Stockholders' Equity - Additional Information (Detail) - USD ($)		9 Months Ended
	Apr. 25, 2018	Sep. 30, 2018	Dec. 31, 2017	Jan. 22, 2016
Class Of Stock [Line Items]
Number of votes per share		one vote per share
Common stock, shares authorized		500,000,000	500,000,000
Common stock, shares issued		90,598,259	86,790,624
Common stock, shares outstanding		90,598,259	86,790,624
Treasury stock, shares		0	0
Preferred Stock, shares authorized		25,000,000	25,000,000
Preferred Stock, shares issued		0	0
Preferred Stock, shares outstanding		0	0
Common stock | Stock Repurchase Program
Class Of Stock [Line Items]
Stock repurchase program, announced date		Jan. 25, 2016
Common stock repurchased, Shares		0
Common stock | Maximum | Stock Repurchase Program
Class Of Stock [Line Items]
Authorized to repurchase of common stock				$ 100,000,000
Common stock | Accredited Investor
Class Of Stock [Line Items]
Aggregate purchase price		$ 200,000,000
Investment Agreement | Common stock | Accredited Investor
Class Of Stock [Line Items]
Shares issued and sold	2,820,464
Aggregate purchase price	$ 200,000,000