Cash Equivalents and Investments (Summary of Cash and Available-For-Sale Securities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2019		Dec. 31, 2018		Jun. 30, 2018	[2]	Dec. 31, 2017	[3]
Schedule of Available-For-Sale Securities [Line Items]
Cash and cash equivalents	$ 284,941	[1],[2]	$ 394,590	[3]	$ 395,653		$ 527,266
Cash [Member]
Schedule of Available-For-Sale Securities [Line Items]
Cash and cash equivalents	55,987		167,871
Money Market Funds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Cash and cash equivalents	$ 228,954		$ 226,719

[1] unaudited

[2] unaudited

[3] Note 1