Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
STANDARDIZED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 10,822.2	$ 10,152.0
RATIO	0.139	0.145
Minimum to qualify as well-capitalized balance	$ 4,659.7	$ 4,205.3
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 12,085.7	$ 11,456.7
RATIO	0.156	0.163
Minimum to qualify as well-capitalized balance	$ 7,766.2	$ 7,008.8
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,822.2	$ 10,152.0
RATIO	0.076	0.087
STANDARDIZED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 9,962.2	$ 8,898.7
RATIO	0.128	0.127
STANDARDIZED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,003.3	$ 8,476.0
RATIO	0.130	0.123
Minimum to qualify as well-capitalized balance	$ 6,147.0	$ 5,504.0
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 11,123.1	$ 9,610.4
RATIO	0.145	0.140
Minimum to qualify as well-capitalized balance	$ 7,683.7	$ 6,880.1
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,003.3	$ 8,476.0
RATIO	0.070	0.073
Minimum to qualify as well-capitalized balance	$ 7,105.0	$ 5,835.4
Minimum to qualify as well-capitalized ratio	0.050	0.050
STANDARDIZED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,003.3	$ 8,476.0
RATIO	0.130	0.123
Minimum to qualify as well-capitalized balance	$ 4,994.4	$ 4,472.0
Minimum to qualify as well-capitalized ratio	0.065	0.065
ADVANCED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 10,822.2	$ 10,152.0
RATIO	0.145	0.150
Minimum to qualify as well-capitalized balance	$ 4,467.6	$ 4,051.6
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 11,825.8	$ 11,332.3
RATIO	0.159	0.168
Minimum to qualify as well-capitalized balance	$ 7,446.0	$ 6,752.7
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,822.2	$ 10,152.0
RATIO	0.076	0.087
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 10,822.2	$ 10,152.0
Supplementary leverage capital to average assets	8.60%	7.60%
ADVANCED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 9,962.2	$ 8,898.7
RATIO	0.134	0.132
ADVANCED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,003.3	$ 8,476.0
RATIO	0.138	0.130
Minimum to qualify as well-capitalized balance	$ 5,805.6	$ 5,206.2
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 10,863.3	$ 9,486.0
RATIO	0.150	0.146
Minimum to qualify as well-capitalized balance	$ 7,257.0	$ 6,507.7
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,003.3	$ 8,476.0
RATIO	0.070	0.073
Minimum to qualify as well-capitalized balance	$ 7,105.0	$ 5,835.4
Minimum to qualify as well-capitalized ratio	0.050	0.050
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 10,003.3	$ 8,476.0
Supplementary leverage capital to average assets	7.70%	6.40%
Supplementary leverage capital required to be well capitalized	$ 3,883.4	$ 3,983.6
Supplementary leverage ratio	0.030	0.030
ADVANCED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,003.3	$ 8,476.0
RATIO	0.138	0.130
Minimum to qualify as well-capitalized balance	$ 4,717.1	$ 4,230.0
Minimum to qualify as well-capitalized ratio	0.065	0.065