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Preferred and Common Stockholder's Equity - Additional Information (Detail) - USD ($)
Sep. 30, 2018
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2016
Class of Stock [Line Items]        
Preferred stock, shares authorized 5,000,000      
Preferred stock, par value $ 0.01      
Preferred stock, shares outstanding 0      
Common stock, shares authorized 150,000,000      
Common stock, par value $ 0.01      
Common stock, shares issued 74,700,000 70,600,000 68,400,000  
Common stock, shares outstanding 74,700,000 70,600,000 68,400,000  
Share repurchase program, remaining authorized repurchase amount $ 160,700,000      
Eighth Share Repurchase Program [Member]        
Class of Stock [Line Items]        
Share repurchase program, authorized amount       $ 200,000,000