Financial Instruments (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financial Instruments [Line Items]
Realized loss or gains	$ 0	$ 0	$ 0	$ 0
US Treasury and Government Short-term Debt Securities [Member]
Financial Instruments [Line Items]
Available for Sale Securities Maturity Period			5 years