Risk-Based Capital Amounts and Ratios for Northern Trust on Consolidated Basis and for Bank (Detail) - USD ($) $ in Millions	Jan. 01, 2018	Dec. 31, 2016	Dec. 31, 2015
Advanced Approach \| Parent Company
Tier 1 Capital
Balance		$ 9,319.9	$ 8,431.3
Ratio		13.70%	12.50%
Minimum to qualify as well-capitalized balance		$ 5,460.6	$ 5,386.8
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 10,281.6	$ 9,529.7
Ratio		15.10%	14.20%
Minimum to qualify as well-capitalized balance		$ 6,825.8	$ 6,733.5
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 9,319.9	$ 8,431.3
Ratio		8.00%	7.50%
Minimum to qualify as well-capitalized balance		$ 5,847.9	$ 5,655.0
Supplementary Leverage
Balance		$ 9,319.9	$ 8,431.3
Ratio		6.80%	6.20%
Minimum to qualify as well-capitalized balance		$ 4,129.9	$ 4,085.0
Advanced Approach \| Parent Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,480.4	$ 8,012.6
Ratio		12.40%	11.90%
Minimum to qualify as well-capitalized balance		$ 4,436.7	$ 4,376.7
Minimum to qualify as well-capitalized ratio		6.50%	6.50%
Advanced Approach \| The Northern Trust Company
Tier 1 Capital
Balance		$ 8,201.4	$ 7,591.7
Ratio		12.40%	11.60%
Minimum to qualify as well-capitalized balance		$ 5,287.6	$ 5,225.2
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 9,271.4	$ 8,538.1
Ratio		14.00%	13.10%
Minimum to qualify as well-capitalized balance		$ 6,609.5	$ 6,531.5
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 8,201.4	$ 7,591.7
Ratio		7.00%	6.70%
Minimum to qualify as well-capitalized balance		$ 5,831.0	$ 5,639.0
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Supplementary Leverage
Balance		$ 8,201.4	$ 7,591.7
Ratio		6.00%	5.60%
Minimum to qualify as well-capitalized balance		$ 4,120.0	$ 4,075.3
Advanced Approach \| The Northern Trust Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,201.4	$ 7,591.7
Ratio		12.40%	11.60%
Minimum to qualify as well-capitalized balance		$ 4,296.2	$ 4,245.5
Standardized Approach \| Parent Company
Tier 1 Capital
Balance		$ 9,319.9	$ 8,431.3
Ratio		12.90%	11.40%
Minimum to qualify as well-capitalized balance		$ 5,761.7	$ 5,917.0
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 10,475.0	$ 9,767.7
Ratio		14.50%	13.20%
Minimum to qualify as well-capitalized balance		$ 7,202.1	$ 7,396.3
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 9,319.9	$ 8,431.3
Ratio		8.00%	7.50%
Minimum to qualify as well-capitalized balance		$ 5,847.9	$ 5,655.0
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Standardized Approach \| Parent Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,480.4	$ 8,012.6
Ratio		11.80%	10.80%
Minimum to qualify as well-capitalized balance		$ 4,681.4	$ 4,807.6
Minimum to qualify as well-capitalized ratio		6.50%	6.50%
Standardized Approach \| The Northern Trust Company
Tier 1 Capital
Balance		$ 8,201.4	$ 7,591.7
Ratio		11.50%	10.40%
Minimum to qualify as well-capitalized balance		$ 5,692.1	$ 5,853.4
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 9,463.4	$ 8,771.4
Ratio		13.30%	12.00%
Minimum to qualify as well-capitalized balance		$ 7,115.1	$ 7,316.8
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 8,201.4	$ 7,591.7
Ratio		7.00%	6.70%
Minimum to qualify as well-capitalized balance		$ 5,831.0	$ 5,639.0
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Standardized Approach \| The Northern Trust Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,201.4	$ 7,591.7
Ratio		11.50%	10.40%
Minimum to qualify as well-capitalized balance		$ 4,624.8	$ 4,755.9
Forecast
Supplementary Leverage
Minimum to qualify as well-capitalized ratio	3.00%

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Supplementary Leverage, Risk Based Capital

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Supplementary Leverage, Risk Based Capital Required To Be Well Capitalized

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Supplementary Leverage, Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets

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Supplementary Leverage, Risk Based Capital to Risk Weighted Assets

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The amount of total capital as defined in the regulations.

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The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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