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Net Capital (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Stanford C. Bernstein & Co.,LLC [Member]  
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 $ 194.9
Alternative net capital requirement 32.5
Stanford C. Bernstein & Co.,LLC [Member] | US Securities And Exchange Commission [Member]  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Net capital 227.4
Stanford C. Bernstein Limited [Member]  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Net capital 56.1
Minimum financial resources/net capital required 25.6
Actual net capital in excess of the minimum net capital required 30.5
AB Investments [Member] | US Securities And Exchange Commission [Member]  
Computation of Net Capital or Financial Resource Requirement [Abstract]  
Net capital 15.6
Minimum financial resources/net capital required 0.3
Actual net capital in excess of the minimum net capital required $ 15.3