Long-Term Investments - Rollforward of Credit (Gain) Loss Recognized in Earnings on Available-for-Sale Auction Rate Securities (Detail) - Failed Auction Security [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at the beginning of the period	$ 48	$ 59	$ 72
Reductions in the amount related to credit gain for which other-than-temporary impairment was not previously recognized	(7)	(11)	(13)
Balance at the end of the period	$ 41	$ 48	$ 59