Consolidated Company-Sponsored Investment Funds (Summary of Balance Sheet Amounts) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 617,547	$ 998,448	$ 1,247,459	$ 994,510
Other assets	297,995	211,893
Total assets	8,548,636	9,295,167
Liabilities	4,502,887	4,630,276
Redeemable non-controlling interest	103,997	601,587
Partners' capital attributable to AB Unitholders	3,940,067	4,061,740
Non-redeemable non-controlling interests in consolidated entities	1,685	1,564
Total liabilities, redeemable non-controlling interest and capital	8,548,636	9,295,167
Company-sponsored investment funds
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	10,438	326,518
Investments	303,301	1,246,283
Other assets	9,095	35,397
Liabilities	8,579	698,101
Redeemable non-controlling interest	101,109	596,223
Non-redeemable non-controlling interests in consolidated entities	658	757
Reportable entities | Company-sponsored investment funds
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	10,438	326,518
Investments	303,301	1,246,283
Other assets	9,095	35,397
Total assets	322,834	1,608,198
Liabilities	8,579	698,101
Redeemable non-controlling interest	101,109	596,223
Partners' capital attributable to AB Unitholders	212,488	313,117
Non-redeemable non-controlling interests in consolidated entities	658	757
Total liabilities, redeemable non-controlling interest and capital	322,834	1,608,198
Reportable entities | VIEs
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	9,736	326,158
Investments	186,386	1,189,835
Other assets	5,544	33,931
Total assets	201,666	1,549,924
Liabilities	6,077	695,997
Redeemable non-controlling interest	80,436	596,241
Partners' capital attributable to AB Unitholders	114,495	256,929
Non-redeemable non-controlling interests in consolidated entities	658	757
Total liabilities, redeemable non-controlling interest and capital	201,666	1,549,924
Reportable entities | VOEs
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	702	360
Investments	116,915	56,448
Other assets	3,551	1,466
Total assets	121,168	58,274
Liabilities	2,502	2,104
Redeemable non-controlling interest	20,673	(18)
Partners' capital attributable to AB Unitholders	97,993	56,188
Non-redeemable non-controlling interests in consolidated entities	0	0
Total liabilities, redeemable non-controlling interest and capital	$ 121,168	$ 58,274