Debt - Expected Future Principal and Interest Payments Based on Debt Balances (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Debt Disclosure [Abstract]
2019, Principal	$ 7,857
2020, Principal	8,571
2021, Principal	8,572
2022, Principal	5,000
Total, Principal	30,000
2019, Interest	2,579
2020, Interest	1,765
2021, Interest	931
2022, Interest	2,560
Total, Interest	7,835
2019, Total	10,436
2020, Total	10,336
2021, Total	9,503
2022, Total	7,560
Total	$ 37,835