Income Taxes - Reconciliation of the Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 29	$ 265	$ 265
Additions attributable to state and local apportionment	2,650
Reductions for tax positions of prior years	(29)	(236)	0
Balance at end of year	$ 2,650	$ 29	$ 265