Income Taxes - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax expense (benefit)	$ 0	$ 0	$ 0
State income tax expense (benefit), continuing operationsi	$ 0	$ 0	$ 0
Statutory tax (benefit) expense	(35.00%)	(35.00%)	(35.00%)
Net operating loss carryforwards	$ 254,300
Deferred tax asset, provisional income tax expense (benefit)	$ 82,500