Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 19,176
Less than 12 Months, Unrealized Losses	(3)	$ (391)
12 Months or Greater, Unrealized Losses		(23)
Total, Fair Value	19,176
Total, Unrealized Losses	(3)	(414)
Less than 12 Months, Fair Value		298,407
12 Months or Greater, Fair Value		12,424
Total, Fair Value		310,831
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	19,176
Less than 12 Months, Unrealized Losses	(3)	(354)
12 Months or Greater, Unrealized Losses		(23)
Total, Fair Value	19,176
Total, Unrealized Losses	$ (3)	(377)
Less than 12 Months, Fair Value		245,283
12 Months or Greater, Fair Value		12,424
Total, Fair Value		257,707
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Unrealized Losses		(37)
Total, Unrealized Losses		(37)
Less than 12 Months, Fair Value		53,124
Total, Fair Value		$ 53,124