Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Amortized Cost	$ 33,869.0	$ 35,631.9
Gross unrealized, gains	79.1	78.7
Gross unrealized, losses	103.2	130.8
Fair Value	33,844.9	35,579.8
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	5,731.6	7,514.5
Gross unrealized, gains	24.2	22.4
Gross unrealized, losses	11.4	14.3
Fair Value	5,744.4	7,522.6
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	766.3	890.8
Gross unrealized, gains	0.3	0.0
Gross unrealized, losses	0.7	5.6
Fair Value	765.9	885.2
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	18,101.4	17,914.1
Gross unrealized, gains	43.1	49.3
Gross unrealized, losses	61.7	70.6
Fair Value	18,082.8	17,892.8
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	139.9	420.0
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	1.2	2.1
Fair Value	138.7	417.9
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,194.3	3,787.4
Gross unrealized, gains	3.6	2.6
Gross unrealized, losses	16.6	24.8
Fair Value	3,181.3	3,765.2
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	833.5	1,148.6
Gross unrealized, gains	1.5	0.8
Gross unrealized, losses	2.9	5.5
Fair Value	832.1	1,143.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	2,202.6	1,343.6
Gross unrealized, gains	4.0	0.9
Gross unrealized, losses	3.8	3.8
Fair Value	2,202.8	1,340.7
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,394.0	2,083.7
Gross unrealized, gains	2.4	2.7
Gross unrealized, losses	2.1	1.3
Fair Value	2,394.3	2,085.1
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	4.4	5.0
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.3	0.3
Fair Value	4.1	4.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	469.0	474.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	2.5	2.5
Fair Value	466.5	471.6
Other
Investment Holdings [Line Items]
Amortized Cost	32.0	50.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.0	0.0
Fair Value	$ 32.0	$ 50.1

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_InvestmentHoldingsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ntrs_CoveredBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ntrs_SubSovereignSupranationalAndNonUsAgencyBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ntrs_OtherAssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AuctionRateSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ntrs_OtherSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: