Net Income per Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Unit [Abstract]
Net income – basic	$ 66,999	$ 59,828	$ 186,184	$ 158,541
Additional allocation of equity in net income attributable to AB resulting from assumed dilutive effect of compensatory options	14	17	32	62
Net income – diluted	$ 67,013	$ 59,845	$ 186,216	$ 158,603
Weighted average units outstanding – basic (in units)	96,350,000	96,376,000	95,415,000	95,939,000
Dilutive effect of compensatory options (in units)	27,000	36,000	23,000	52,000
Weighted average units outstanding – diluted (in units)	96,377,000	96,412,000	95,438,000	95,991,000
Basic net income per unit (in dollars per unit)	$ 0.70	$ 0.62	$ 1.95	$ 1.65
Diluted net income per unit (in dollars per unit)	$ 0.70	$ 0.62	$ 1.95	$ 1.65
Anti-dilutive units excluded from diluted net income (in units)	29,056	29,056	29,056	29,056