Net Income Per Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income - basic	$ 80,022		$ 59,828		$ 52,274		$ 46,439		$ 59,880		$ 65,900		$ 58,457		$ 58,160		$ 238,563	$ 242,397	$ 207,422
Additional allocation of equity in net income attributable to AB resulting from assumed dilutive effect of compensatory options																	79	447	680
Net income - diluted																	$ 238,642	$ 242,844	$ 208,102
Weighted average units outstanding - basic (in units)																	95,884,000	97,041,000	94,733,000
Dilutive effect of compensatory options (in units)																	44,000	251,000	430,000
Weighted average units outstanding - diluted (in units)																	95,928,000	97,292,000	95,163,000
Basic net income per unit (in dollars per unit)	$ 0.84	[1]	$ 0.62	[1]	$ 0.54	[1]	$ 0.49	[1]	$ 0.63	[1]	$ 0.68	[1]	$ 0.59	[1]	$ 0.60	[1]	$ 2.49	$ 2.50	$ 2.19
Diluted net income per unit (in dollars per unit)	$ 0.84	[1]	$ 0.62	[1]	$ 0.54	[1]	$ 0.49	[1]	$ 0.63	[1]	$ 0.68	[1]	$ 0.59	[1]	$ 0.60	[1]	$ 2.49	$ 2.50	$ 2.19
Antidilutive securities (in units)																	29,056	49,784	1,970,741

[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.