Changes in Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance, Amount	$ 244,641
Other comprehensive loss before reclassifications	122
Ending Balance, Amount	599,003
Cumulative Translation Adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance, Amount	(397)
Other comprehensive loss before reclassifications	(304)
Ending Balance, Amount	(701)
Unrealized Gain on Investments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance, Amount	(326)
Other comprehensive loss before reclassifications	426
Ending Balance, Amount	100
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance, Amount	(723)
Ending Balance, Amount	$ (601)