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Summary of the Organization, Description of Business and Significant Accounting Policies - Additional Information (Detail)
3 Months Ended 9 Months Ended 12 Months Ended
Jan. 12, 2018
USD ($)
Dec. 01, 2017
USD ($)
Mar. 06, 2017
Dec. 31, 2017
USD ($)
$ / shares
Sep. 30, 2018
USD ($)
Funds
shares
Sep. 30, 2018
USD ($)
Funds
shares
Dec. 31, 2017
Sep. 30, 2017
USD ($)
shares
Partnership Organization and Basis of Presentation [Line Items]                
Number of Hennessy funds to which company provides investment advisory services | Funds           14    
Number of mutual funds | Funds         28 28    
Management contracts impairment amount         $ 0 $ 0   $ 0
Federal corporate income tax rate         21.00% 24.50% 35.00% 35.00%
Blended statutory tax rate           24.50%    
Reduction to income tax expense due to Tax Cuts and Jobs Act of 2017       $ 4,000,000        
Income tax expense non cash benefit,diluted earnings per share | $ / shares       $ 0.54        
Effective tax rates           11.90%   35.70%
Stock split ratio     1.5          
Shares available for issuance           50.00%    
Share based compensation percentage criteria description           More than 50% of the outstanding shares    
Shares grant period               10 years
Maximum aggregate fair market value of shares               $ 100,000
Maximum common stock issuable under plan | shares         22,500,000 22,500,000   22,500,000
Dividend Reinvestment and Stock Purchase Plan [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Stock issued during period shares | shares           3,588   2,148
Maximum common stock issuable under plan | shares         1,550,000 1,550,000    
Shares available for issuance under plan | shares         1,546,963 1,546,963    
Manning and Napier Group LLC and Rainier Investment Management LLC [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Business combination, cash paid $ 2,100,000 $ 1,000,000       $ 3,100,000    
Business combination, assets acquired and liabilities assumed, assets $ 253,000,000 $ 122,000,000     $ 375,000,000 $ 375,000,000    
Business combination, agreement date           May 10, 2017    
Total capitalized costs related to purchase         3,500,000 $ 3,500,000    
Business acquisition, effective date of acquisition Jan. 12, 2018 Dec. 01, 2017            
Restricted Stock Units (RSUs) [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Maximum Vesting Period           4 years    
Rate of restricted stock units vest under plan           25.00%    
Granted, Number of Restricted Share Units | shares           127,825   130,900
Compensation expense related to non-vested awards not yet recognized         $ 5,010,000 $ 5,010,000    
Weighted-average vesting period for compensation expenses not yet recognized           2 years 10 months 25 days    
Minimum [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Estimated useful lives of assets           1 year    
Percentage of exercise price of fair value               110.00%
Minimum [Member] | Incentive Stock Option Plan [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Percentage of shareholder ownership to meet incentive stock option               10.00%
Maximum [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Unrealized gains           $ 1,000   $ 1,000
Estimated useful lives of assets           10 years    
Shares grant period               10 years
Maximum Vesting Period               5 years
Maximum [Member] | Stock Appreciation Right [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Aggregate number of shares | shares               75,000
Maximum [Member] | 2013 Omnibus Incentive Plan [Member]                
Partnership Organization and Basis of Presentation [Line Items]                
Number of options available for grant | shares         3,948,573 3,948,573