Stock-Based Compensation - Summary of Company's Stock Option Plan Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding, Number of Options	340,000	350,000	240,000	340,000	240,000	340,000	240,000	240,000	130,000
Granted, Number of options		110,000	110,000
Exercised, Number of Options	10,000
Exercisable, Number of Options						92,500	47,500
Outstanding, Weighted Average Exercise Price	$ 43.21	$ 42.16	$ 37.19	$ 43.21	$ 37.19	$ 43.21	$ 37.19	$ 37.19	$ 34.82
Granted, Weighted Average Exercise Price		$ 53.00	$ 40.00
Exercised, Weighted Average Exercise Price	$ 6.30
Exercisable, Weighted-Average Exercise Price						$ 36.36	$ 25.81
Outstanding, Weighted-Average Remaining Contractual Term		8 years 6 months	8 years 9 months 18 days	8 years 4 months 24 days	8 years 7 months 6 days	8 years 1 month 6 days	8 years 3 months 18 days	8 years 9 months 18 days	8 years 2 months 12 days
Exercisable, Weighted-Average Remaining Contractual Term						7 years	6 years
Outstanding, Aggregate Intrinsic Value	$ 445	$ 1,329	$ 637	$ 445	$ 749	$ 1,057	$ 1,574	$ 3,278	$ 472
Exercisable, Aggregate Intrinsic Value						$ 627	$ 852