Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) $ in Millions	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
STANDARDIZED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 10,397.4	$ 11,142.2
RATIO	0.118	0.129
Minimum to qualify as well-capitalized balance	$ 5,286.5	$ 5,177.6
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 12,245.5	$ 12,126.8
RATIO	0.139	0.141
Minimum to qualify as well-capitalized balance	$ 8,810.8	$ 8,629.3
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,397.4	$ 11,142.2
RATIO	0.071	0.069
STANDARDIZED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 9,539.7	$ 10,277.1
RATIO	0.108	0.119
STANDARDIZED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,111.2	$ 10,315.7
RATIO	0.116	0.120
Minimum to qualify as well-capitalized balance	$ 6,996.2	$ 6,866.1
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 11,766.8	$ 11,158.4
RATIO	0.135	0.130
Minimum to qualify as well-capitalized balance	$ 8,745.3	$ 8,582.6
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,111.2	$ 10,315.7
RATIO	0.069	0.064
Minimum to qualify as well-capitalized balance	$ 7,323.8	$ 8,019.5
Minimum to qualify as well-capitalized ratio	0.050	0.050
STANDARDIZED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,111.2	$ 10,315.7
RATIO	0.116	0.120
Minimum to qualify as well-capitalized balance	$ 5,684.4	$ 5,578.7
Minimum to qualify as well-capitalized ratio	0.065	0.065
ADVANCED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 10,397.4	$ 11,142.2
RATIO	0.125	0.143
Minimum to qualify as well-capitalized balance	$ 4,990.9	$ 4,668.4
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 12,045.9	$ 11,942.0
RATIO	0.145	0.153
Minimum to qualify as well-capitalized balance	$ 8,318.1	$ 7,780.7
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,397.4	$ 11,142.2
RATIO	0.071	0.069
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 10,397.4	$ 11,142.2
Supplementary leverage capital to average assets	7.90%	8.20%
ADVANCED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 9,539.7	$ 10,277.1
RATIO	0.115	0.132
ADVANCED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,111.2	$ 10,315.7
RATIO	0.124	0.135
Minimum to qualify as well-capitalized balance	$ 6,506.1	$ 6,110.2
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 11,567.2	$ 10,973.7
RATIO	0.142	0.144
Minimum to qualify as well-capitalized balance	$ 8,132.6	$ 7,637.8
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,111.2	$ 10,315.7
RATIO	0.069	0.064
Minimum to qualify as well-capitalized balance	$ 7,323.8	$ 8,019.5
Minimum to qualify as well-capitalized ratio	0.050	0.050
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 10,111.2	$ 10,315.7
Supplementary leverage capital to average assets	7.70%	7.60%
Supplementary leverage capital required to be well capitalized	$ 3,931.5	$ 4,081.7
Supplementary leverage ratio	0.030	0.030
ADVANCED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,111.2	$ 10,315.7
RATIO	0.124	0.135
Minimum to qualify as well-capitalized balance	$ 5,286.2	$ 4,964.5
Minimum to qualify as well-capitalized ratio	0.065	0.065