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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Stockholders Equity Note Disclosure [Line Items]
Common stock, shares issued	1,587,000	3,144,531	52,606,700		52,606,700		52,078,258
Common stock issue price per share	$ 87.00	$ 64.00
Exercised number of shares					303,343
Net proceeds from public offering	$ 131,100	$ 189,600				$ 320,713
Stock-based compensation expense			$ 4,225	$ 3,175	$ 12,492	$ 9,459
Incentive options, term					10 years
Stock options, outstanding			723,914	959,916	723,914	959,916	957,559
Closing price of common stock			$ 147.54		$ 147.54
Aggregate intrinsic value of stock options exercised					$ 30,700	$ 5,200
Weighted average grant date fair value of share-based awards granted					$ 48.13	$ 30.07
Total fair value of stock options vested					$ 2,600	$ 2,900
Total unrecognized compensation cost			$ 47,000		$ 47,000
Unrecognized compensation cost, weighted average remaining requisite service period					3 years 7 months 9 days
Number of unvested options and restricted stock units			1,833,239		1,833,239
2018 Plan
Stockholders Equity Note Disclosure [Line Items]
Common stock shares reserved for Issuance								2,778,000
Incentive options, vesting period			2,325,314		2,325,314
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
Employee Stock Option | Maximum | Vest Over Three 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					9 years
Unvested Options
Stockholders Equity Note Disclosure [Line Items]
Incentive options, vesting period					5 years
Restricted Stock Units and Performance Stock Units
Stockholders Equity Note Disclosure [Line Items]
Restricted stock units, outstanding			675,567	740,213	675,567	740,213	734,984
Closing price of common stock			$ 147.54		$ 147.54
Aggregate intrinsic value of restricted stock units vested					$ 25,000	$ 15,500
Weighted average grant date fair value of restricted stock units granted					$ 66.00	$ 33.46
Total grant date fair value of restricted stock units vested					$ 9,800	$ 7,600
Underwriters | Common Stock
Stockholders Equity Note Disclosure [Line Items]
Exercised number of shares	207,000	410,156