Summary of Potential Number of Shares Vesting under Restricted Stock Unit Awards (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) shares
Disclosure Of Restricted Stock Unit [Abstract]
Number of shares vesting at IRR of at least 7% | shares	76,065
Number of shares vesting at IRR of at least 9.5% | shares	152,129
Number of shares vesting at IRR of at least 13% | shares	228,194
Value at grant at IRR of at least 7% | $	$ 2,967
Value at grant at IRR of at least 9.5% | $	5,934
Value at grant at IRR of at least 13% | $	$ 8,901