Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Segment Capital_Repayment CapitalClass	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($) Segment	Jan. 01, 2020 USD ($)	Dec. 31, 2017 USD ($)
Basis Of Presentation [Line Items]
Number of operating segments | Segment	2	2	2
Period for partner's capital redemption in the event of death	6 months
Number of annual capital repayments for withdrawal of limited partners | Segment	3
Period to begin repaying capital upon withdrawal for limited partners	90 days
Number of annual capital repayments for withdrawal of subordinated limited partners | Capital_Repayment	6
Period to begin repaying capital upon withdrawal for subordinated limited partners	90 days
Fair value of level III, assets	$ 0	$ 0
Fair value of level III, liabilities	0	0
Liability for the future payments due to financial advisors	$ 268,000,000
Retirement and transition employment agreement term	5 years
Retirement transition plan employment term	2 years
Retirement transition plan non-compete term	3 years
Retirement transition plan compensation paid term	4 years
Retirement transition plan accrued future payments	$ 109,000,000	102,000,000
Retirement transition plan accrued future payments expected to be paid in 2021	52,000,000
Net income	$ 0	$ 0	$ 0
Number of classes of capital | CapitalClass	3
Limited partnership's minimum annual return rate	7.50%	7.50%	7.50%
Impact of new accounting standards on opening partnership capital	$ 3,589,000,000	$ 3,364,000,000	$ 2,855,000,000		$ 2,795,000,000
Accounting Standards Update 2016-13 [Member]
Basis Of Presentation [Line Items]
Impact of new accounting standards on opening partnership capital				$ 0
Building [Member]
Basis Of Presentation [Line Items]
Equipment, property and improvements estimated useful lives	30 years
Branch Office Space [Member]
Basis Of Presentation [Line Items]
Lessee, operating lease, description	The Partnership leases branch office space under numerous operating leases from non-affiliates and financial advisors. Branch offices are generally leased for terms of five years and generally contain a renewal option.
Lessee, operating lease term of contract	5 years
Lessee Operating Lease Home Office Spaces [Member]
Basis Of Presentation [Line Items]
Lessee, operating lease, description	The Partnership also leases home office spaces and land from non-affiliates with terms ranging from 12 to 30 years.
Minimum [Member]
Basis Of Presentation [Line Items]
Retirement transition plan compensation expense term	1 year
Minimum [Member] | Equipment [Member]
Basis Of Presentation [Line Items]
Equipment, property and improvements estimated useful lives	3 years
Minimum [Member] | Lessee Operating Lease Home Office Spaces [Member]
Basis Of Presentation [Line Items]
Lessee, operating lease term of contract	12 years
Maximum [Member]
Basis Of Presentation [Line Items]
Retirement transition plan compensation expense term	2 years
Retirement transition plan compensation on transitioned assets term	4 years
Maximum [Member] | Equipment [Member]
Basis Of Presentation [Line Items]
Equipment, property and improvements estimated useful lives	10 years
Maximum [Member] | Lessee Operating Lease Home Office Spaces [Member]
Basis Of Presentation [Line Items]
Lessee, operating lease term of contract	30 years