				(1.0	)			—				—
	Interest Income		—				—				—				—
	Other Operating Expense		(1.1	)			(1.6	)			—				—
	Total	$	(1.1	)		$	(2.6	)		$	—			$	—
	(In Millions)	Foreign Exchange								Interest Rate Swap
		Contracts (Before Tax)								Contracts (Before Tax)
	Nine Months Ended September 30,	2013				2012				2013				2012
	Net Gain/(Loss) Recognized in AOCI	$	(4.0	)		$	4.7			$	—			$	—
	Net Gain/(Loss) Reclassified from AOCI to Earnings
	Other Operating Income		(1.8	)			(3.3	)			—				—
	Interest Income		—				—				—				(0.2	)
	Other Operating Expense		(3.9	)			(0.2	)			—				—
	Total	$	(5.7	)		$	(3.5	)		$	—			$	(0.2	)
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 and nine months ended September 30, 2013 and 2012.
		Hedging Gain/(Loss)
		Recognized in OCI (Before Tax)
		Three Months Ended September 30,								Nine Months Ended September 30,
	(In Millions)	2013				2012				2013				2012
	Foreign Exchange Contracts	$	(73.8	)		$	(35.4	)		$	(9.5	)		$	(8.1	)
	Sterling Denominated Subordinated Debt		(15.0	)			(9.4	)			0.2				(9.3	)
	Total	$	(88.8	)		$	(44.8	)		$	(9.3	)		$	(17.4	)
Types and Classifications of Risk Management Derivative Instruments Not Formally Designated as Hedges, Including Notional 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.
		30-Sep-13												31-Dec-12
		Notional				Fair Value								Notional				Fair Value
	(In Millions)	Value				Asset				Liability				Value				Asset			Liability
	Credit Default Swap Contracts	$	—			$	—			$	—			$	42.5			$	—		$	1
	Foreign Exchange Contracts		144				1.9				0.9				1,189.80				10.3			3
	Total	$	144			$	1.9			$	0.9			$	1,232.30			$	10.3		$	4
Gains/Losses of Derivative Financial Instruments	The following table provides the location and amount of gains and losses recorded in the consolidated statement of income for the three and nine months ended September 30, 2013 and 2012.
		Location of		Amount of Derivative Gain/(Loss)
				Recognized in Income
		Derivative Gain/		Three Months Ended								Nine Months Ended
		(Loss) Recognized		September 30,								September 30,
	(In Millions)	in Income		2013				2012				2013				2012
	Credit Default Swap Contracts	Other Operating Income		$	—			$	(0.6	)		$	(0.1	)		$	(2.5	)
	Foreign Exchange Contracts	Other Operating Income			6.7				5				(4.3	)			3.8
	Total			$	6.7			$	4.4			$	(4.4	)		$	1.3

Offsetting_of_Assets_and_Liabi1

Offsetting of Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2013
Consolidated Balance Sheet for Securities Purchased under Agreements to Sell and Derivative Assets	The following tables provide information regarding the offsetting of derivative assets and securities purchased under agreements to resell within the consolidated balance sheet as of September 30, 2013 and December 31, 2012.
	30-Sep-13
	(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)		$	2,880.10	$	1,593.70	$	1,286.40	$	—	$	1,286.40
	Interest Rate Swaps OTC			237.6		46.8		190.8		—		190.8
	Interest Rate Swaps Exchange Cleared			5.2		4.3		0.9		—		0.9
	Interest Rate Options OTC			—		—		—		—		—
	Cross Product Netting Adjustment			—		73		—		—		—
	Cross Product Collateral Adjustment			—		300.5		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			3,122.90		2,018.30		1,104.60		—		1,104.60
	Total Derivatives Not Subject to a Master Netting Arrangement			299		—		299		—		299
	Total Derivatives			3,421.90		2,018.30		1,403.60		—		1,403.60
	Securities Purchased under Agreements to Resell (2)		$	500	$	—	$	500	$	500	$	—
	-1	Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $3,549.3 million as of September 30, 2013.
	-2	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheet. Federal funds sold totaled $34.6 million as of September 30, 2013.
	-3	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of September 30, 2013.
	31-Dec-12
	(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		$	1,756.60	$	877.1	$	879.5	$	—	$	879.5
	Interest Rate Swaps OTC			310.3		68.3		242		—		242
	Cross Product Netting Adjustment			—		37.1		—		—		—
	Cross Product Collateral Adjustment			—		118.6		—		—		—
	Total			2,066.90		1,101.10		965.8		—		965.8
	Securities Purchased under Agreements to Resell (2)		$	35.4	$	—	$	35.4	$	35.4	$	—
	-1	Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $2,964.4 million as of December 31, 2012.
	-2	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheet. Federal funds sold totaled $25.4 million as of December 31, 2012.
	-3	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of December 31, 2012.
Consolidated Balance Sheet for Securities Sold under Agreements to Repurchase and Derivative Liabilities	The following tables provide information regarding the offsetting of derivative liabilities and securities sold under agreements to repurchase within the consolidated balance sheet as of September 30, 2013 and December 31, 2012.
	30-Sep-13
	(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		$	2,550.30	$	1,593.70	$	956.6	$	—	$	956.6
	Interest Rate Swaps OTC			168.7		46.8		121.9		—		121.9
	Interest Rate Swaps Exchange Cleared			4.3		4.3		—		—		—
	Interest Rate Options OTC			—		—		—		—		—
	Credit Default Swaps OTC			—		—		—		—		—
	Cross Product Netting Adjustment			—		73		—		—		—
	Cross Product Collateral Adjustment			—		495.9		—		—		—
	Total Derivatives Subject to a Master Netting Arrangement			2,723.30		2,213.70		509.6		—		509.6
	Total Derivatives Not Subject to a Master Netting Arrangement			651.7		—		651.7		—		651.7
	Total Derivatives			3,375.00		2,213.70		1,161.30		—		1,161.30
	Securities Sold under Agreements to Repurchase		$	469	$	—	$	469	$	469	$	—
	-1	Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,235.0 million as of September 30, 2013.
	-2	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of September 30, 2013.
	31-Dec-12
	(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		$	1,772.70	$	877.1	$	895.6	$	—	$	895.6
	Interest Rate Swaps OTC			249.3		68.3		181		—		181
	Credit Default Swaps OTC			1		—		1		—		1
	Cross Product Netting Adjustment			—		37.1		—		—		—
	Cross Product Collateral Adjustment			—		425		—		—		—
	Total			2,023.00		1,407.50		615.5		—		615.5
	Securities Sold under Agreements to Repurchase		$	699.8	$	—	$	699.8	$	699.8	$	—
	-1	Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $1,961.7 million as of December 31, 2012.
	-2	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of December 31, 2012.

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities portfolio, value	$27,501.60		$27,501.60		$28,643.50
Trading account securities	9.6		9.6		8
Other operating expenses	67.9	63	215.5	199
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio	783		783		696
Available for sale securities portfolio, value	25,900		25,900		26,800
Loans | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value	38.1	27.7	38.1	27.7
Change in specific allowance	-0.6	5.7	-0.6	-8.5
Other Real Estate Owned | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value	1.6	1	1.6	1
Other operating expenses		0.2	0.7	0.4
External Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio	780		780		689
Available for sale securities portfolio, value	25,700		25,700		26,500
Trading account securities	9.6		9.6		8
Internal Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio	3		3		7
Available for sale securities portfolio, value	$158.80		$158.80		$307.10

Valuation_Techniques_Significa

Valuation Techniques, Significant Unobservable Inputs, and Quantitative Information (Detail) (Fair Value, Inputs, Level 3, USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument, fair value	98.7
Auction Rate Securities | Group One
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique	Discounted Cash Flow
Unobservable Input	Remaining lives
Auction Rate Securities | Group Two
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique	Discounted Cash Flow
Unobservable Input	Discount rates
Auction Rate Securities | Lower Limit
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument discount rate	0.20%
Financial instrument remaining lives	2.7
Auction Rate Securities | Upper Limit
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument discount rate	8.10%
Financial instrument remaining lives	8.6
Contingent Consideration
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument, fair value	54
Financial instrument discount rate	10.50%
Contingent Consideration | Group One
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique	Discounted Cash Flow
Unobservable Input	Discount rate
Contingent Consideration | Group Two
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Valuation Technique	Discounted Cash Flow
Unobservable Input	Business growth rates
Contingent Consideration | Lower Limit
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument business growth rates	19.00%
Contingent Consideration | Upper Limit
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Financial instrument business growth rates	35.00%

Assets_and_Liabilities_Measure

Assets and Liabilities Measured at Fair Value on Recurring Basis Segregated by Fair Value Hierarchy Level (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	$27,501.60		$28,643.50
Trading Account	9.6		8
Contingent Consideration	54		50.1
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading	27,511.20		28,651.50
Contingent Consideration	54		50.1
Fair Value, Measurements, Recurring | Securities Investment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	27,501.60		28,643.50
Trading Account	9.6		8
Fair Value, Measurements, Recurring | Securities Investment | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,524.50		1,784.60
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	14.4		14.1
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	17,024.60		18,638.80
Fair Value, Measurements, Recurring | Securities Investment | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	3,491		2,618.40
Fair Value, Measurements, Recurring | Securities Investment | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	1,805.60		1,748
Fair Value, Measurements, Recurring | Securities Investment | Supranational, Sovereign and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	721.1
Fair Value, Measurements, Recurring | Securities Investment | Supranational Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale			1,060.70
Fair Value, Measurements, Recurring | Securities Investment | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	53.5		92
Fair Value, Measurements, Recurring | Securities Investment | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	2,535.70		2,283.90
Fair Value, Measurements, Recurring | Securities Investment | Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	98.7		97.8
Fair Value, Measurements, Recurring | Securities Investment | Others
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	232.5		305.2
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	1,403.60		965.8
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,179.10		1,756.60
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	242.8		310.3
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1,161.30		615.5
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,202		1,772.70
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	173		249.3
Fair Value, Measurements, Recurring | Other Liabilities | Credit Default Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities			1
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	1,524.50		1,784.60
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	1,524.50		1,784.60
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	1,524.50		1,784.60
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	25,888		26,769.10
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	25,878.40		26,761.10
Trading Account	9.6		8
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	14.4		14.1
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	17,024.60		18,638.80
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	3,491		2,618.40
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	1,805.60		1,748
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Supranational, Sovereign and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale	721.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Securities Investment | Supranational Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale			1,060.70
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	53.5		92
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	2,535.70		2,283.90
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	232.5		305.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,421.90		2,066.90
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	3,179.10		1,756.60
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	242.8		310.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,375		2,023
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	3,202		1,772.70
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	173		249.3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Other Liabilities | Credit Default Swap Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities			1
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	98.7		97.8
Contingent Consideration	54		50.1
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	98.7		97.8
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	98.7		97.8
Fair Value, Measurements, Recurring | Netting and Collateral | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	-2,018.30	[1]	-1,101.10	[2]
Fair Value, Measurements, Recurring | Netting and Collateral | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	($2,213.70)	[1]	($1,407.50)	[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 September 30, 2013, derivative assets and liabilities shown above also include reductions of $300.5 million and $495.9 million, 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, 2012, derivative assets and liabilities shown above also include reductions of $118.6 million and $425.0 million, 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) (Fair Value, Measurements, Recurring, Netting and Collateral, USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
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	$300.50	$118.60
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	$495.90	$425

