Leases - Leases included in the Condensed Consolidated Statement of Financial Condition (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease right-of-use assets	$ 416,007	$ 360,185
Operating lease liabilities	$ 503,174	465,907
Operating Lease, Liability, Statement of Financial Position [Extensible List]	ab:LeaseLiability
Property and equipment, gross	$ 5,167	3,825
Amortization of right-of-use assets	(2,719)	(1,317)
Property and equipment, net	2,448	2,508
Finance lease liabilities	$ 2,375	$ 2,544