Related Party Transactions - Summary of Revenues, Expenses and Balance Sheet Items for AXA and Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Investment advisory and services fees	$ 2,200,400	$ 1,933,471	$ 1,973,837
Bernstein research services	449,919	479,875	493,463
Distribution revenues	412,063	384,405	427,156
Other revenues	98,040	99,859	101,169
Total revenues	3,323,686	3,037,902	3,024,048
Expenses:
Commissions and distribution payments to financial intermediaries	420,350	371,607	393,033
General and administrative	481,488	426,147	431,635
Other	204,392	208,538	223,415
Total expenses	2,524,611	2,305,614	2,389,628
Balance Sheet:
Institutional investment advisory and services fees receivable	124,164	104,376
AXA and its subsidiaries [Member]
Revenues:
Investment advisory and services fees	157,430	150,016	149,035
Bernstein research services	403	583	694
Distribution revenues	13,387	12,145	11,541
Other revenues	1,130	969	912
Total revenues	172,350	163,713	162,182
Expenses:
Commissions and distribution payments to financial intermediaries	19,202	16,077	16,140
General and administrative	12,428	16,315	17,680
Other	1,696	1,653	1,483
Total expenses	33,326	34,045	$ 35,303
Balance Sheet:
Institutional investment advisory and services fees receivable	13,806	11,826
Prepaid expenses	2,905	1,461
Other due to AXA and its subsidiaries	(19,666)	(5,325)
Total	$ (2,955)	$ 7,962