Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) $ in Millions	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
STANDARDIZED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 11,870.2	$ 11,023.9
RATIO	0.133	0.123
Minimum to qualify as well-capitalized balance	$ 5,336.4	$ 5,371.6
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 13,423.2	$ 12,733.9
RATIO	0.151	0.142
Minimum to qualify as well-capitalized balance	$ 8,894.0	$ 8,952.7
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 11,870.2	$ 11,023.9
RATIO	0.081	0.081
STANDARDIZED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 11,038.2	$ 10,179.4
RATIO	0.124	0.114
STANDARDIZED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 9,983.8	$ 10,806.2
RATIO	0.114	0.122
Minimum to qualify as well-capitalized balance	$ 7,019.4	$ 7,099.5
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 11,241.7	$ 12,279.3
RATIO	0.128	0.138
Minimum to qualify as well-capitalized balance	$ 8,774.2	$ 8,874.4
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 9,983.8	$ 10,806.2
RATIO	0.069	0.080
Minimum to qualify as well-capitalized balance	$ 7,262.3	$ 6,764.0
Minimum to qualify as well-capitalized ratio	0.050	0.050
STANDARDIZED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 9,983.8	$ 10,806.2
RATIO	0.114	0.122
Minimum to qualify as well-capitalized balance	$ 5,703.2	$ 5,768.4
Minimum to qualify as well-capitalized ratio	0.065	0.065
ADVANCED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 11,870.2	$ 11,023.9
RATIO	0.156	0.145
Minimum to qualify as well-capitalized balance	$ 4,555.3	$ 4,558.8
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 13,217.3	$ 12,514.7
RATIO	0.174	0.165
Minimum to qualify as well-capitalized balance	$ 7,592.1	$ 7,598.0
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 11,870.2	$ 11,023.9
RATIO	0.081	0.081
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 11,870.2	$ 11,023.9
Supplementary leverage capital to average assets	8.90%	8.60%
ADVANCED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 11,038.2	$ 10,179.4
RATIO	0.145	0.134
ADVANCED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 9,983.8	$ 10,806.2
RATIO	0.136	0.146
Minimum to qualify as well-capitalized balance	$ 5,894.2	$ 5,918.3
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 11,035.8	$ 12,060.2
RATIO	0.150	0.163
Minimum to qualify as well-capitalized balance	$ 7,367.8	$ 7,397.9
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 9,983.8	$ 10,806.2
RATIO	0.069	0.080
Minimum to qualify as well-capitalized balance	$ 7,262.3	$ 6,764.0
Minimum to qualify as well-capitalized ratio	0.050	0.050
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 9,983.8	$ 10,806.2
Supplementary leverage capital to average assets	7.50%	8.50%
Supplementary leverage capital required to be well capitalized	$ 3,974.3	$ 3,825.6
Supplementary leverage ratio	0.030	0.030
ADVANCED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 9,983.8	$ 10,806.2
RATIO	0.136	0.146
Minimum to qualify as well-capitalized balance	$ 4,789.1	$ 4,808.6
Minimum to qualify as well-capitalized ratio	0.065	0.065