Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	$ 1,064,133	$ 837,395	$ 2,061,598	$ 1,744,715
Not subject to contracts with customers:
Dividend and interest income, net of interest expense	7,924	9,257	15,464	20,403
Investment gains (losses)	4,181	24,189	6,109	(20,117)
Other revenues	584	608	917	604
Total revenue not from contract with customer	12,689	34,054	22,490	890
Net revenues	1,076,822	871,449	2,084,088	1,745,605
Base fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	723,717	569,296	1,411,408	1,182,883
Performance-based fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	53,907	8,907	69,682	17,045
Bernstein research services
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	105,655	113,609	224,676	242,832
All-in-management fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	86,073	75,828	171,255	162,185
12b-1 fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	20,653	17,830	40,687	37,283
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	48,812	26,441	91,196	51,488
Shareholder servicing fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	22,383	19,694	44,078	40,537
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers	$ 2,933	$ 5,790	$ 8,616	$ 10,462