Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									$ 3,688,518	$ 3,430,316	$ 3,315,390
Dividend and interest income, net of interest expense									35,273	47,216	45,827
Investment (losses) gains									(16,401)	38,659	2,653
Other revenues									1,146	2,241	3,491
Revenue not subject to contract with customer									20,018	88,116	51,971
Net revenues	$ 1,062,893	$ 900,038	$ 871,449	$ 874,156	$ 987,304	$ 877,867	$ 857,799	$ 795,462	3,708,536	3,518,432	3,367,361
Base fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									2,462,810	2,372,429	2,244,068
Performance-based fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									132,626	99,615	118,143
Bernstein research services
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									459,744	407,911	439,432
All-in-management fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									331,268	291,999	254,477
12b-1 fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									75,973	80,268	87,166
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									122,540	82,776	76,919
Shareholder servicing fees
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									82,317	77,394	75,974
Other
Disaggregation of Revenue [Line Items]
Revenue subject to contracts with customers									$ 21,240	$ 17,924	$ 19,211