Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	$ 1,087,770	$ 890,976	$ 3,149,368	$ 2,635,691
Not subject to contracts with customers:
Dividend and interest income, net of interest expense	7,834	7,603	23,298	28,006
Investment (losses) gains	(2,724)	1,106	3,385	(19,011)
Other revenues	(48)	353	869	957
Total revenue not from contract with customer	5,062	9,062	27,552	9,952
Net revenues	1,092,832	900,038	3,176,920	2,645,643
Base fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	758,524	623,593	2,169,932	1,806,476
Performance-based fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	18,273	6,946	87,955	23,991
Bernstein research services
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	113,340	98,514	338,016	341,346
All-in-management fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	89,737	83,466	260,992	245,651
12b-1 fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	21,681	19,068	62,368	56,351
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	59,194	33,159	150,390	84,647
Shareholder servicing fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	23,495	20,753	67,573	61,290
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	$ 3,526	$ 5,477	$ 12,142	$ 15,939