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Net Capital (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Stanford C. Bernstein & Co.,LLC  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Minimum net capital required per stock exchange and SEC rules $ 1.0
Alternative method of calculating aggregate debit items arising from customer transactions permitted by rule, percent 2.00%
Alternative excess net capital $ 223.5
Alternative net capital requirement 34.9
Stanford C. Bernstein & Co.,LLC | US SEC  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Net capital 258.4
Stanford C. Bernstein Limited  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Net capital 56.4
Minimum financial resources/net capital required 23.2
Actual net capital in excess of the minimum net capital required 33.2
AB Investments | US SEC  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Net capital 23.9
Minimum financial resources/net capital required 0.3
Actual net capital in excess of the minimum net capital required $ 23.6