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, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 10,652	$ 10,826	$ 9,296
Federal tax on partnership gross business income	27,197	27,674	24,520
State income taxes	532	576	451
Credit for UBT paid by AB	(10,652)	(10,826)	(9,296)
Income taxes	$ 27,729	$ 28,250	$ 24,971
AB Holding' effective tax rate [Abstract]
UBT statutory rate, percent	4.00%	4.00%	4.00%
Federal tax on partnership gross business income, percent	10.20%	10.20%	10.50%
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	10.40%	10.40%	10.70%