Preferred and Common Stockholder's Equity - Additional Information (Detail) - USD ($)	6 Months Ended
	Dec. 31, 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	73,200,000	70,600,000	68,400,000
Common stock, shares outstanding	73,200,000	70,600,000	68,400,000
Share repurchase program, remaining authorized repurchase amount	$ 110,700,000
Payments for repurchase of company stock	$ 50,000,000
Eighth Share Repurchase Program [Member]
Class of Stock [Line Items]
Share repurchase program, authorized amount				$ 200,000,000