Net Capital (Details) $ in Millions	Dec. 31, 2020 USD ($)
Stanford C. Bernstein & Co.,LLC
Schedule Of Regulatory Capital Agencies And Requirements [Line Items]
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	$ 243.0
Alternative net capital requirement	34.0
Stanford C. Bernstein & Co.,LLC | US SEC
Schedule Of Regulatory Capital Agencies And Requirements [Line Items]
Net capital	277.0
Stanford C. Bernstein Limited
Schedule Of Regulatory Capital Agencies And Requirements [Line Items]
Net capital	53.4
Minimum financial resources/net capital required	47.5
Actual net capital in excess of the minimum net capital required	5.9
AB Investments | US SEC
Schedule Of Regulatory Capital Agencies And Requirements [Line Items]
Net capital	32.1
Minimum financial resources/net capital required	0.3
Actual net capital in excess of the minimum net capital required	$ 31.8