Related Party Transactions - Summary of Revenues, Expenses and Balance Sheet Items for AXA and Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Revenue subject to contracts with customers	$ 3,430,316	$ 3,315,390	$ 3,156,149
Other revenues	97,559	98,676	97,135
Total revenues	3,575,637	3,419,760	3,323,686
Expenses:
General and administrative	484,750	448,996	481,488
Other	219,860	222,630	213,275
Total expenses	2,694,995	2,542,047	2,524,611
Balance Sheet:
Institutional investment advisory and services fees receivable	148,744	127,462
EQH
Revenues:
Other revenues	1,013	1,104	864
Total revenues	110,329	105,914	99,314
Expenses:
General and administrative	2,466	2,615	2,610
Other	2,759	1,485	1,696
Total expenses	9,181	8,064	8,134
Balance Sheet:
Institutional investment advisory and services fees receivable	8,716	9,751
Prepaid expenses	238	364
Other due to EQH, AXA and their respective subsidiaries	(2,111)	(1,842)
EQH Facility	(560,000)	0
Total	(553,157)	8,273
AXA
Revenues:
Other revenues	482	625	266
Total revenues	78,581	79,003	73,036
Expenses:
General and administrative	11,501	12,391	9,818
Other	0	0	0
Total expenses	28,194	29,994	25,192
Balance Sheet:
Institutional investment advisory and services fees receivable	10,842	7,861
Prepaid expenses	0	0
Other due to EQH, AXA and their respective subsidiaries	(5,234)	(5,417)
EQH Facility	0	0
Total	5,608	2,444
Investment advisory and services fees
Revenues:
Revenue subject to contracts with customers	2,472,044	2,362,211	2,201,305
Investment advisory and services fees | EQH
Revenues:
Revenue subject to contracts with customers	109,316	104,810	98,450
Investment advisory and services fees | AXA
Revenues:
Revenue subject to contracts with customers	65,086	64,347	58,980
Bernstein research services
Revenues:
Revenue subject to contracts with customers	407,911	439,432	449,919
Bernstein research services | EQH
Revenues:
Revenue subject to contracts with customers	0	0	0
Bernstein research services | AXA
Revenues:
Revenue subject to contracts with customers	45	134	403
Distribution related
Revenues:
Revenue subject to contracts with customers	455,043	418,562	412,063
Expenses:
Commissions and distribution payments to financial intermediaries	487,965	427,186	411,467
Distribution related | EQH
Revenues:
Revenue subject to contracts with customers	0	0	0
Expenses:
Commissions and distribution payments to financial intermediaries	3,956	3,964	3,828
Distribution related | AXA
Revenues:
Revenue subject to contracts with customers	12,968	13,897	13,387
Expenses:
Commissions and distribution payments to financial intermediaries	$ 16,693	$ 17,603	$ 15,374