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Commitments and Contingencies - Additional Information (Detail) - USD ($)
3 Months Ended 5 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Jun. 23, 2016
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2017
Jun. 24, 2016
Jan. 15, 2016
Dec. 31, 2015
Jun. 26, 2015
Mar. 31, 2015
Commitment And Contingencies [Line Items]                      
Rental expense relating to operating leases $ 9,311,000 $ 8,345,000   $ 25,065,000 $ 25,704,000            
Unfunded commitments for capital contributions 5,134,000     5,134,000              
Investment in private equity funds 86,532,000     86,532,000         $ 86,287,000    
Other Commitment                     $ 5,500,000
Loans and Leases Receivable, Gross   $ 3,500,000     $ 3,500,000            
Remaining Loan Commitment 2,000,000     2,000,000              
Loss Contingency, Damages Sought, Value       $ 300,000,000              
Protego [Member]                      
Commitment And Contingencies [Line Items]                      
Percentage of the return proceeds received payable (as a percent)       90.00%              
Scenario, Forecast [Member]                      
Commitment And Contingencies [Line Items]                      
Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions           20.00%          
First Republic Bank [Member]                      
Commitment And Contingencies [Line Items]                      
Maximum borrowing capacity               $ 50,000,000   $ 75,000,000  
Amount outstanding during period     $ 50,000,000                
PNC Bank [Member]                      
Commitment And Contingencies [Line Items]                      
Maximum borrowing capacity             $ 30,000,000        
Other Assets [Member]                      
Commitment And Contingencies [Line Items]                      
Loans and Leases Receivable, Gross 3,500,000     $ 3,500,000              
Private Equity Funds [Member]                      
Commitment And Contingencies [Line Items]                      
Investment in private equity funds 19,880,000     19,880,000         26,473,000    
Private Equity Funds [Member] | Discovery Fund [Member]                      
Commitment And Contingencies [Line Items]                      
Investment in private equity funds 7,459,000     7,459,000         $ 6,632,000    
Private Equity Funds [Member] | Discovery Fund [Member] | Protego [Member]                      
Commitment And Contingencies [Line Items]                      
Goodwill 10,150,000     10,150,000              
Investment in private equity funds $ 7,459,000     $ 7,459,000