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Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Supplementary Leverage Capital [Abstract]    
Supplementary leverage, risk based capital required to be well capitalized to risk weighted assets 3.00%  
ADVANCED APPROACH | Parent Company    
Tier 1 Capital    
BALANCE $ 9,596.7 $ 9,473.4
RATIO 15.00% 14.80%
Minimum to qualify as well-capitalized balance $ 3,834.9 $ 3,841.1
Minimum to qualify as well-capitalized ratio 6.00% 6.00%
Total Capital    
BALANCE $ 10,803.8 $ 10,707.4
RATIO 16.90% 16.70%
Minimum to qualify as well-capitalized balance $ 6,391.5 $ 6,401.9
Minimum to qualify as well-capitalized ratio 10.00% 10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
BALANCE $ 9,596.7 $ 9,473.4
RATIO 8.00% 7.80%
Supplementary Leverage Capital [Abstract]    
Supplementary leverage capital $ 9,596.7 $ 9,473.4
Supplementary leverage capital to average assets 7.00% 6.80%
ADVANCED APPROACH | Parent Company | COMMON STOCK    
Tier 1 Capital    
BALANCE $ 8,729.8 $ 8,626.3
RATIO 13.70% 13.50%
ADVANCED APPROACH | The Northern Trust Company    
Tier 1 Capital    
BALANCE $ 8,722.5 $ 8,517.8
RATIO 14.10% 13.70%
Minimum to qualify as well-capitalized balance $ 4,932.2 $ 4,963.3
Minimum to qualify as well-capitalized ratio 8.00% 8.00%
Total Capital    
BALANCE $ 9,732.5 $ 9,527.8
RATIO 15.80% 15.40%
Minimum to qualify as well-capitalized balance $ 6,165.3 $ 6,204.2
Minimum to qualify as well-capitalized ratio 10.00% 10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
BALANCE $ 8,722.5 $ 8,517.8
RATIO 7.30% 7.00%
Minimum to qualify as well-capitalized balance $ 5,998.6 $ 6,057.9
Minimum to qualify as well-capitalized ratio 5.00% 5.00%
Supplementary Leverage Capital [Abstract]    
Supplementary leverage capital $ 8,722.5 $ 8,517.8
Supplementary leverage capital to average assets 6.40% 6.10%
Supplementary leverage capital required to be well capitalized $ 4,077.2  
Supplementary leverage capital required for capital adequacy to average assets 3.00%  
ADVANCED APPROACH | The Northern Trust Company | COMMON STOCK    
Tier 1 Capital    
BALANCE $ 8,722.5 $ 8,517.8
RATIO 14.10% 13.70%
Minimum to qualify as well-capitalized balance $ 4,007.4 $ 4,032.7
Minimum to qualify as well-capitalized ratio 6.50% 6.50%
Standardized Approach | Parent Company    
Tier 1 Capital    
BALANCE $ 9,596.7 $ 9,473.4
RATIO 14.10% 13.80%
Minimum to qualify as well-capitalized balance $ 4,070.0 $ 4,117.0
Minimum to qualify as well-capitalized ratio 6.00% 6.00%
Total Capital    
BALANCE $ 10,942.0 $ 10,861.2
RATIO 16.10% 15.80%
Minimum to qualify as well-capitalized balance $ 6,783.7 $ 6,861.6
Minimum to qualify as well-capitalized ratio 10.00% 10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
BALANCE $ 9,596.7 $ 9,473.4
RATIO 8.00% 7.80%
Standardized Approach | Parent Company | COMMON STOCK    
Tier 1 Capital    
BALANCE $ 8,729.8 $ 8,626.3
RATIO 12.90% 12.60%
Standardized Approach | The Northern Trust Company    
Tier 1 Capital    
BALANCE $ 8,722.5 $ 8,517.8
RATIO 13.10% 12.60%
Minimum to qualify as well-capitalized balance $ 5,336.4 $ 5,423.8
Minimum to qualify as well-capitalized ratio 8.00% 8.00%
Total Capital    
BALANCE $ 9,870.7 $ 9,681.6
RATIO 14.80% 14.30%
Minimum to qualify as well-capitalized balance $ 6,670.6 $ 6,779.7
Minimum to qualify as well-capitalized ratio 10.00% 10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)    
BALANCE $ 8,722.5 $ 8,517.8
RATIO 7.30% 7.00%
Minimum to qualify as well-capitalized balance $ 5,998.6 $ 6,057.9
Minimum to qualify as well-capitalized ratio 5.00% 5.00%
Standardized Approach | The Northern Trust Company | COMMON STOCK    
Tier 1 Capital    
BALANCE $ 8,722.5 $ 8,517.8
RATIO 13.10% 12.60%
Minimum to qualify as well-capitalized balance $ 4,335.9 $ 4,406.8
Minimum to qualify as well-capitalized ratio 6.50% 6.50%