Regulatory Capital Requirements - Risk-Based Capital Amounts and Ratios (Detail) $ in Millions	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
STANDARDIZED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 11,142.2	$ 10,822.2
RATIO	0.129	0.139
Minimum to qualify as well-capitalized balance	$ 5,177.6	$ 4,659.7
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 12,126.8	$ 12,085.7
RATIO	0.141	0.156
Minimum to qualify as well-capitalized balance	$ 8,629.3	$ 7,766.2
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 11,142.2	$ 10,822.2
RATIO	0.069	0.076
STANDARDIZED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,277.1	$ 9,962.2
RATIO	0.119	0.128
STANDARDIZED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,315.7	$ 10,003.3
RATIO	0.120	0.130
Minimum to qualify as well-capitalized balance	$ 6,866.1	$ 6,147.0
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 11,158.4	$ 11,123.1
RATIO	0.130	0.145
Minimum to qualify as well-capitalized balance	$ 8,582.6	$ 7,683.7
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,315.7	$ 10,003.3
RATIO	0.064	0.070
Minimum to qualify as well-capitalized balance	$ 8,019.5	$ 7,105.0
Minimum to qualify as well-capitalized ratio	0.050	0.050
STANDARDIZED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,315.7	$ 10,003.3
RATIO	0.120	0.130
Minimum to qualify as well-capitalized balance	$ 5,578.7	$ 4,994.4
Minimum to qualify as well-capitalized ratio	0.065	0.065
ADVANCED APPROACH | Parent Company
Tier 1 Capital
BALANCE	$ 11,142.2	$ 10,822.2
RATIO	0.143	0.145
Minimum to qualify as well-capitalized balance	$ 4,668.4	$ 4,467.6
Minimum to qualify as well-capitalized ratio	0.060	0.060
Total Capital
BALANCE	$ 11,942.0	$ 11,825.8
RATIO	0.153	0.159
Minimum to qualify as well-capitalized balance	$ 7,780.7	$ 7,446.0
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 11,142.2	$ 10,822.2
RATIO	0.069	0.076
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 11,142.2	$ 10,822.2
Supplementary leverage capital to average assets	8.20%	8.60%
ADVANCED APPROACH | Parent Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,277.1	$ 9,962.2
RATIO	0.132	0.134
ADVANCED APPROACH | The Northern Trust Company
Tier 1 Capital
BALANCE	$ 10,315.7	$ 10,003.3
RATIO	0.135	0.138
Minimum to qualify as well-capitalized balance	$ 6,110.2	$ 5,805.6
Minimum to qualify as well-capitalized ratio	0.080	0.080
Total Capital
BALANCE	$ 10,973.7	$ 10,863.3
RATIO	0.144	0.150
Minimum to qualify as well-capitalized balance	$ 7,637.8	$ 7,257.0
Minimum to qualify as well-capitalized ratio	0.100	0.100
Leverage (Tier 1 Capital to Adjusted Average Fourth Quarter Assets)
BALANCE	$ 10,315.7	$ 10,003.3
RATIO	0.064	0.070
Minimum to qualify as well-capitalized balance	$ 8,019.5	$ 7,105.0
Minimum to qualify as well-capitalized ratio	0.050	0.050
Supplementary Leverage Capital [Abstract]
Supplementary leverage capital	$ 10,315.7	$ 10,003.3
Supplementary leverage capital to average assets	7.60%	7.70%
Supplementary leverage capital required to be well capitalized	$ 4,081.7	$ 3,883.4
Supplementary leverage ratio	0.030	0.030
ADVANCED APPROACH | The Northern Trust Company | Common Stock
Tier 1 Capital
BALANCE	$ 10,315.7	$ 10,003.3
RATIO	0.135	0.138
Minimum to qualify as well-capitalized balance	$ 4,964.5	$ 4,717.1
Minimum to qualify as well-capitalized ratio	0.065	0.065