Related Party Transactions - Summary of Revenues, Expenses and Balance Sheet Items for AXA and Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Revenue subject to contracts with customers	$ 3,315,390	$ 3,156,149	$ 2,892,144
Other revenues	98,676	97,135	99,859
Total revenues	3,419,760	3,323,686	3,037,902
Expenses:
Commissions and distribution payments to financial intermediaries	427,186	411,467	363,603
General and administrative	448,996	481,488	426,147
Other	222,630	213,275	216,542
Total expenses	2,542,047	2,524,611	2,305,614
Balance Sheet:
Institutional investment advisory and services fees receivable	127,462	130,119
AXA, EQH and its subsidiaries
Revenues:
Other revenues	1,729	1,130	969
Total revenues	184,917	172,350	163,713
Expenses:
Commissions and distribution payments to financial intermediaries	21,567	19,202	16,077
General and administrative	15,006	12,428	16,315
Other	1,485	1,696	1,653
Total expenses	38,058	33,326	34,045
Balance Sheet:
Institutional investment advisory and services fees receivable	17,612	13,806
Prepaid expenses	364	2,905
Other due to AXA, EQH and their subsidiaries	(7,259)	(19,666)
Total	10,717	(2,955)
Investment advisory and services fees
Revenues:
Revenue subject to contracts with customers	2,362,211	2,201,305	1,933,471
Investment advisory and services fees | AXA, EQH and its subsidiaries
Revenues:
Revenue subject to contracts with customers	169,157	157,430	150,016
Bernstein research services
Revenues:
Revenue subject to contracts with customers	439,432	449,919	479,875
Bernstein research services | AXA, EQH and its subsidiaries
Revenues:
Revenue subject to contracts with customers	134	403	583
Distribution revenues
Revenues:
Revenue subject to contracts with customers	418,562	412,063	384,405
Distribution revenues | AXA, EQH and its subsidiaries
Revenues:
Revenue subject to contracts with customers	$ 13,897	$ 13,387	$ 12,145