Changes_in_Level_3_Assets_Deta

Changes in Level 3 Assets (Detail) (Auction Rate Securities, USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Auction Rate Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Assets Beginning Balance	$99.10		$105.20		$97.80		$178.30
Total Gains and (Losses):
Included in Earnings		[1]	0.3	[1]	0.1	[1]	-22	[1]
Included in Other Comprehensive Income	0.3	[2]	-1.1	[2]	3	[2]	4.9	[2]
Purchases, Issues, Sales, and Settlements
Sales			-0.1				-54.7
Settlements	-0.7		-4.8		-2.2		-7
Fair Value Assets Ending Balance	$98.70		$99.50		$98.70		$99.50
[1]	Realized gains for the three month period ended September 30, 2012 of $0.3 million represent gains from redemptions by issuers. Realized gains for the nine month period ended September 30, 2013 of $0.1 million represent gains from redemptions by issuers. Realized losses for the nine month period ended September 30, 2012 of $22.0 million include $20.8 million of losses from sales of securities and $1.6 million of impairment losses, partially offset by $0.4 million of gains from redemptions by issuers. Gains on redemptions are recorded in interest income and sales and impairment losses are recorded in investment security gains (losses), within the consolidated statement of income.
[2]	Unrealized losses related to auction rate securities are included in net unrealized gains (losses) on securities available for sale within the consolidated statement of comprehensive income.

Changes_in_Level_3_Assets_Pare

Changes in Level 3 Assets (Parenthetical) (Detail) (Auction Rate Securities, USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized Gains (losses) on securities	$0.30	$0.10	($22)
Sales of Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized Gains (losses) on securities			-20.8
Impairments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized Gains (losses) on securities			-1.6
Redemptions by Issuers
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Realized Gains (losses) on securities			$0.40

Changes_in_Level_3_Liabilities

Changes in Level 3 Liabilities (Detail) (Contingent Consideration, USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Contingent Consideration
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Liabilities Beginning Balance	$52.70		$57.20		$50.10		$56.80
Total (Gains) and Losses:
Included in Earnings	1.3	[1]	0.1	[1]	3.9	[1]	0.8	[1]
Included in Other Comprehensive Income		[2]	-0.2	[2]			-0.5	[2]
Purchases, Issues, Sales, and Settlements
Settlements			-8.2				-8.2
Fair Value Liabilities Ending Balance	54		48.9		54		48.9
Purchases, Issues, Sales, and Settlements
Settlements			-8.2				-8.2
Fair Value Liabilities Ending Balance	54		48.9		54		48.9
Unrealized (Gain) Losses Included in Earnings Related to Financial Instruments Held at September 30	$1.30	[1]	$1.30	[1]	$3.90	[1]	$3.50	[1]
[1]	Gains (losses) are recorded in other operating income (expense) within the consolidated statement of income.
[2]	Unrealized foreign currency related losses are included in foreign currency translation adjustments within the consolidated statement of comprehensive income.

Valuation_Techniques_Significa1

Valuation Techniques, Significant Unobservable Inputs, Quantitative Information Non-recurring Basis (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation Technique	Market Approach
Unobservable Input	Discount to reflect realizable value
Loans | Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instrument, fair value	38.1
Other Real Estate Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation Technique	Market Approach
Unobservable Input	Discount to reflect realizable value
Other Real Estate Owned | Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial Instrument, fair value	1.6
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	40.00%
Upper Limit | Other Real Estate Owned
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Range of Discounts Applied	40.00%

Book_and_Fair_Values_of_Financ

Book and Fair Values of Financial Instruments (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Assets
Cash and Due from Banks	$2,690.70		$3,752.70		$3,396.70	$4,315.30
Federal Funds Sold and Resell Agreements	534.6		60.8
Interest-Bearing Deposits with Banks	17,383.90		18,803.50
Federal Reserve Deposits and Other Interest-Bearing	8,452.80		7,619.70
Securities
Available for Sale	27,501.60		28,643.50
Held to Maturity	3,053.60		2,382
Trading Account	9.6		8
Loans (excluding Leases)
Client Security Settlement Receivables	1,630.20		2,049.10
Deposits
Savings Certificates and Other Time	1,939.60		2,466.10
Non U.S. Offices Interest-Bearing	45,017.20		39,720.20
Federal Funds Purchased	1,053.60		780.2
Securities Sold Under Agreements to Repurchase	469		699.8
Other Borrowings	1,804.70		367.4
Senior Notes	1,996.50		2,405.80
Other Liabilities
Contingent Consideration	54		50.1
Carrying (Reported) Amount, Fair Value Disclosure
Assets
Cash and Due from Banks	2,690.70		3,752.70
Federal Funds Sold and Resell Agreements	534.6		60.8
Interest-Bearing Deposits with Banks	17,383.90		18,803.50
Federal Reserve Deposits and Other Interest-Bearing	8,452.80		7,619.70
Securities
Available for Sale	27,501.60	[1]	28,643.50	[2]
Held to Maturity	3,053.60		2,382
Trading Account	9.6		8
Loans (excluding Leases)
Held for Investment	27,797		28,165.40
Held for Sale	2.8		11.7
Client Security Settlement Receivables	1,630.20		2,049.10
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	194.7		197.6
Community Development Investments	217.8		253.2
Employee Benefit and Deferred Compensation	131		121.3
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	31,204.90		39,221.50
Savings Certificates and Other Time	1,939.60		2,466.10
Non U.S. Offices Interest-Bearing	45,017.20		39,720.20
Federal Funds Purchased	1,053.60		780.2
Securities Sold Under Agreements to Repurchase	469		699.8
Other Borrowings	1,804.70		367.4
Senior Notes	1,996.50		2,405.80
Long Term Debt (excluding Leases)
Subordinated Debt	820.5		1,045.40
Federal Home Loan Bank Borrowings	135		335
Floating Rate Capital Debt	277.1		277
Other Liabilities
Standby Letters of Credit	55.6		60.5
Contingent Consideration	54		50.1
Loan Commitments	38.1		38.9
Carrying (Reported) Amount, Fair Value Disclosure | Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	17		21.3
Liabilities	36.7		42.3
Carrying (Reported) Amount, Fair Value Disclosure | Asset And Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	107.9		129.7
Liabilities	44.4		75.3
Carrying (Reported) Amount, Fair Value Disclosure | Asset And Liability Management | Credit Default Swap Contracts
Derivative Instruments
Liabilities			1
Carrying (Reported) Amount, Fair Value Disclosure | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	3,162.10		1,735.30
Liabilities	3,165.30		1,730.40
Carrying (Reported) Amount, Fair Value Disclosure | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	134.9		180.6
Liabilities	128.6		174
Estimate of Fair Value, Fair Value Disclosure
Assets
Cash and Due from Banks	2,690.70		3,752.70
Federal Funds Sold and Resell Agreements	534.6		60.8
Interest-Bearing Deposits with Banks	17,383.90		18,803.50
Federal Reserve Deposits and Other Interest-Bearing	8,452.80		7,619.70
Securities
Available for Sale	27,501.60	[1]	28,643.50	[2]
Held to Maturity	3,053.20		2,394.80
Trading Account	9.6		8
Loans (excluding Leases)
Held for Investment	27,752.50		28,220.20
Held for Sale	2.8		11.7
Client Security Settlement Receivables	1,630.20		2,049.10
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	194.7		197.6
Community Development Investments	230		275.1
Employee Benefit and Deferred Compensation	126.2		126.1
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	31,204.90		39,221.50
Savings Certificates and Other Time	1,943.90		2,746.70
Non U.S. Offices Interest-Bearing	45,017.20		39,720.20
Federal Funds Purchased	1,053.60		780.2
Securities Sold Under Agreements to Repurchase	469		699.8
Other Borrowings	1,804.70		367.4
Senior Notes	2,002.30		2,513.40
Long Term Debt (excluding Leases)
Subordinated Debt	841.1		1,065.30
Federal Home Loan Bank Borrowings	138.6		345.4
Floating Rate Capital Debt	229.3		228
Other Liabilities
Standby Letters of Credit	55.6		60.5
Contingent Consideration	54		50.1
Loan Commitments	38.1		38.9
Estimate of Fair Value, Fair Value Disclosure | Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	17		21.3
Liabilities	36.7		42.3
Estimate of Fair Value, Fair Value Disclosure | Asset And Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	107.9		129.7
Liabilities	44.4		75.3
Estimate of Fair Value, Fair Value Disclosure | Asset And Liability Management | Credit Default Swap Contracts
Derivative Instruments
Liabilities			1
Estimate of Fair Value, Fair Value Disclosure | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	3,162.10		1,735.30
Liabilities	3,165.30		1,730.40
Estimate of Fair Value, Fair Value Disclosure | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	134.9		180.6
Liabilities	128.6		174
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 1
Assets
Cash and Due from Banks	2,690.70		3,752.70
Securities
Available for Sale	1,524.50	[1]	1,784.60	[2]
Other Assets
Employee Benefit and Deferred Compensation	82.3		86.7
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	31,204.90		39,221.50
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2
Assets
Federal Funds Sold and Resell Agreements	534.6		60.8
Interest-Bearing Deposits with Banks	17,383.90		18,803.50
Federal Reserve Deposits and Other Interest-Bearing	8,452.80		7,619.70
Securities
Available for Sale	25,878.40	[1]	26,761.10	[2]
Held to Maturity	3,053.20		2,394.80
Trading Account	9.6		8
Loans (excluding Leases)
Client Security Settlement Receivables	1,630.20		2,049.10
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	194.7		197.6
Community Development Investments	230		275.1
Employee Benefit and Deferred Compensation	43.9		39.4
Deposits
Savings Certificates and Other Time	1,943.90		2,746.70
Non U.S. Offices Interest-Bearing	45,017.20		39,720.20
Federal Funds Purchased	1,053.60		780.2
Securities Sold Under Agreements to Repurchase	469		699.8
Other Borrowings	1,804.70		367.4
Senior Notes	2,002.30		2,513.40
Long Term Debt (excluding Leases)
Subordinated Debt	841.1		1,065.30
Federal Home Loan Bank Borrowings	138.6		345.4
Floating Rate Capital Debt	229.3		228
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Asset And Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	17		21.3
Liabilities	36.7		42.3
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Asset And Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	107.9		129.7
Liabilities	44.4		75.3
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Asset And Liability Management | Credit Default Swap Contracts
Derivative Instruments
Liabilities			1
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	3,162.10		1,735.30
Liabilities	3,165.30		1,730.40
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 2 | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	134.9		180.6
Liabilities	128.6		174
Estimate of Fair Value, Fair Value Disclosure | Fair Value, Inputs, Level 3
Securities
Available for Sale	98.7	[1]	97.8	[2]
Loans (excluding Leases)
Held for Investment	27,752.50		28,220.20
Held for Sale	2.8		11.7
Other Liabilities
Standby Letters of Credit	55.6		60.5
Contingent Consideration	54		50.1
Loan Commitments	$38.10		$38.90
[1]	Refer to the table located on page 37 for the disaggregation of available for sale securities.
[2]	Refer to the table located on page 38 for the disaggregation of available for sale securities.

