Consolidated Company-Sponsored Investment Funds (Summary of Balance Sheet Amounts) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Condensed Balance Sheet Statements, Captions [Line Items]
Other assets	$ 284,835	$ 297,940
Total assets	8,555,271	8,789,098
Liabilities	4,398,256	4,724,080
Redeemable non-controlling interest	291,524	148,809
Partners' capital attributable to AB Unitholders	3,865,491	3,915,260
Total liabilities, redeemable non-controlling interest and capital	8,555,271	8,789,098
Company-sponsored investment funds
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	17,009	13,118
Investments	536,282	351,696
Other assets	36,400	22,840
Liabilities	38,073	22,610
Redeemable non-controlling interest	291,524	145,921
Reportable entities | Company-sponsored investment funds
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	17,009	13,118
Investments	536,282	351,696
Other assets	36,400	22,840
Total assets	589,691	387,654
Liabilities	38,073	22,610
Redeemable non-controlling interest	291,524	145,921
Partners' capital attributable to AB Unitholders	260,094	219,123
Total liabilities, redeemable non-controlling interest and capital	589,691	387,654
Reportable entities | VIEs
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	14,647	11,880
Investments	372,668	217,840
Other assets	20,418	6,024
Total assets	407,733	235,744
Liabilities	17,892	5,215
Redeemable non-controlling interest	244,327	117,523
Partners' capital attributable to AB Unitholders	145,515	113,006
Total liabilities, redeemable non-controlling interest and capital	407,734	235,744
Reportable entities | VOEs
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	2,362	1,238
Investments	163,614	133,856
Other assets	15,982	16,816
Total assets	181,958	151,910
Liabilities	20,181	17,395
Redeemable non-controlling interest	47,197	28,398
Partners' capital attributable to AB Unitholders	114,579	106,117
Total liabilities, redeemable non-controlling interest and capital	$ 181,957	$ 151,910