Risk-Based Capital Amounts and Ratios for Northern Trust on Consolidated Basis and for Bank (Detail) - USD ($) $ in Millions	Jan. 01, 2018	Dec. 31, 2015	Dec. 31, 2014
Advanced Approach | Parent Company
Tier 1 Capital
Balance		$ 8,431.3	$ 8,318.0
Ratio		12.50%	13.20%
Minimum to qualify as well-capitalized balance		$ 5,386.8	$ 3,773.8
Minimum to qualify as well-capitalized ratio		8.00%	6.00%
Total Capital
Balance		$ 9,529.7	$ 9,449.2
Ratio		14.20%	15.00%
Minimum to qualify as well-capitalized balance		$ 6,733.5	$ 6,289.7
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 8,431.3
Ratio		7.50%
Minimum to qualify as well-capitalized balance		$ 5,655.0
Minimum to qualify as well-capitalized ratio		5.00%
Supplementary Leverage
Balance		$ 8,431.3
Ratio		6.20%
Minimum to qualify as well-capitalized balance		$ 4,085.0
Advanced Approach | Parent Company | COMMON STOCK
Tier 1 Capital
Balance		$ 8,012.6	$ 7,813.3
Ratio		11.90%	12.40%
Minimum to qualify as well-capitalized balance		$ 4,376.7
Minimum to qualify as well-capitalized ratio		6.50%
Advanced Approach | The Northern Trust Company
Tier 1 Capital
Balance		$ 7,591.7	$ 7,327.3
Ratio		11.60%	12.00%
Minimum to qualify as well-capitalized balance		$ 5,225.2	$ 3,665.8
Minimum to qualify as well-capitalized ratio		8.00%	6.00%
Total Capital
Balance		$ 8,538.1	$ 8,420.4
Ratio		13.10%	13.80%
Minimum to qualify as well-capitalized balance		$ 6,531.5	$ 6,109.7
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 7,591.7
Ratio		6.70%
Minimum to qualify as well-capitalized balance		$ 5,639.0
Minimum to qualify as well-capitalized ratio		5.00%
Supplementary Leverage
Balance		$ 7,591.7
Ratio		5.60%
Minimum to qualify as well-capitalized balance		$ 4,075.3
Advanced Approach | The Northern Trust Company | COMMON STOCK
Tier 1 Capital
Balance		$ 7,591.7	$ 7,327.3
Ratio		11.60%	12.00%
Minimum to qualify as well-capitalized balance		$ 4,245.5
Minimum to qualify as well-capitalized ratio		6.50%
Standardized Approach | Parent Company
Tier 1 Capital
Balance		$ 8,431.3	$ 8,317.6
Ratio		11.40%	13.30%
Minimum to qualify as well-capitalized balance		$ 5,917.0	$ 3,759.1
Minimum to qualify as well-capitalized ratio		8.00%	6.00%
Total Capital
Balance		$ 9,767.7	$ 9,723.0
Ratio		13.20%	15.50%
Minimum to qualify as well-capitalized balance		$ 7,396.3	$ 6,265.1
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 8,431.3	$ 8,317.6
Ratio		7.50%	7.80%
Minimum to qualify as well-capitalized balance		$ 5,655.0	$ 5,340.7
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Standardized Approach | Parent Company | COMMON STOCK
Tier 1 Capital
Balance		$ 8,012.6	$ 7,813.3
Ratio		10.80%	12.50%
Minimum to qualify as well-capitalized balance		$ 4,807.6
Minimum to qualify as well-capitalized ratio		6.50%
Standardized Approach | The Northern Trust Company
Tier 1 Capital
Balance		$ 7,591.7	$ 7,327.3
Ratio		10.40%	11.80%
Minimum to qualify as well-capitalized balance		$ 5,853.4	$ 3,738.0
Minimum to qualify as well-capitalized ratio		8.00%	6.00%
Total Capital
Balance		$ 8,771.4	$ 8,695.1
Ratio		12.00%	14.00%
Minimum to qualify as well-capitalized balance		$ 7,316.8	$ 6,229.9
Minimum to qualify as well-capitalized ratio		10.00%	10.00%
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
Balance		$ 7,591.7	$ 7,327.3
Ratio		6.70%	6.90%
Minimum to qualify as well-capitalized balance		$ 5,639.0	$ 5,324.9
Minimum to qualify as well-capitalized ratio		5.00%	5.00%
Standardized Approach | The Northern Trust Company | COMMON STOCK
Tier 1 Capital
Balance		$ 7,591.7	$ 7,327.3
Ratio		10.40%	11.80%
Minimum to qualify as well-capitalized balance		$ 4,755.9
Minimum to qualify as well-capitalized ratio		6.50%
Forecast
Supplementary Leverage
Minimum to qualify as well-capitalized ratio	3.00%