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Risk-Based Capital Amounts and Ratios for Northern Trust on Consolidated Basis and for Bank (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2014
Parent Company    
Tier 1 Capital    
Balance $ 7,853.2us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Ratio 13.40%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Minimum to qualify as well-capitalized balance 3,526.4us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Minimum to qualify as well-capitalized ratio 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Total Capital    
Balance 9,294.9us-gaap_Capital
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Ratio 15.80%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Minimum to qualify as well-capitalized balance 5,877.4us-gaap_CapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Minimum to qualify as well-capitalized ratio 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
Balance 7,853.2us-gaap_TierOneLeverageCapital
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Ratio 7.90%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Minimum to qualify as well-capitalized balance 4,953.7us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Minimum to qualify as well-capitalized ratio 5.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [1]  
Parent Company | COMMON STOCK    
Tier 1 Capital    
Balance 7,853.2us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [1]  
Ratio 12.90%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [1]  
The Northern Trust Company    
Tier 1 Capital    
Balance 6,765.6us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Ratio 11.50%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Minimum to qualify as well-capitalized balance 3,515.2us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Minimum to qualify as well-capitalized ratio 6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Total Capital    
Balance 8,366.2us-gaap_Capital
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Ratio 14.30%us-gaap_CapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Minimum to qualify as well-capitalized balance 5,858.8us-gaap_CapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Minimum to qualify as well-capitalized ratio 10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
Balance 6,765.6us-gaap_TierOneLeverageCapital
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Ratio 6.80%us-gaap_TierOneLeverageCapitalToAverageAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Minimum to qualify as well-capitalized balance 4,939.9us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
Minimum to qualify as well-capitalized ratio 5.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [1]  
The Northern Trust Company | COMMON STOCK    
Tier 1 Capital    
Balance 6,765.6us-gaap_TierOneRiskBasedCapital
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [1]  
Ratio 11.50%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [1]  
Advanced Approach | Parent Company    
Tier 1 Capital    
Balance   8,318.0us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Ratio   13.20%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Minimum to qualify as well-capitalized balance   3,773.8us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Minimum to qualify as well-capitalized ratio   6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Total Capital    
Balance   9,449.2us-gaap_Capital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Ratio   15.00%us-gaap_CapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Minimum to qualify as well-capitalized balance   6,289.7us-gaap_CapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Minimum to qualify as well-capitalized ratio   10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [2]
Advanced Approach | Parent Company | COMMON STOCK    
Tier 1 Capital    
Balance   7,813.3us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2]
Ratio   12.40%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2]
Advanced Approach | The Northern Trust Company    
Tier 1 Capital    
Balance   7,327.3us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Ratio   12.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Minimum to qualify as well-capitalized balance   3,665.8us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Minimum to qualify as well-capitalized ratio   6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Total Capital    
Balance   8,420.4us-gaap_Capital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Ratio   13.80%us-gaap_CapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Minimum to qualify as well-capitalized balance   6,109.7us-gaap_CapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Minimum to qualify as well-capitalized ratio   10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [2]
Advanced Approach | The Northern Trust Company | COMMON STOCK    
Tier 1 Capital    
Balance   7,327.3us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2]
Ratio   12.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_AdvancedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [2]
Standardized Approach | Parent Company    
Tier 1 Capital    
Balance   8,317.6us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Ratio   13.30%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Minimum to qualify as well-capitalized balance   3,759.1us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Minimum to qualify as well-capitalized ratio   6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Total Capital    
Balance   9,723.0us-gaap_Capital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Ratio   15.50%us-gaap_CapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Minimum to qualify as well-capitalized balance   6,265.1us-gaap_CapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Minimum to qualify as well-capitalized ratio   10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
Balance   8,317.6us-gaap_TierOneLeverageCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Ratio   7.80%us-gaap_TierOneLeverageCapitalToAverageAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Minimum to qualify as well-capitalized balance   5,340.7us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Minimum to qualify as well-capitalized ratio   5.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember		 [3]
Standardized Approach | Parent Company | COMMON STOCK    
Tier 1 Capital    
Balance   7,813.3us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [3]
Ratio   12.50%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [3]
Standardized Approach | The Northern Trust Company    
Tier 1 Capital    
Balance   7,327.3us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Ratio   11.80%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Minimum to qualify as well-capitalized balance   3,738.0us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Minimum to qualify as well-capitalized ratio   6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Total Capital    
Balance   8,695.1us-gaap_Capital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Ratio   14.00%us-gaap_CapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Minimum to qualify as well-capitalized balance   6,229.9us-gaap_CapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Minimum to qualify as well-capitalized ratio   10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
Balance   7,327.3us-gaap_TierOneLeverageCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Ratio   6.90%us-gaap_TierOneLeverageCapitalToAverageAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Minimum to qualify as well-capitalized balance   5,324.9us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Minimum to qualify as well-capitalized ratio   5.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember		 [3]
Standardized Approach | The Northern Trust Company | COMMON STOCK    
Tier 1 Capital    
Balance   $ 7,327.3us-gaap_TierOneRiskBasedCapital
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [3]
Ratio   11.80%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntrs_StandardizedApproachMember
/ dei_LegalEntityAxis
= ntrs_WhollyOwnedSubsidiaryMember
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember		 [3]
[1] The December 31, 2013, capital balances and ratios were calculated in accordance with Basel I requirements.
[2] Effective with the second quarter of 2014, Northern Trust exited its parallel run. Accordingly, the December 31, 2014, capital balances and ratios are calculated in compliance with the Basel III Advanced Approach final rules released by the Federal Reserve Board on July 2, 2013.
[3] Standardized Approach capital components in 2014 are determined by Basel III phased-in requirements and risk-weighted assets are determined by Basel I requirements. The December 31, 2014, ratios calculated under the Standardized Approach comply with the final rules released by the Federal Reserve Board on July 2, 2013.