Reconciliation_of_Amortized_Co

Reconciliation of Amortized Cost to Fair Values of Securities Available for Sale (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investment Holdings [Line Items]
Amortized Cost	$27,445.30	$28,422.20
Gross Unrealized Gains	158.1	242.8
Gross Unrealized Losses	101.8	21.5
Fair Value	27,501.60	28,643.50
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	1,498.70	1,747.90
Gross Unrealized Gains	25.8	36.7
Fair Value	1,524.50	1,784.60
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	13.9	13.9
Gross Unrealized Gains	0.5	0.2
Fair Value	14.4	14.1
U.S. Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,979.10	18,520.60
Gross Unrealized Gains	81.6	122.2
Gross Unrealized Losses	36.1	4
Fair Value	17,024.60	18,638.80
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,537	2,602.40
Gross Unrealized Gains	8.5	18.1
Gross Unrealized Losses	54.5	2.1
Fair Value	3,491	2,618.40
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,774.20	1,697.10
Gross Unrealized Gains	33.7	51
Gross Unrealized Losses	2.3	0.1
Fair Value	1,805.60	1,748
Supranational, Sovereign and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	718.5
Gross Unrealized Gains	4.1
Gross Unrealized Losses	1.5
Fair Value	721.1
Supranational Bonds
Investment Holdings [Line Items]
Amortized Cost		1,053.90
Gross Unrealized Gains		7
Gross Unrealized Losses		0.2
Fair Value		1,060.70
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	58.1	102.4
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	4.7	10.8
Fair Value	53.5	92
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,535.70	2,280
Gross Unrealized Gains	1.2	4.3
Gross Unrealized Losses	1.2	0.4
Fair Value	2,535.70	2,283.90
Auction Rate Securities
Investment Holdings [Line Items]
Amortized Cost	98.1	99.6
Gross Unrealized Gains	2.1	2.1
Gross Unrealized Losses	1.5	3.9
Fair Value	98.7	97.8
Others
Investment Holdings [Line Items]
Amortized Cost	232	304.4
Gross Unrealized Gains	0.5	0.8
Fair Value	$232.50	$305.20

Reconciliation_of_Amortized_Co1

Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$3,053.60	$2,382
Gross Unrealized Gains	13.3	21.5
Gross Unrealized Losses	13.7	8.7
Fair Value	3,053.20	2,394.80
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	248.8	329.3
Gross Unrealized Gains	11.3	17.2
Gross Unrealized Losses
Fair Value	260.1	346.5
U.S. Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	41.3	112.9
Gross Unrealized Gains	1.2	3.8
Gross Unrealized Losses
Fair Value	42.5	116.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	219.3	205
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	219.3	205
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,497.40	1,667.60
Gross Unrealized Gains		0.2
Gross Unrealized Losses	0.2	0.6
Fair Value	1,497.20	1,667.20
Supranational and Sovereign Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	989.5
Gross Unrealized Gains	0.8
Gross Unrealized Losses	1.9
Fair Value	988.4
Others
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	57.3	67.2
Gross Unrealized Gains		0.3
Gross Unrealized Losses	11.6	8.1
Fair Value	$45.70	$59.40

Remaining_Maturity_of_Securiti

Remaining Maturity of Securities Available for Sale (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Available for Sale - Amortized Cost
Due in One Year or Less	$9,630.10
Due After One Year Through Five Years	14,551.90
Due After Five Years Through Ten Years	2,264.20
Due After Ten Years	999.1
Total	27,445.30
Held to Maturity - Amortized Cost
Due in One Year or Less	1,844.30
Due After One Year Through Five Years	1,134.80
Due After Five Years Through Ten Years	37.7
Due After Ten Years	36.8
Total	3,053.60	2,382
Available for Sale - Fair Value
Due in One Year or Less	9,645.70
Due After One Year Through Five Years	14,617.70
Due After Five Years Through Ten Years	2,242.10
Due After Ten Years	996.1
Total	27,501.60
Held to Maturity - Fair Value
Due in One Year or Less	1,846.20
Due After One Year Through Five Years	1,138.70
Due After Five Years Through Ten Years	40
Due After Ten Years	28.3
Fair Value	$3,053.20	$2,394.80

Securities_Additional_Informat

Securities - Additional Information (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
	Investment				Investment
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net	($2,200,000)	[1]	$200,000	[1]	($1,900,000)	[1]	($1,700,000)	[1]
Net impairment losses recognized in earnings	0		200,000		0		3,300,000
Gross proceeds from sale of securities	316,400,000		218,200,000		398,100,000		2,700,000,000
Gross realized securities gains	100,000		356,000		400,000		23,500,000
Gross realized securities losses	2,300,000		1,000		2,300,000		21,900,000
Number of securities in an unrealized loss position	428				428
Total Fair Value	8,823,400,000				8,823,400,000				3,825,800,000
Total Unrealized Losses	115,500,000				115,500,000				30,200,000
12 Months or Longer Unrealized Losses	18,000,000				18,000,000				24,000,000
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	36,100,000				36,100,000
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	11,600,000				11,600,000
Auction Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net impairment losses recognized in earnings							1,600,000
Total Unrealized Losses	1,500,000				1,500,000
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	54,500,000				54,500,000
Percent of corporate debt portfolio	51.00%				51.00%
Non-Agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net impairment losses recognized in earnings			148,600				1,700,000
Non-Agency RMBS | Financing Receivable, Current | Lower Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	5.00%				5.00%
Non-Agency RMBS | Financing Receivable, Current | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	30.00%				30.00%
Non-Agency RMBS | Financing Receivable, 30 to 59 Days Past Due
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	30.00%				30.00%
Non-Agency RMBS | Financing Receivable, 60 to 89 Days Past Due
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	80.00%				80.00%
Non-Agency RMBS | Financing Receivable, 90 Days and Greater Past Due
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	90.00%				90.00%
Non-Agency RMBS | Other Real Estate Owned and Loans in Foreclosure
Schedule of Trading Securities and Other Trading Assets [Line Items]
Expected loss on subprime, Alt-A, prime and 2nd lien portfolios developed using default roll rate	100.00%				100.00%
Residential Mortgage-Backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	4,700,000				4,700,000
Number of securities in an unrealized loss position for more than 12 months	6				6
12 Months or Longer Unrealized Losses	45,500,000				45,500,000
Residential Mortgage-Backed | Sub-prime and Alt-A Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Residential mortgage-backed securities total amortized cost	58,100,000				58,100,000
Residential mortgage-backed securities fair value	$53,500,000				$53,500,000
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%				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				5
[1]	Changes in Other-Than-Temporary-Impairment (OTTI) Losses $ - $ 0.4 $ - $ (2.7 ) Noncredit-related OTTI Losses Recorded in/(Reclassified from) OCI - (0.6 ) - (0.6 ) Other Security Gains (Losses), net (2.2 ) 0.4 (1.9 ) 1.6 Investment Security Gains (Losses), net $ (2.2 ) $ 0.2 $ (1.9 ) $ (1.7 )

