Income Taxes - Principal Reasons for the Difference Between the Effective Tax Rate and Statutory Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Credit for UBT	4.00%	4.00%	4.00%
Federal tax rate	3.50%	3.50%	3.50%
UBT statutory rate, percent	4.00%	4.00%	4.00%
AB Holding's effective tax expense [Abstract]
UBT statutory rate	$ 12,336	$ 10,652	$ 10,826
Federal tax on partnership gross business income	28,522	27,197	27,674
State income taxes	502	532	576
Credit for UBT paid by AB	(12,336)	(10,652)	(10,826)
Total income taxes	$ 29,024	$ 27,729	$ 28,250
AB Holding' effective tax rate [Abstract]
UBT statutory rate, percent	4.00%	4.00%	4.00%
Federal tax on partnership gross business income, percent	9.20%	10.20%	10.20%
State income taxes, percent	0.20%	0.20%	0.20%
Credit for UBT paid by AB, percent	(4.00%)	(4.00%)	(4.00%)
Income tax effective tax rate, percent	9.40%	10.40%	10.40%