Summary of Potential Number of Shares Vesting under Restricted Stock Unit Awards (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) shares
Disclosure Of Restricted Stock Unit [Abstract]
Number of shares vesting at IRR of at least 7% | shares	74,867
Number of shares vesting at IRR of at least 9.5% | shares	149,735
Number of shares vesting at IRR of at least 13% | shares	224,602
Value at grant at IRR of at least 7% | $	$ 2,922
Value at grant at IRR of at least 9.5% | $	5,844
Value at grant at IRR of at least 13% | $	$ 8,766