Securities_Continuous_Unrealiz

Securities Continuous Unrealized Loss Position (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$8,359.60	$2,394.50
Less than 12 Months Unrealized Losses	97.5	6.2
12 Months or Longer Fair Value	463.8	1,431.30
12 Months or Longer Unrealized Losses	18	24
Total Fair Value	8,823.40	3,825.80
Total Unrealized Losses	115.5	30.2
U.S. Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	3,776.10	482.2
Less than 12 Months Unrealized Losses	32.3	1
12 Months or Longer Fair Value	302.7	1,171.80
12 Months or Longer Unrealized Losses	3.8	3
Total Fair Value	4,078.80	1,654
Total Unrealized Losses	36.1	4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,911.20	441.5
Less than 12 Months Unrealized Losses	54.4	2
12 Months or Longer Fair Value	25	50
12 Months or Longer Unrealized Losses	0.1	0.1
Total Fair Value	1,936.20	491.5
Total Unrealized Losses	54.5	2.1
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	207.4	20.1
Less than 12 Months Unrealized Losses	2.3	0.1
Total Fair Value	207.4	20.1
Total Unrealized Losses	2.3	0.1
Supranational Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		113.8
Less than 12 Months Unrealized Losses		0.2
Total Fair Value		113.8
Total Unrealized Losses		0.2
Supranational, Sovereign and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	549.6
Less than 12 Months Unrealized Losses	3.4
Total Fair Value	549.6
Total Unrealized Losses	3.4
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	45.5	84.7
12 Months or Longer Unrealized Losses	4.7	10.8
Total Fair Value	45.5	84.7
Total Unrealized Losses	4.7	10.8
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	1,113.90	146.1
Less than 12 Months Unrealized Losses	1.1	0.1
12 Months or Longer Fair Value	45.1	40
12 Months or Longer Unrealized Losses	0.1	0.3
Total Fair Value	1,159	186.1
Total Unrealized Losses	1.2	0.4
Certificates of Deposit
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	735.7	1,178.80
Less than 12 Months Unrealized Losses	0.2	0.6
Total Fair Value	735.7	1,178.80
Total Unrealized Losses	0.2	0.6
Auction Rate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	27.8	2.7
Less than 12 Months Unrealized Losses	0.4	0.3
12 Months or Longer Fair Value	16.5	41
12 Months or Longer Unrealized Losses	1.1	3.6
Total Fair Value	44.3	43.7
Total Unrealized Losses	1.5	3.9
Others
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	37.9	9.3
Less than 12 Months Unrealized Losses	3.4	1.9
12 Months or Longer Fair Value	29	43.8
12 Months or Longer Unrealized Losses	8.2	6.2
Total Fair Value	66.9	53.1
Total Unrealized Losses	$11.60	$8.10

Book_Values_Ultimate_Default_R

Book Values, Ultimate Default Rates, and Loss Severity Rates for Non-Agency Residential Mortgage-Backed Securities Portfolio, by Security Type (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$27,445.30	$28,422.20
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58.1
Weighted Average Ultimate Default Rates	40.10%
Loss Severity Rates - Weighted Average	74.80%
Non-Agency RMBS | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	32.90%
Non-Agency RMBS | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	99.00%
Non-Agency RMBS | Prime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7.4
Weighted Average Ultimate Default Rates	18.90%
Loss Severity Rates - Weighted Average	41.10%
Non-Agency RMBS | Prime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	32.90%
Non-Agency RMBS | Prime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	45.00%
Non-Agency RMBS | Alt-A
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12.4
Weighted Average Ultimate Default Rates	43.10%
Loss Severity Rates - Weighted Average	66.20%
Non-Agency RMBS | Alt-A | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	66.20%
Non-Agency RMBS | Alt-A | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	66.20%
Non-Agency RMBS | Subprime
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26.7
Weighted Average Ultimate Default Rates	48.80%
Loss Severity Rates - Weighted Average	77.60%
Non-Agency RMBS | Subprime | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	72.50%
Non-Agency RMBS | Subprime | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	82.10%
Non-Agency RMBS | 2nd Lien
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$11.60
Weighted Average Ultimate Default Rates	33.10%
Loss Severity Rates - Weighted Average	99.00%
Non-Agency RMBS | 2nd Lien | Lower Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	98.90%
Non-Agency RMBS | 2nd Lien | Upper Limit
Schedule of Available-for-sale Securities [Line Items]
Loss Severity Rates	99.00%

Credit_Related_Impairment_Loss

Credit Related Impairment Losses Recognized in Earnings on Other Than Temporarily Impaired Securities (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Changes in OTTI Losses		[1]	$0.40	[1]		[1]	($2.70)	[1]
Noncredit-related Losses Recorded in / (Reclassified from) OCI		[2]	-0.6	[2]		[2]	-0.6	[2]
Net Impairment Losses Recognized in Earnings	$0		($0.20)		$0		($3.30)
[1]	For initial other-than-temporary impairments in the respective 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 other-than-temporary impairments in the respective period, the balance includes the portion of the excess of amortized cost over the fair value of the impaired securities that was recorded in 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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative Credit-Related Losses on Securities Held - Beginning of Period	$8.80	$43.20	$42.30	$68.20
Plus: Losses on Newly Identified Impairments				1.6
Additional Losses on Previously Identified Impairments		0.2		1.7
Less: Current and Prior Period Losses on Securities Sold During the Period			-33.5	-28.1
Cumulative Credit-Related Losses on Securities Held - End of Period	$8.80	$43.40	$8.80	$43.40

Debt_Securities_Held_for_which

Debt Securities Held for which Other-Than-Temporary Impairment Loss has been Recognized in Current Period or Previously (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
OtherThanTemporary Impairment Non Credit Losses Recognized In Earnings [Line Items]
Fair Value	$38.90	$51.50
Amortized Cost Basis	44.3	59
Noncredit-related Losses Recognized in OCI	-5.4	-7.5
Tax Effect	2	2.8
Amount Recorded in OCI	($3.40)	($4.70)

Amounts_Outstanding_for_Loans_

Amounts Outstanding for Loans and Leases by Segment and Class (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	$29,064.80	$29,504.50
Allowance for Credit Losses Assigned to Loans and Leases	-287.2	-297.9
Net Loans and Leases	28,777.60	29,206.60
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial and Institutional	7,237	7,468.50
Commercial Real Estate	2,821.80	2,859.80
Lease Financing, net	982.1	1,035
Non-U.S.	1,155.20	1,192.30
Other	285.1	341.6
Total Loans and Leases	12,481.20	12,897.20
Allowance for Credit Losses Assigned to Loans and Leases	-152.1	-166.1
Personal
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential Real Estate	10,359.50	10,375.20
Private Client	6,139.10	6,130.10
Other	85	102
Total Loans and Leases	16,583.60	16,607.30
Allowance for Credit Losses Assigned to Loans and Leases	($135.10)	($131.80)

Loans_and_Leases_Additional_In

Loans and Leases - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
			Y
Accounts, Notes, Loans and Financing Receivable [Line Items]
Draw period for credit line products			10
Other U.S. loans and non-U.S. loans, short duration advances	$1,500,000,000		$1,500,000,000		$1,500,000,000
Demand deposits	54,700,000		54,700,000		224,700,000
Loans held for sale	2,800,000		2,800,000		11,700,000
Maximum limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures	250,000		250,000
Interest income that would have been recorded for nonperforming loans	2,800,000	2,900,000	8,000,000	8,700,000
Unfunded loan commitments and standby letters of credit		4,300,000		4,300,000	2,100,000
Total Loans and Leases	29,064,800,000		29,064,800,000		29,504,500,000
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
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
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	87.00%		87.00%		86.00%
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	2,000,000,000		2,000,000,000		2,300,000,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	65,800,000		65,800,000		49,800,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loans and Leases	100,100,000		100,100,000		74,700,000
Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number loans modified in troubled debt restructurings in the previous 12 months which became nonperforming	0		0
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan, lower limit	65.00%		65.00%
Collateral required on Residential real estate loan, upper limit	80.00%		80.00%
Residential Real Estate | Re-Defaulted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number loans modified in troubled debt restructurings in the previous 12 months which became nonperforming		0		2
Recorded Investment				89,400
Unpaid Principal Balance				$89,400

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 $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$29,064.80	$29,504.50
1 to 3 Category
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	13,860.40	13,332
4 to 5 Category
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	14,292.90	15,240.40
6 to 9 Category (Watch List)
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	911.5	932.1
Commercial
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	7,237	7,468.50
Commercial Real Estate	2,821.80	2,859.80
Lease Financing, net	982.1	1,035
Non-U.S.	1,155.20	1,192.30
Other	285.1	341.6
Total Loans and Leases	12,481.20	12,897.20
Commercial | 1 to 3 Category
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	4,384.70	4,291.80
Commercial Real Estate	992.9	888.6
Lease Financing, net	674.6	647.1
Non-U.S.	523.4	542.7
Other	107.5	167.2
Total Loans and Leases	6,683.10	6,537.40
Commercial | 4 to 5 Category
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	2,679	3,040.60
Commercial Real Estate	1,652.20	1,710.90
Lease Financing, net	303	382.3
Non-U.S.	631.1	646.6
Other	177.6	174.4
Total Loans and Leases	5,442.90	5,954.80
Commercial | 6 to 9 Category (Watch List)
Financing Receivable, Recorded Investment [Line Items]
Commercial and Institutional	173.3	136.1
Commercial Real Estate	176.7	260.3
Lease Financing, net	4.5	5.6
Non-U.S.	0.7	3
Total Loans and Leases	355.2	405
Personal
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	10,359.50	10,375.20
Private Client	6,139.10	6,130.10
Other	85	102
Total Loans and Leases	16,583.60	16,607.30
Personal | 1 to 3 Category
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	3,289.70	3,003.30
Private Client	3,855.50	3,741.30
Other	32.1	50
Total Loans and Leases	7,177.30	6,794.60
Personal | 4 to 5 Category
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	6,535.40	6,868.20
Private Client	2,261.70	2,365.40
Other	52.9	52
Total Loans and Leases	8,850	9,285.60
Personal | 6 to 9 Category (Watch List)
Financing Receivable, Recorded Investment [Line Items]
Residential Real Estate	534.4	503.7
Private Client	21.9	23.4
Total Loans and Leases	$556.30	$527.10

