Net Income Per Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	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
Earnings Per Share [Abstract]
Net income - basic	$ 93,196		$ 66,999		$ 56,926		$ 62,259		$ 80,022		$ 59,828		$ 52,274		$ 46,439		$ 279,380	$ 238,563	$ 242,397
Additional allocation of equity in net income attributable to AB resulting from assumed dilutive effect of compensatory options																	56	79	447
Net income - diluted																	$ 279,436	$ 238,642	$ 242,844
Weighted average units outstanding - basic (in units)																	96,870,000	95,884,000	97,041,000
Dilutive effect of compensatory options (in units)																	27,000	44,000	251,000
Weighted average units outstanding - diluted (in units)																	96,897,000	95,928,000	97,292,000
Basic net income per unit (in dollars per unit)	$ 0.97	[1]	$ 0.70	[1]	$ 0.59	[1]	$ 0.63	[1]	$ 0.84	[1]	$ 0.62	[1]	$ 0.54	[1]	$ 0.49	[1]	$ 2.88	$ 2.49	$ 2.50
Diluted net income per unit (in dollars per unit)	$ 0.97	[1]	$ 0.70	[1]	$ 0.59	[1]	$ 0.63	[1]	$ 0.84	[1]	$ 0.62	[1]	$ 0.54	[1]	$ 0.49	[1]	$ 2.88	$ 2.49	$ 2.50
Antidilutive securities (in units)																	29,056	29,056	49,784

[1] Basic and diluted net income per unit are computed independently for each of the periods presented. Accordingly, the sum of the quarterly net income per unit amounts may not agree to the total for the year.