Leases - Leases included in the Condensed Consolidated Statement of Financial Condition (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Operating Leases
Operating lease right-of-use assets	$ 300,366	$ 360,185
Operating lease liabilities	392,875	465,907
Finance Leases
Property and equipment, gross	5,167	3,825
Amortization of right-of-use assets	(2,293)	(1,317)
Property and equipment, net	2,874	2,508
Finance lease liabilities	$ 2,822	$ 2,544
Operating Lease, Liability, Statement of Financial Position [Extensible List]	ab:LeaseLiability
Finance Lease, Liability, Statement of Financial Position [Extensible List]	ab:LeaseLiability