Balances_and_Delinquency_Statu

Balances and Delinquency Status of Performing and Nonperforming Loans and Leases by Segment and Class as well as Total Other Real Estate Owned and Nonperforming Asset Balances (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Financing Receivable, Recorded Investment [Line Items]
Other Real Estate Owned	$13.90	$20.30
Total Nonperforming Assets	284	275.1
Total Loans and Leases	29,064.80	29,504.50
Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	12,481.20	12,897.20
Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	7,237	7,468.50
Commercial | Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	2,821.80	2,859.80
Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	982.1	1,035
Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	1,155.20	1,192.30
Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	285.1	341.6
Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	16,583.60	16,607.30
Personal | Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	10,359.50	10,375.20
Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	6,139.10	6,130.10
Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	85	102
Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Current	28,670	29,073.90
30-59 Days Past Due	74	96.2
60-89 Days Past Due	25.9	60.6
90 Days or More Past Due	24.8	19
Total Loans and Leases	28,794.70	29,249.70
Performing Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Current	12,362.40	12,784.30
30-59 Days Past Due	22.9	13.3
60-89 Days Past Due	5.5	18.6
90 Days or More Past Due	12	3
Total Loans and Leases	12,402.80	12,819.20
Performing Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Current	7,194.10	7,433.40
30-59 Days Past Due	12.2	6.4
60-89 Days Past Due	2.8	5.5
90 Days or More Past Due	3.8	1.6
Total Loans and Leases	7,212.90	7,446.90
Performing Financing Receivable | Commercial | Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Current	2,745.90	2,782
30-59 Days Past Due	10.7	6.9
60-89 Days Past Due	2.7	13.1
90 Days or More Past Due	8.2	1.4
Total Loans and Leases	2,767.50	2,803.40
Performing Financing Receivable | Commercial | Lease Financing, net
Financing Receivable, Recorded Investment [Line Items]
Current	982.1	1,035
Total Loans and Leases	982.1	1,035
Performing Financing Receivable | Commercial | Non-U.S.
Financing Receivable, Recorded Investment [Line Items]
Current	1,155.20	1,192.30
Total Loans and Leases	1,155.20	1,192.30
Performing Financing Receivable | Commercial | Other
Financing Receivable, Recorded Investment [Line Items]
Current	285.1	341.6
Total Loans and Leases	285.1	341.6
Performing Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Current	16,307.60	16,289.60
30-59 Days Past Due	51.1	82.9
60-89 Days Past Due	20.4	42
90 Days or More Past Due	12.8	16
Total Loans and Leases	16,391.90	16,430.50
Performing Financing Receivable | Personal | Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Current	10,135.90	10,096.30
30-59 Days Past Due	11.7	68.1
60-89 Days Past Due	12.5	25.7
90 Days or More Past Due	9.6	10.5
Total Loans and Leases	10,169.70	10,200.60
Performing Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Current	6,086.70	6,091.30
30-59 Days Past Due	39.4	14.8
60-89 Days Past Due	7.9	16.3
90 Days or More Past Due	3.2	5.5
Total Loans and Leases	6,137.20	6,127.90
Performing Financing Receivable | Personal | Other
Financing Receivable, Recorded Investment [Line Items]
Current	85	102
Total Loans and Leases	85	102
Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	270.1	254.8
Nonperforming Financing Receivable | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	78.4	78
Nonperforming Financing Receivable | Commercial | Commercial and Institutional
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	24.1	21.6
Nonperforming Financing Receivable | Commercial | Commercial Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	54.3	56.4
Nonperforming Financing Receivable | Personal
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	191.7	176.8
Nonperforming Financing Receivable | Personal | Residential Real Estate
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	189.8	174.6
Nonperforming Financing Receivable | Personal | Private Client
Financing Receivable, Recorded Investment [Line Items]
Total Loans and Leases	$1.90	$2.20

Impaired_Loans_by_Segment_and_

Impaired Loans by Segment and Class (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Recorded Investment	$310.80				$310.80				$269.80
Unpaid principal balance	372.2				372.2				325.4
Specific allowance	17.8				17.8				18
Average recorded investment	294.7	[1]	253.3	[1]	278.3	[1]	237.2	[1]
Interest income recognized	0.8		0.3		2.8		1.2
Commercial
Financing Receivable, Impaired [Line Items]
Recorded Investment	109.5				109.5				125
Unpaid principal balance	127				127				144.8
Specific allowance	13.7				13.7				11
Average recorded investment	102.5	[1]	114.2	[1]	100.6	[1]	105	[1]
Interest income recognized	0.3		0.1		0.9		0.4
Commercial | Commercial and Institutional
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	11.3				11.3				15.1
With a related specific reserve, recorded investment	14				14				8.1
With no related specific reserve, unpaid principal balance	14.5				14.5				19.3
With a related specific reserve, unpaid principal balance	16.2				16.2				10.2
Specific allowance	5				5				2.8
With no related specific reserve, average recorded investment	10.7	[1]	27	[1]	11.3	[1]	25.2	[1]
With a related specific reserve, average recorded investment	14.1	[1]	5.2	[1]	12.5	[1]	6.5	[1]
With no related specific reserve, interest income recognized			0.1		0.1		0.2
With a related specific reserve, interest income recognized
Commercial | Commercial Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	48.6				48.6				64.9
With a related specific reserve, recorded investment	31.1				31.1				32.3
With no related specific reserve, unpaid principal balance	59.2				59.2				76.9
With a related specific reserve, unpaid principal balance	32.6				32.6				33.8
Specific allowance	8.7				8.7				8.2
With no related specific reserve, average recorded investment	42	[1]	54.7	[1]	40.6	[1]	49.6	[1]
With a related specific reserve, average recorded investment	31.2	[1]	22.1	[1]	31.6	[1]	20.2	[1]
With no related specific reserve, interest income recognized	0.2				0.6		0.2
With a related specific reserve, interest income recognized
Commercial | Lease Financing, net
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	4.5				4.5				4.6
With no related specific reserve, unpaid principal balance	4.5				4.5				4.6
With no related specific reserve, average recorded investment	4.5	[1]	5.2	[1]	4.6	[1]	3.5	[1]
With no related specific reserve, interest income recognized	0.1				0.2
Personal
Financing Receivable, Impaired [Line Items]
Recorded Investment	201.3				201.3				144.8
Unpaid principal balance	245.2				245.2				180.6
Specific allowance	4.1				4.1				7
Average recorded investment	192.2	[1]	139.1	[1]	177.7	[1]	132.2	[1]
Interest income recognized	0.5		0.2		1.9		0.8
Personal | Residential Real Estate
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	177.2				177.2				131.3
With a related specific reserve, recorded investment	10				10				11.8
With no related specific reserve, unpaid principal balance	220.4				220.4				165.7
With a related specific reserve, unpaid principal balance	10.7				10.7				13
Specific allowance	3.3				3.3				6.1
With no related specific reserve, average recorded investment	168.1	[1]	120.7	[1]	155.2	[1]	114	[1]
With a related specific reserve, average recorded investment	10	[1]	15.3	[1]	9.4	[1]	15.3	[1]
With no related specific reserve, interest income recognized	0.3		0.2		1.4		0.8
With a related specific reserve, interest income recognized
Personal | Private Client
Financing Receivable, Impaired [Line Items]
With no related specific reserve, recorded investment	13.3				13.3				0.8
With a related specific reserve, recorded investment	0.8				0.8				0.9
With no related specific reserve, unpaid principal balance	13.3				13.3				1
With a related specific reserve, unpaid principal balance	0.8				0.8				0.9
Specific allowance	0.8				0.8				0.9
With no related specific reserve, average recorded investment	13.3	[1]	2.2	[1]	12	[1]	1.8	[1]
With a related specific reserve, average recorded investment	0.8	[1]	0.9	[1]	1.1	[1]	1.1	[1]
With no related specific reserve, interest income recognized	0.2				0.5
With a related specific reserve, interest income recognized
[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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
	Contract	Contract	Contract	Contract
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	45	24	146	95
Recorded Investment	$29,500,000	$4,700,000	$91,900,000	$41,200,000
Unpaid Principal Balance	36,400,000	5,300,000	105,500,000	46,800,000
Commercial
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	7	1	18	10
Recorded Investment	22,800,000	2,700,000	27,900,000	30,300,000
Unpaid Principal Balance	25,000,000	2,700,000	32,900,000	32,500,000
Commercial | Commercial and Institutional
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases			7	2
Recorded Investment			500,000	500,000
Unpaid Principal Balance			1,000,000	1,100,000
Commercial | Commercial Real Estate Portfolio Segment
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	7	1	11	7
Recorded Investment	22,800,000	2,700,000	27,400,000	24,600,000
Unpaid Principal Balance	25,000,000	2,700,000	31,900,000	26,200,000
Commercial | Lease Financing, Net
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases				1
Recorded Investment				5,200,000
Unpaid Principal Balance				5,200,000
Personal
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	38	23	128	85
Recorded Investment	6,700,000	2,000,000	64,000,000	10,900,000
Unpaid Principal Balance	11,400,000	2,600,000	72,600,000	14,300,000
Personal | Residential Real Estate
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases	38	23	122	84
Recorded Investment	6,700,000	2,000,000	51,200,000	10,100,000
Unpaid Principal Balance	11,400,000	2,600,000	59,800,000	13,500,000
Personal | Private Client
Financing Receivable, Modifications [Line Items]
Number of Loans and Leases			6	1
Recorded Investment			12,800,000	800,000
Unpaid Principal Balance			$12,800,000	$800,000

Changes_in_Total_Allowance_for

Changes in Total Allowance for Credit Losses by Segment (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Period	$320.70	$329.90	$327.60	$328.90
Charge-Offs	-11.6	-16.3	-39.8	-46.9
Recoveries	3.3	4.4	14.7	26
Net (Charge-Offs) Recoveries	-8.3	-11.9	-25.1	-20.9
Provision for Credit Losses	5	10	15	20
Effect of Foreign Exchange Rates	0.1
Balance at End of Period	317.5	328	317.5	328
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Period	184.1	196.3	194.2	211
Charge-Offs	-1.7	-7.2	-6.9	-15.4
Recoveries	1.7	2.4	8.1	13.6
Net (Charge-Offs) Recoveries		-4.8	1.2	-1.8
Provision for Credit Losses	-4.6	6.9	-15.8	-10.8
Effect of Foreign Exchange Rates	0.1
Balance at End of Period	179.6	198.4	179.6	198.4
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance at Beginning of Period	136.6	133.6	133.4	117.9
Charge-Offs	-9.9	-9.1	-32.9	-31.5
Recoveries	1.6	2	6.6	12.4
Net (Charge-Offs) Recoveries	-8.3	-7.1	-26.3	-19.1
Provision for Credit Losses	9.6	3.1	30.8	30.8
Balance at End of Period	$137.90	$129.60	$137.90	$129.60

