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Stock Compensation - Assumptions Used in Determination of Grant Date Fair Values Using Black-Scholes Option Pricing Model (Detail)
6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Document Fiscal Year Focus 2017  
Employee Stock Option [Member] | HGI [Member] | Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate 1.79542% 1.6545%
Expected option term 5 years 5 years
Volatility 35.11387% 37.40%
Employee Stock Option [Member] | HGI [Member] | Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate 2.24703% 1.7425%
Expected option term 6 years 6 months 5 years 6 months
Volatility 37.54231% 37.90%
Employee Stock Option [Member] | HGI [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Assumed dividend yield 0.00% 0.00%