Summary of Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended								12 Months Ended
		Feb. 11, 2021	Dec. 31, 2020	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Distribution Made to Limited Liability Company (LLC) Member [Line Items]
Subsequent cash distribution, distribution declared (in dollars per unit)	[1],[2]			$ 0.97	$ 0.69	$ 0.61	$ 0.64	$ 0.85	$ 0.63	$ 0.56	$ 0.49
Total cash distributions per unit paid to unitholders (in dollars per unit)												$ 2.79	$ 2.32	$ 2.88
Long-Term Incentive Compensation Plans [Abstract]
AB Holding units purchased in period (in units)												5,400,000	6,000,000.0
Dollar amount paid for AB holding units acquired												$ 149,000	$ 172,600
Open-market purchases of AB holding units (in units)												3,100,000	2,900,000
Dollar amount paid for open-market purchases of AB holding units												$ 74,000	$ 82,700
Restricted AB holding unit awards to employees (in units)			5,000,000.0										5,400,000
Options to buy AB holding units outstanding, number (in units)												5,182	500,000
Proceeds from exercise of compensatory options to buy AB holding units												$ 147	$ 11,511	$ 16,589
Employees and Eligible Directors
Long-Term Incentive Compensation Plans [Abstract]
Restricted AB holding unit awards to employees (in units)												5,700,000	7,700,000
Subsequent Event
Distribution Made to Limited Liability Company (LLC) Member [Line Items]
Subsequent cash distribution, distribution declared (in dollars per unit)		$ 0.97

[1] Cash distributions reflect the impact of AB’s non-GAAP adjustments.

[2] Declared and paid during the following quarter.