Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	$ 890,976	$ 861,148	$ 2,635,691	$ 2,465,095
Not subject to contracts with customers:
Dividend and interest income, net of interest expense	7,603	11,904	28,006	33,439
Investment gains (losses)	1,106	4,433	(19,011)	31,117
Other revenues	353	382	957	1,477
Total revenue not from contract with customer	9,062	16,719	9,952	66,033
Net revenues	900,038	877,867	2,645,643	2,531,128
Base fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	623,593	608,765	1,806,476	1,746,072
Performance-based fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	6,946	7,619	23,991	23,270
Bernstein research services
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	98,514	102,014	341,346	298,240
All-in-management fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	83,466	77,110	245,651	207,377
12b-1 fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	19,068	20,287	56,351	60,055
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	33,159	21,238	84,647	60,059
Shareholder servicing fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	20,753	20,020	61,290	57,413
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	$ 5,477	$ 4,095	$ 15,939	$ 12,609