Quarterly report pursuant to Section 13 or 15(d)

Notes Receivable from Financial Advisors (Detail)

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Notes Receivable from Financial Advisors (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Percentage bearing variable interest rate 10.00%    
Allowance for doubtful accounts $ 1,452   $ 1,208
Contract acquisition costs, net $ 77,803 $ 61,340 $ 0
Minimum      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans amortized over repayment period 3 years    
Maximum      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans amortized over repayment period 5 years    
Accounting Standards Update 2014-09      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Contract acquisition costs, net   $ 40,566