Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) - USD ($) $ in Millions	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
ADVANCED APPROACH \| Parent Company
Tier 1 Capital
Balance		$ 9,473.4	$ 9,319.9
Ratio		14.80%	13.70%
Minimum to qualify as well-capitalized balance		$ 5,121.5	$ 5,460.6
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 10,707.4	$ 10,281.6
Ratio		16.70%	15.10%
Minimum to qualify as well-capitalized balance		$ 6,401.9	$ 6,825.8
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 9,473.4	$ 9,319.9
Ratio		7.80%	8.00%
Minimum to qualify as well-capitalized balance		$ 6,075.9	$ 5,847.9
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Supplementary Leverage
Balance		$ 9,473.4	$ 9,319.9
Ratio		6.80%	6.80%
Minimum to qualify as well-capitalized balance		$ 4,175.7	$ 4,129.9
ADVANCED APPROACH \| Parent Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,626.3	$ 8,480.4
Ratio		13.50%	12.40%
Minimum to qualify as well-capitalized balance		$ 4,161.2	$ 4,436.7
Minimum to qualify as well-capitalized ratio		6.50%	6.50%
ADVANCED APPROACH \| The Northern Trust Company
Tier 1 Capital
Balance		$ 8,517.8	$ 8,201.4
Ratio		13.70%	12.40%
Minimum to qualify as well-capitalized balance		$ 4,963.3	$ 5,287.6
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 9,527.8	$ 9,271.4
Ratio		15.40%	14.00%
Minimum to qualify as well-capitalized balance		$ 6,204.2	$ 6,609.5
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 8,517.8	$ 8,201.4
Ratio		7.00%	7.00%
Minimum to qualify as well-capitalized balance		$ 6,057.9	$ 5,831.0
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Supplementary Leverage
Balance		$ 8,517.8	$ 8,201.4
Ratio		6.10%	6.00%
Minimum to qualify as well-capitalized balance		$ 4,164.7	$ 4,120.0
ADVANCED APPROACH \| The Northern Trust Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,517.8	$ 8,201.4
Ratio		13.70%	12.40%
Minimum to qualify as well-capitalized balance		$ 4,032.7	$ 4,296.2
Minimum to qualify as well-capitalized ratio		6.50%	6.50%
Standardized Approach \| Parent Company
Tier 1 Capital
Balance		$ 9,473.4	$ 9,319.9
Ratio		13.80%	12.90%
Minimum to qualify as well-capitalized balance		$ 5,489.3	$ 5,761.7
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 10,861.2	$ 10,475.0
Ratio		15.80%	14.50%
Minimum to qualify as well-capitalized balance		$ 6,861.6	$ 7,202.1
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 9,473.4	$ 9,319.9
Ratio		7.80%	8.00%
Minimum to qualify as well-capitalized balance		$ 6,075.9	$ 5,847.9
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Standardized Approach \| Parent Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,626.3	$ 8,480.4
Ratio		12.60%	11.80%
Minimum to qualify as well-capitalized balance		$ 4,460.1	$ 4,681.4
Minimum to qualify as well-capitalized ratio		6.50%	6.50%
Standardized Approach \| The Northern Trust Company
Tier 1 Capital
Balance		$ 8,517.8	$ 8,201.4
Ratio		12.60%	11.50%
Minimum to qualify as well-capitalized balance		$ 5,423.8	$ 5,692.1
Minimum to qualify as well-capitalized ratio		8.00%	8.00%
Total Capital
Balance		$ 9,681.6	$ 9,463.4
Ratio		14.30%	13.30%
Minimum to qualify as well-capitalized balance		$ 6,779.7	$ 7,115.1
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 8,517.8	$ 8,201.4
Ratio		7.00%	7.00%
Minimum to qualify as well-capitalized balance		$ 6,057.9	$ 5,831.0
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Standardized Approach \| The Northern Trust Company \| COMMON STOCK
Tier 1 Capital
Balance		$ 8,517.8	$ 8,201.4
Ratio		12.60%	11.50%
Minimum to qualify as well-capitalized balance		$ 4,406.8	$ 4,624.8
Minimum to qualify as well-capitalized ratio		6.50%	6.50%
Forecast
Supplementary Leverage
Minimum to qualify as well-capitalized ratio	3.00%

Supplementary Leverage [Abstract]

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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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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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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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The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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

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