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Share-Based Payment Awards - Key Assumptions Used to Apply Option Pricing Model For Options Granted (Detail) - Inducement Award Grants [Member]
6 Months Ended 12 Months Ended
Dec. 31, 2018
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Option life (in years) 6 years    
Stock volatility, minimum 59.00% 60.00%  
Stock volatility, maximum 61.00% 62.00%  
Stock volatility     70.00%
Risk-free interest rate, minimum 2.81% 2.88%  
Risk-free interest rate, maximum 2.94% 2.91%  
Risk-free interest rate     2.13%
Expected dividends 0.00% 0.00% 0.00%
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Option life (in years)   5 years 6 months 5 years 6 months
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Option life (in years)   6 years 6 years 3 months