Allowances_for_Credit_Losses_a

Allowances for Credit Losses and Recorded Investments in Loans and Leases by Segment (Detail) (USD $)	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	$311		$269.80
Evaluated for Inherent Impairment	28,753.80		29,234.70
Total Loans and Leases	29,064.80		29,504.50
Specifically Evaluated for Impairment	17.8		18
Evaluated for Inherent Impairment	269.4		279.9
Allowance Assigned to Loans and Leases	287.2		297.9
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	30.3		29.7
Total Allowance for Credit Losses	317.5	320.7	327.6	328	329.9	328.9
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	109.7		125
Evaluated for Inherent Impairment	12,371.50		12,772.20
Total Loans and Leases	12,481.20		12,897.20
Specifically Evaluated for Impairment	13.7		11
Evaluated for Inherent Impairment	138.4		155.1
Allowance Assigned to Loans and Leases	152.1		166.1
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	27.5		28.1
Total Allowance for Credit Losses	179.6	184.1	194.2	198.4	196.3	211
Personal
Financing Receivable, Allowance for Credit Losses [Line Items]
Specifically Evaluated for Impairment	201.3		144.8
Evaluated for Inherent Impairment	16,382.30		16,462.50
Total Loans and Leases	16,583.60		16,607.30
Specifically Evaluated for Impairment	4.1		7
Evaluated for Inherent Impairment	131		124.8
Allowance Assigned to Loans and Leases	135.1		131.8
Allowance for Unfunded Exposures Commitments and Standby Letters of Credit	2.8		1.6
Total Allowance for Credit Losses	$137.90	$136.60	$133.40	$129.60	$133.60	$117.90

Pledged_Assets_Additional_Info

Pledged Assets - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$28,300,000,000		$28,300,000,000		$27,200,000,000
Fair value of accepted collateral	500,000,000		500,000,000		35,400,000
Amount of securities repledged or sold collateral	0		0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	900,000,000	1,100,000,000	900,000,000	1,100,000,000
US Government Corporations and Agencies Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	19,400,000,000		19,400,000,000		19,000,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	256,300,000		256,300,000		334,000,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	8,700,000,000		8,700,000,000		7,900,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	4,500,000,000		4,500,000,000		3,400,000,000
Available for Sale Investment Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$460,400,000		$460,400,000		$690,100,000

Carrying_Amounts_Of_Goodwill_D

Carrying Amounts Of Goodwill (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Goodwill [Line Items]
Total Goodwill	$537.70	$537.80
Corporate and Institutional Services
Goodwill [Line Items]
Total Goodwill	466.2	466.3
Wealth Management
Goodwill [Line Items]
Total Goodwill	$71.50	$71.50

Other_Intangible_Assets_Subjec

Other Intangible Assets Subject to Amortization (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$251.30	$252.10
Less: Accumulated Amortization	163.5	148.1
Net Book Value	$87.80	$104

Goodwill_and_Other_Intangibles2

Goodwill and Other Intangibles - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Goodwill and Intangible Assets Disclosure [Line Items]
Estimated future amortization expense for 2013	$5.60		$5.60
Estimated future amortization expense for 2014	19.4		19.4
Estimated future amortization expense for 2015	11.8		11.8
Estimated future amortization expense for 2016	9.2		9.2
Estimated future amortization expense for 2017	9.2		9.2
Other Intangible Assets
Goodwill and Intangible Assets Disclosure [Line Items]
Amortization expense related to other intangible assets	$5.20	$5.20	$15.50	$15

Earnings_Contribution_of_North

Earnings Contribution of Northern Trust's Business Units (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	$648		$601.90		$1,936		$1,782.90
Foreign Exchange Trading Income	62.8		44		193.6		165.3
Other Noninterest Income	99.4		81		231.3		222.1
Net Interest Income (FTE)	244.8	[1]	256.9	[1]	706.5	[2]	787.5	[2]
Revenue	1,055	[1]	983.8	[1]	3,067.40	[2]	2,957.80	[2]
Provision for Credit Losses	5		10		15		20
Noninterest Expense	740.7		696.4		2,199.30		2,137.30
Income (loss) before Income Taxes	309.3	[1]	277.4	[1]	853.1	[2]	800.5	[2]
Provision (Benefit) for Taxes	102.8	[1]	98.6	[1]	291.5	[2]	280.9	[2]
Net Income	206.5		178.8		561.6		519.6
Percentage of Consolidated Net Income	100.00%		100.00%		100.00%		100.00%
Average Assets	95,212.50		92,709.90		93,223.50		93,413.60
Corporate and Institutional Services
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	359.8		334.4		1,072.70		989.8
Foreign Exchange Trading Income	61.8		41.3		189.8		155.3
Other Noninterest Income	44.3		50.2		130.1		145.9
Net Interest Income (FTE)	70.1	[1]	68	[1]	200.2	[2]	217	[2]
Revenue	536	[1]	493.9	[1]	1,592.80	[2]	1,508	[2]
Provision for Credit Losses	0.4		-1.6		-1.1		-1.6
Noninterest Expense	412		394.5		1,206.70		1,190
Income (loss) before Income Taxes	123.6	[1]	101	[1]	387.2	[2]	319.6	[2]
Provision (Benefit) for Taxes	38.8	[1]	32.7	[1]	123	[2]	102.1	[2]
Net Income	84.8		68.3		264.2		217.5
Percentage of Consolidated Net Income	41.00%		38.00%		47.00%		42.00%
Average Assets	53,653.50		50,638.60		52,323.80		49,642.80
Wealth Management
Segment Reporting Information [Line Items]
Trust, Investment and Other Servicing Fees	288.2		267.5		863.3		793.1
Foreign Exchange Trading Income	1		2.7		3.8		10
Other Noninterest Income	54.3		23.3		95.8		70.1
Net Interest Income (FTE)	136.3	[1]	157.4	[1]	425.3	[2]	476.9	[2]
Revenue	479.8	[1]	450.9	[1]	1,388.20	[2]	1,350.10	[2]
Provision for Credit Losses	4.6		11.6		16.1		21.6
Noninterest Expense	297.3		285.2		900.9		878.4
Income (loss) before Income Taxes	177.9	[1]	154.1	[1]	471.2	[2]	450.1	[2]
Provision (Benefit) for Taxes	67	[1]	58.1	[1]	177.8	[2]	170.2	[2]
Net Income	110.9		96		293.4		279.9
Percentage of Consolidated Net Income	54.00%		54.00%		52.00%		54.00%
Average Assets	22,923.60		23,530.70		22,863.10		23,527.60
Treasury and Other
Segment Reporting Information [Line Items]
Other Noninterest Income	0.8		7.5		5.4		6.1
Net Interest Income (FTE)	38.4	[1]	31.5	[1]	81	[2]	93.6	[2]
Revenue	39.2	[1]	39	[1]	86.4	[2]	99.7	[2]
Noninterest Expense	31.4		16.7		91.7		68.9
Income (loss) before Income Taxes	7.8	[1]	22.3	[1]	-5.3	[2]	30.8	[2]
Provision (Benefit) for Taxes	-3	[1]	7.8	[1]	-9.3	[2]	8.6	[2]
Net Income	10.8		14.5		4		22.2
Percentage of Consolidated Net Income	5.00%		8.00%		1.00%		4.00%
Average Assets	$18,635.40		$18,540.60		$18,036.60		$20,243.20
[1]	Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $7.8 million for 2013 and $11.3 million for 2012.
[2]	Stated on a fully taxable equivalent basis (FTE). Total consolidated includes FTE adjustments of $23.3 million for 2013 and $31.4 million for 2012.

Earnings_Contribution_of_North1

Earnings Contribution of Northern Trust's Business Units (Parenthetical) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Fully taxable equivalent basis adjustments	$7.80	$11.30	$23.30	$31.40

Components_of_Accumulated_Othe

Components of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			($283)	($345.60)
Net Change	12.8	58.2	-54.9	129.7
Ending Balance	-337.9	-215.9	-337.9	-215.9
Net Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			101	39.8
Net Change			-74.4	67.3
Ending Balance	26.6	107.1	26.6	107.1
Net Unrealized Gains (Losses) on Cash Flow Hedge Designations
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			-1.4	-7
Net Change			1.2	5.5
Ending Balance	-0.2	-1.5	-0.2	-1.5
Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			10.5	-9.5
Net Change			-2.8	24.1
Ending Balance	7.7	14.6	7.7	14.6
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			-393.1	-368.9
Net Change			21.1	32.8
Ending Balance	($372)	($336.10)	($372)	($336.10)

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Loss) Changes During Period (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income	$12.80	$58.20	($54.90)	$129.70
Net Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	9.4	57.1	-119.2	107.7
Tax Effect	-3.6	-21.5	44.8	-40.4
Other Comprehensive Income	5.8	35.6	-74.4	67.3
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		4.9	2.1	15
Tax Effect		-1.8	-0.8	-5.6
Other Comprehensive Income		3.1	1.3	9.4
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	7.3	57.3	-123.2	105.9
Tax Effect	-2.8	-21.6	46.3	-39.7
Other Comprehensive Income	4.5	35.7	-76.9	66.2
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	2.1	-5.1	1.9	-13.2
Tax Effect	-0.8	1.9	-0.7	4.9
Other Comprehensive Income	1.3	-3.2	1.2	-8.3
Net Unrealized Gains (Losses) on Cash Flow Hedge Designations
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	6.4	4.6	1.7	8.4
Tax Effect	-2.1	-1.5	-0.5	-2.9
Other Comprehensive Income	4.3	3.1	1.2	5.5
Net Unrealized Gains (Losses) on Cash Flow Hedge Designations | Reclassification Adjustment for (Gains) Losses Included in Net Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	1.1	2.6	5.7	3.7
Tax Effect	-0.4	-1	-2.1	-1.4
Other Comprehensive Income	0.7	1.6	3.6	2.3
Net Unrealized Gains (Losses) on Cash Flow Hedge Designations | Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	5.3	2	-4	4.7
Tax Effect	-1.7	-0.5	1.6	-1.5
Other Comprehensive Income	3.6	1.5	-2.4	3.2
Foreign Currency Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-33.8	-2.7	-4	13.9
Tax Effect	29.4	16.9	1.2	10.2
Other Comprehensive Income	-4.4	14.2	-2.8	24.1
Foreign Currency Adjustments | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	55	42.1	5.3	31.3
Tax Effect	-4		-2.3
Other Comprehensive Income	51	42.1	3	31.3
Foreign Currency Adjustments | Net Investment Hedge Gains (Losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	-88.8	-44.8	-9.3	-17.4
Tax Effect	33.4	16.9	3.5	10.2
Other Comprehensive Income	-55.4	-27.9	-5.8	-7.2
Net Pension and Other Postretirement Benefit Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax	11.3	8.5	33.7	52.3
Tax Effect	-4.2	-3.2	-12.6	-19.5
Other Comprehensive Income	7.1	5.3	21.1	32.8
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	11.3	8.5	33.7	25.6
Tax Effect	-4.2	-3.2	-12.6	-9.4
Other Comprehensive Income	7.1	5.3	21.1	16.2
Net Pension and Other Postretirement Benefit Adjustments | Net Actuarial Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Before Tax				26.7
Tax Effect				-10.1
Other Comprehensive Income				$16.60

