Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended
	Jul. 19, 2019	May 03, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Stockholders Equity Note Disclosure [Line Items]
Common stock, shares issued	1,587,000	3,144,531	52,052,581		52,052,581		43,917,378
Common stock issue price per share	$ 87.00	$ 64.00
Exercised number of shares					83,306
Net proceeds from public offering	$ 131,100	$ 189,600			$ 320,713
Stock-based compensation expense			$ 3,175	$ 2,779	$ 9,459	$ 7,672
Stock options, outstanding			959,916		959,916		998,226
Closing price of common stock			$ 76.69		$ 76.69
Aggregate intrinsic value of stock options exercised					$ 5,200	$ 3,500
Weighted average grant date fair value of share-based awards granted					$ 30.07	$ 18.56
Total fair value of stock options vested					$ 2,900	$ 2,100
Total unrecognized compensation cost			$ 37,700		$ 37,700
Unrecognized compensation cost, weighted average remaining requisite service period					4 years 1 month 17 days
Number of unvested options and restricted stock units			1,676,976		1,676,976
2018 Plan
Stockholders Equity Note Disclosure [Line Items]
Common stock shares reserved for Issuance							2,778,000
Incentive options, vesting period			2,571,611		2,571,611
Employee Stock Option | Minimum
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting period					3 years
Employee Stock Option | Minimum | Vest Over Three Year
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting percentage					20.00%
Employee Stock Option | Maximum
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting period					5 years
Incentive options, term					10 years
Employee Stock Option | Maximum | Vest Over Five Year
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting percentage					33.00%
Non-Employee Directors
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting period					1 year
Option To Purchase Common Stock
Stockholders Equity Note Disclosure [Line Items]
Stock options, outstanding			959,916	1,033,312	959,916	1,033,312
Restricted Stock Units (RSUs)
Stockholders Equity Note Disclosure [Line Items]
Restricted stock units, outstanding			740,213	707,716	740,213	707,716	707,413
Closing price of common stock			$ 76.69		$ 76.69
Aggregate intrinsic value of restricted stock units vested					$ 15,500	$ 5,200
Weighted average grant date fair value of restricted stock units granted					$ 33.46	$ 30.21
Total grant date fair value of restricted stock units vested					$ 7,600	$ 4,100
Unvested Options
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting period					5 years
Underwriters | Common Stock
Stockholders Equity Note Disclosure [Line Items]
Exercised number of shares	207,000	410,156