Fair Value Measurements - Schedule of Change in Valuation of Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Equity securities and warrants beginning balance	$ 4,668	$ 2,768	$ 2,768	$ 2,768
Purchase of additional common stock		0	500	0
Investment impairment charges	(4,449)		(4,678)
Equity securities and warrants ending balance	$ 219	2,768	219	2,768
ASU 2016-01 [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Impact on remeasurement of equity investments recorded within other current assets or long term assets		$ 0	$ 1,629	$ 0