Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
STANDARDIZED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 11,023.9	$ 10,397.4
RATIO	0.123	0.118
Minimum to qualify as well-capitalized balance	$ 5,371.6	$ 5,286.5
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 12,733.9	$ 12,245.5
RATIO	0.142	0.139
Minimum to qualify as well-capitalized balance	$ 8,952.7	$ 8,810.8
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 11,023.9	$ 10,397.4
RATIO	0.081	0.071
STANDARDIZED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,179.4	$ 9,539.7
RATIO	0.114	0.108
STANDARDIZED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,806.2	$ 10,111.2
RATIO	0.122	0.116
Minimum to qualify as well-capitalized balance	$ 7,099.5	$ 6,996.2
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 12,279.3	$ 11,766.8
RATIO	0.138	0.135
Minimum to qualify as well-capitalized balance	$ 8,874.4	$ 8,745.3
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,806.2	$ 10,111.2
RATIO	0.080	0.069
Minimum to qualify as well-capitalized balance	$ 6,764.0	$ 7,323.8
Minimum to qualify as well-capitalized ratio	0.050	0.050
STANDARDIZED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,806.2	$ 10,111.2
RATIO	0.122	0.116
Minimum to qualify as well-capitalized balance	$ 5,768.4	$ 5,684.4
Minimum to qualify as well-capitalized ratio	0.065	0.065
ADVANCED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 11,023.9	$ 10,397.4
RATIO	0.145	0.125
Minimum to qualify as well-capitalized balance	$ 4,558.8	$ 4,990.9
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 12,514.7	$ 12,045.9
RATIO	0.165	0.145
Minimum to qualify as well-capitalized balance	$ 7,598.0	$ 8,318.1
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 11,023.9	$ 10,397.4
RATIO	0.081	0.071
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 11,023.9	$ 10,397.4
Supplementary leverage capital to average assets	8.60%	7.90%
ADVANCED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,179.4	$ 9,539.7
RATIO	0.134	0.115
ADVANCED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,806.2	$ 10,111.2
RATIO	0.146	0.124
Minimum to qualify as well-capitalized balance	$ 5,918.3	$ 6,506.1
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 12,060.2	$ 11,567.2
RATIO	0.163	0.142
Minimum to qualify as well-capitalized balance	$ 7,397.9	$ 8,132.6
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,806.2	$ 10,111.2
RATIO	0.080	0.069
Minimum to qualify as well-capitalized balance	$ 6,764.0	$ 7,323.8
Minimum to qualify as well-capitalized ratio	0.050	0.050
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 10,806.2	$ 10,111.2
Supplementary leverage capital to average assets	8.50%	7.70%
Supplementary leverage capital required to be well capitalized	$ 3,825.6	$ 3,931.5
Supplementary leverage ratio	0.030	0.030
ADVANCED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,806.2	$ 10,111.2
RATIO	0.146	0.124
Minimum to qualify as well-capitalized balance	$ 4,808.6	$ 5,286.2
Minimum to qualify as well-capitalized ratio	0.065	0.065