Reclassification_Adjustments_o

Reclassification Adjustments out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
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	$2.10	$1.90
CASH FLOW HEDGES | Foreign Exchange Contracts | Other Operating Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	1.1	5.7
Pension and Other Postretirement Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gross Reclassification Adjustment	11.3	33.7
Pension and Other Postretirement Adjustments | Employee Benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of Net Actuarial (Gain) Loss	11.9	35.8
Amortization of Prior Service Cost	($0.60)	($2.10)

Computations_of_Net_Income_Per

Computations of Net Income Per Common Share (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic Net Income Per Common Share
Average Number of Common Shares Outstanding	239,930,074	240,237,014	239,614,868	240,740,803
Net Income Applicable to Common Stock	$206.50	$178.80	$561.60	$519.60
Less: Earnings Allocated to Participating Securities	3.5	2.8	9.1	7.6
Earnings Allocated to Common Shares Outstanding	203	176	552.5	512
Basic Net Income Per Common Share	$0.85	$0.73	$2.31	$2.13
Diluted Net Income Per Common Share
Average Number of Common Shares Outstanding	239,930,074	240,237,014	239,614,868	240,740,803
Plus: Dilutive Effect of Share-based Compensation	1,400,578	460,048	1,242,829	464,383
Average Common and Potential Common Shares	241,330,652	240,697,062	240,857,697	241,205,186
Earnings Allocated to Common and Potential Common Shares	$203	$176	$552.50	$512
Diluted Net Income Per Common Share	$0.84	$0.73	$2.29	$2.12

Net_Income_Per_Common_Share_Co2

Net Income Per Common Share Computations - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents not included in the computation of diluted earnings per share because their inclusion would have been antidilutive	3,371,680	11,890,286	3,783,018	12,412,541

Components_of_Net_Interest_Inc

Components of Net Interest Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Interest Income
Loans and Leases	$185.40	$206.90	$558.50	$630.40
Securities - Taxable	60.9	63.2	167.6	191
- Non-Taxable	2.7	4.1	9	13.9
Interest-Bearing Deposits with Banks	36.3	44.8	105.8	138.8
Federal Reserve Deposits and Other	5.8	4.1	12.2	11.5
Total Interest Income	291.1	323.1	853.1	985.6
Interest Expense
Deposits	25.8	43.8	79.9	122.9
Federal Funds Purchased	0.4	0.1	1	1.1
Securities Sold Under Agreements to Repurchase	0.1	0.1	0.3	0.3
Other Borrowings	0.8	1.2	2.5	3.3
Senior Notes	18.4	18.6	56.9	52.4
Long-Term Debt	8	13	27.5	47.3
Floating Rate Capital Debt	0.6	0.7	1.8	2.2
Total Interest Expense	54.1	77.5	169.9	229.5
Net Interest Income	$237	$245.60	$683.20	$756.10

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Taxes [Line Items]
Income tax expense	$95	$87.30	$268.20	$249.50
Effective tax rate	31.50%	32.80%	32.30%	32.40%

Postretirement_Health_Care_Pla

Postretirement Health Care Plan Net Periodic (Benefit) Expense (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
U.S. Plan
Defined Benefit plan Net periodic benefit cost
Service Cost	$7.50	$8.90	$22.70	$26.50
Interest Cost	10.5	10.3	31.6	31.1
Expected Return on Plan Assets	-23.1	-21.7	-69.2	-65.3
Amortization
Net Actuarial (Gain) Loss	10.3	8.6	30.9	25.8
Prior Service Cost	-0.1	-0.1	-0.3	-0.3
Net Periodic Pension Expense (Benefit)	5.1	6	15.7	17.8
Non U.S. Plans
Defined Benefit plan Net periodic benefit cost
Interest Cost	1.6	1.6	4.9	4.7
Expected Return on Plan Assets	-1.5	-1.7	-4.6	-4.9
Amortization
Net Actuarial (Gain) Loss	0.2	0.2	0.7	0.6
Net Periodic Pension Expense (Benefit)	0.3	0.1	1	0.4
Supplemental Plan
Defined Benefit plan Net periodic benefit cost
Service Cost	0.4	0.8	1.2	2.2
Interest Cost	1.1	1.2	3.3	3.4
Amortization
Net Actuarial (Gain) Loss	1.7	1.5	5.1	4.5
Prior Service Cost	0.2	0.2	0.4	0.4
Net Periodic Pension Expense (Benefit)	3.4	3.7	10	10.5
Other Postretirement Plan
Defined Benefit plan Net periodic benefit cost
Service Cost		0.1	0.1	0.2
Interest Cost	0.3	0.3	0.9	1
Amortization
Net Actuarial (Gain) Loss	-0.3	-0.7	-0.9	-1.6
Prior Service Cost	-0.7	-1.2	-2.2	-3.8
Net Periodic Pension Expense (Benefit)	($0.70)	($1.50)	($2.10)	($4.20)

Compensation_Expense_for_Share

Compensation Expense for Share-Based Payment Arrangements and Associated Tax Impacts (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	$17.90	$16.60	$56.70	$58
Tax Benefits Recognized	6.8	6.2	21.3	21.8
Stock and Stock Unit Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	12.1	10.2	36.4	33.9
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	3.7	5.7	14.9	22.3
Performance Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-Based Compensation Expense	$2.10	$0.70	$5.40	$1.80

Variable_Interest_Entities_Add

Variable Interest Entities - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Tax Credit Structures
Variable Interest Entity [Line Items]
Carrying amounts	$213.30	$248.20
Liabilities related to unfunded commitments	22.4	33.1
Leveraged Leasing Transactions
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	80.00%	80.00%
Carrying amounts	$669.40	$673.60

Contingent_Liabilities_Additio

Contingent Liabilities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Sep. 30, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit outstanding						$4,300,000,000	$4,600,000,000
Minimum securities lending related collateral percentage						100.00%	100.00%
Possible loss in future periods in excess of accrual						117,000,000
Gain on redemption of Visa common stock				167,900,000
Remaining Visa shares held by Northern Trust, original cost basis				0
Visa indemnified litigation benefit (charges) recorded	23,100,000	33,000,000	17,800,000	76,100,000	-150,000,000
Fair value of contingent consideration						54,000,000	50,100,000
Indemnification Agreement
Commitments and Contingencies Disclosure [Line Items]
Collateralized securities loaned, subject to indemnification						84,700,000,000	69,700,000,000
Credit loss liability						$0	$0

Derivative_Financial_Instrumen2

Derivative Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012
							Net Investment Hedges	Net Investment Hedges	Client Related and Trading	Client Related and Trading
Derivative [Line Items]
Derivative assets recorded on the consolidated balance sheet, reduced as a result of master netting agreements	$1,700		$1,700			$982.50
Derivative liabilities recorded on the consolidated balance sheet, reduced as a result of master netting agreements	1,700		1,700			982.5
Reduction in derivative assets due to cash collateral received from derivative counterparties	300.5		300.5			118.6
Reduction in derivative liabilities due to cash collateral deposited with derivative counterparties	495.9		495.9			425
This amount represents additional cash collateral received from derivative counterparties not offset against derivative assets	152.8		152.8			1.6
This amount represents additional cash collateral deposited with derivative counterparties not offset against derivative liabilities	50.7		50.7			73.3
Securities posted as collateral					12.9
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position	104.3		104.3			178.9
Collateral deposited with derivative counterparties for derivatives instruments with credit-risk-related contingent features that are in a liability position	39.3		39.3			155.4
Termination payments that could have been required for derivative instruments with credit-risk-related contingent features	65		65			23.5
Percentage of derivatives outstanding related to client-related and trading activities									96.00%	96.00%
Sterling denominated debt designated as hedges of foreign exchange risk							249.9	242.3
Changes recorded in the fair value of the hedged items in a fair value hedge	-0.3	0.2	-0.9	0.5
The net gain/loss recognized in earnings during the period representing the amount of hedge ineffectiveness for fair value hedges of available for sale investment securities, senior notes, or subordinated debt	-0.3	-0.1	-0.8	0.4
Maximum length of time being hedged for exposure to variability in future cash flows of forecasted foreign currency denominated transactions			23 months
The net gain/loss recognized in earnings during the period representing the amount of hedge ineffectiveness cash flow hedges	0	0	0	0
Net foreign exchange contract losses reclassified from AOCI	0	0	0	0
Estimated net gain (loss) to be reclassified into earnings within the next twelve months relating to cash flow hedges			-1
Ineffectiveness from net investment hedges	$0	$0	$0	$0

Notional_and_Fair_Value_Amount

Notional and Fair Value Amounts of Client-related and Trading Derivative Financial Instruments (Detail) (Client Related and Trading, USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Derivative [Line Items]
Notional Value	$257,845.40	$218,193.30
Fair Value Asset	3,297	1,915.90
Fair Value Liability	3,293.90	1,904.40
Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	252,806.20	213,246.70
Fair Value Asset	3,162.10	1,735.30
Fair Value Liability	3,165.30	1,730.40
Interest Rate Contracts
Derivative [Line Items]
Notional Value	5,039.20	4,946.60
Fair Value Asset	134.9	180.6
Fair Value Liability	$128.60	$174

Gains_and_Losses_Recorded_in_C

Gains and Losses Recorded in Consolidated Statement of Income Related to Client Related and Trading Derivative Financial Instruments (Detail) (Client Related and Trading, USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$67.20	$47.70	$205	$174.20
Foreign Exchange Contracts | Foreign Exchange Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	62.8	44	193.6	165.3
Interest Rate Contracts | Security Commissions and Trading Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$4.40	$3.70	$11.40	$8.90

