Leases - Leases included in the Condensed Consolidated Statement of Financial Condition (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Leases [Abstract]
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	ab:LeaseRightOfUseAssets
Operating Leases
Operating lease right-of-use assets	$ 314,422	$ 360,185
Operating lease liabilities	413,196	465,907
Finance Leases
Property and equipment, gross	5,167	3,825
Amortization of right-of-use assets	(1,866)	(1,317)
Property and equipment, net	3,301	2,508
Finance lease liabilities	$ 3,281	$ 2,544
Operating Lease, Liability, Statement of Financial Position [Extensible List]	ab:LeaseLiability
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	ab:LeaseRightOfUseAssets
Finance Lease, Liability, Statement of Financial Position [Extensible List]	ab:LeaseLiability