Summary of Potential Number of Shares Vesting under Restricted Stock Unit Awards (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) shares
Disclosure Of Restricted Stock Unit [Abstract]
Number of shares vesting at IRR of at least 7% | shares	58,545
Number of shares vesting at IRR of at least 9.5% | shares	117,089
Number of shares vesting at IRR of at least 13% | shares	175,634
Value at grant at IRR of at least 7% | $	$ 2,481
Value at grant at IRR of at least 9.5% | $	4,961
Value at grant at IRR of at least 13% | $	$ 7,442