Types_and_Classifications_of_D

Types and Classifications of Derivative Instruments Designated as Hedges (Detail) (Asset And Liability Management, USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Derivative [Line Items]
Notional Value	$6,199.10	$6,637.40
Fair Value Asset	123	140.7
Fair Value Liability	80.2	114.6
FAIR VALUE HEDGES | Senior Notes and Long- Term Subordinated Debt | Interest Rate Swap Contracts
Derivative [Line Items]
Notional Value	500	900
Fair Value Asset	84.2	126.3
Fair Value Liability		0.2
FAIR VALUE HEDGES | Available for Sale Investment Securities | Interest Rate Swap Contracts
Derivative [Line Items]
Notional Value	3,646.90	3,617
Fair Value Asset	23.7	3.4
Fair Value Liability	44.4	75.1
CASH FLOW HEDGES | Forecasted Foreign Currency Denominated Transactions | Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	390.9	669
Fair Value Asset	8	8.7
Fair Value Liability	9	11.5
Net Investment Hedges | Net Investments in Non-U.S. Affiliates | Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	1,661.30	1,451.40
Fair Value Asset	7.1	2.3
Fair Value Liability	$26.80	$27.80

Gains_and_Losses_Recorded_in_C1

Gains and Losses Recorded in Consolidated Statement of Income Related to Fair Value Hedges (Detail) (Asset And Liability Management, FAIR VALUE HEDGES, USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	($20)	($14.40)	$5.50	$78.30
Available for Sale Investment Securities | Interest Rate Swap Contracts | Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	-16.4	-23.3	21.6	-93.5
Senior Notes and Long- Term Subordinated Debt | Interest Rate Swap Contracts | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	($3.60)	$8.90	($16.10)	$171.80

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		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Net Gain/(Loss) Recognized in AOCI	$5.30	$2	($4)	$4.70
Net Gain/(Loss) Reclassified from AOCI to Earnings	-1.1	-2.6	-5.7	-3.5
Foreign Exchange Contracts | Net Gain/(Loss) Reclassified from AOCI to Earnings
Derivative Instruments, Gain (Loss) [Line Items]
Other Operating Income		-1	-1.8	-3.3
Other Operating Expense	-1.1	-1.6	-3.9	-0.2
Interest Rate Swap Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Net Gain/(Loss) Reclassified from AOCI to Earnings				-0.2
Interest Rate Swap Contracts | Net Gain/(Loss) Reclassified from AOCI to Earnings
Derivative Instruments, Gain (Loss) [Line Items]
Interest Income				($0.20)

Net_Investment_Hedge_Gains_and

Net Investment Hedge Gains and Losses Recognized in AOCI (Detail) (Net Investment Hedges, USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative [Line Items]
Hedging Instrument Gain/(Loss) Recognized in AOCI (Before Tax)	($88.80)	($44.80)	($9.30)	($17.40)
Foreign Exchange Contracts
Derivative [Line Items]
Hedging Instrument Gain/(Loss) Recognized in AOCI (Before Tax)	-73.8	-35.4	-9.5	-8.1
Sterling Denominated Subordinated Debt
Derivative [Line Items]
Hedging Instrument Gain/(Loss) Recognized in AOCI (Before Tax)	($15)	($9.40)	$0.20	($9.30)

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, USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Derivative [Line Items]
Notional Value	$144	$1,232.30
Fair Value Asset	1.9	10.3
Fair Value Liability	0.9	4
Credit Default Swap Contracts
Derivative [Line Items]
Notional Value		42.5
Fair Value Liability		1
Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	144	1,189.80
Fair Value Asset	1.9	10.3
Fair Value Liability	$0.90	$3

Gains_and_Losses_Recorded_in_C2

Gains and Losses Recorded in Consolidated Statement of Income related to Derivative Instruments Not Formally Designated as Hedges (Detail) (Nondesignated, USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$6.70	$4.40	($4.40)	$1.30
Credit Default Swap Contracts | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income		-0.6	-0.1	-2.5
Foreign Exchange Contracts | Others Operating Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain/(Loss) Recognized in Income	$6.70	$5	($4.30)	$3.80

Offsetting_of_Derivative_Asset

Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheet (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Derivatives
Derivative [Line Items]
Gross Recognized Assets	$3,421.90	[1]	$2,066.90	[2]
Gross Amounts Offset	2,018.30	[1]	1,101.10	[2]
Net Amount Presented	1,403.60	[1]	965.8	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	1,403.60	[1],[3]	965.8	[2],[4]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	3,122.90	[1]
Gross Amounts Offset	2,018.30	[1]
Net Amount Presented	1,104.60	[1]
Gross Amounts Not Offset		[1]
Net Amount	1,104.60	[1],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Assets	299	[1]
Gross Amounts Offset		[1]
Net Amount Presented	299	[1]
Gross Amounts Not Offset		[1]
Net Amount	299	[1],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	2,880.10	[1]	1,756.60	[2]
Gross Amounts Offset	1,593.70	[1]	877.1	[2]
Net Amount Presented	1,286.40	[1]	879.5	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	1,286.40	[1],[3]	879.5	[2],[4]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets	237.6	[1]	310.3	[2]
Gross Amounts Offset	46.8	[1]	68.3	[2]
Net Amount Presented	190.8	[1]	242	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	190.8	[1],[3]	242	[2],[4]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Assets	5.2	[1]
Gross Amounts Offset	4.3	[1]
Net Amount Presented	0.9	[1]
Gross Amounts Not Offset		[1]
Net Amount	0.9	[1],[3]
Derivatives | Interest Rate Option | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Assets		[1]
Gross Amounts Offset		[1]
Net Amount Presented		[1]
Gross Amounts Not Offset		[1]
Net Amount		[1],[3]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Assets		[1]		[2]
Gross Amounts Offset	73	[1]	37.1	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]		[2],[4]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Recognized Assets		[1]		[2]
Gross Amounts Offset	300.5	[1]	118.6	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]		[2],[4]
Securities Purchased under Agreements to Resell
Derivative [Line Items]
Gross Recognized Assets	500	[5]	35.4	[6]
Gross Amounts Offset		[5]		[6]
Net Amount Presented	500	[5]	35.4	[6]
Gross Amounts Not Offset	500	[5]	35.4	[6]
Net Amount		[3],[5]		[4],[6]
[1]	Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $3,549.3 million as of September 30, 2013.
[2]	Derivative assets are reported in other assets in the consolidated balance sheet. Other assets (excluding derivative assets) totaled $2,964.4 million as of December 31, 2012.
[3]	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of September 30, 2013.
[4]	Northern Trust did not possess any cash collateral that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of December 31, 2012.
[5]	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheet. Federal funds sold totaled $34.6 million as of September 30, 2013.
[6]	Securities purchased under agreements to resell are reported in federal funds sold and securities purchased under agreements to resell in the consolidated balance sheet. Federal funds sold totaled $25.4 million as of December 31, 2012.

Offsetting_of_Derivative_Asset1

Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell Within Consolidated Balance Sheet (Parenthetical) (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Derivative [Line Items]
Other Assets (excluding Derivative Assets)	$3,549.30	$2,964.40
Federal Funds Sold	$34.60	$25.40

Offsetting_of_Derivative_Liabi

Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Derivatives
Derivative [Line Items]
Gross Recognized Liabilities	$3,375	[1]	$2,023	[2]
Gross Amounts Offset	2,213.70	[1]	1,407.50	[2]
Net Amount Presented	1,161.30	[1]	615.5	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	1,161.30	[1],[3]	615.5	[2],[4]
Derivatives | Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	2,723.30	[1]
Gross Amounts Offset	2,213.70	[1]
Net Amount Presented	509.6	[1]
Gross Amounts Not Offset		[1]
Net Amount	509.6	[1],[3]
Derivatives | Not Subject to Master Netting Arrangement
Derivative [Line Items]
Gross Recognized Liabilities	651.7	[1]
Gross Amounts Offset		[1]
Net Amount Presented	651.7	[1]
Gross Amounts Not Offset		[1]
Net Amount	651.7	[1],[3]
Derivatives | Foreign Exchange Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	2,550.30	[1]	1,772.70	[2]
Gross Amounts Offset	1,593.70	[1]	877.1	[2]
Net Amount Presented	956.6	[1]	895.6	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	956.6	[1],[3]	895.6	[2],[4]
Derivatives | Interest Rate Swaps | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities	168.7	[1]	249.3	[2]
Gross Amounts Offset	46.8	[1]	68.3	[2]
Net Amount Presented	121.9	[1]	181	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount	121.9	[1],[3]	181	[2],[4]
Derivatives | Interest Rate Swaps | Exchange Cleared
Derivative [Line Items]
Gross Recognized Liabilities	4.3	[1]
Gross Amounts Offset	4.3	[1]
Net Amount Presented		[1]
Gross Amounts Not Offset		[1]
Net Amount		[1],[3]
Derivatives | Interest Rate Option | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities		[1]
Gross Amounts Offset		[1]
Net Amount Presented		[1]
Gross Amounts Not Offset		[1]
Net Amount		[1],[3]
Derivatives | Credit Default Swap Contracts | Over the Counter Derivative
Derivative [Line Items]
Gross Recognized Liabilities		[1]	1	[2]
Gross Amounts Offset		[1]		[2]
Net Amount Presented		[1]	1	[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]	1	[2],[4]
Derivatives | Cross Product Netting Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[1]		[2]
Gross Amounts Offset	73	[1]	37.1	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]		[2],[4]
Derivatives | Cross Product Collateral Adjustment
Derivative [Line Items]
Gross Recognized Liabilities		[1]		[2]
Gross Amounts Offset	495.9	[1]	425	[2]
Net Amount Presented		[1]		[2]
Gross Amounts Not Offset		[1]		[2]
Net Amount		[1],[3]		[2],[4]
Securities Sold under Agreements to Repurchase
Derivative [Line Items]
Gross Recognized Liabilities	469		699.8
Gross Amounts Offset
Net Amount Presented	469		699.8
Gross Amounts Not Offset	469		699.8
Net Amount		[3]		[4]
[1]	Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $2,235.0 million as of September 30, 2013.
[2]	Derivative liabilities are reported in other liabilities in the consolidated balance sheet. Other liabilities (excluding derivative liabilities) totaled $1,961.7 million as of December 31, 2012.
[3]	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of September 30, 2013.
[4]	Northern Trust did not place any cash collateral with counterparties that was not offset in the consolidated balance sheet that could have been used to offset the net amounts presented in the consolidated balance sheet as of December 31, 2012.

Offsetting_of_Derivative_Liabi1

Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase Within Consolidated Balance Sheet (Parenthetical) (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Derivative [Line Items]
Other liabilities	$2,235	$1,961.70