Cash Equivalents and Investments (Summary of Cash and Available-For-Sale Securities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	$ 193,451	$ 394,590
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	193,451	394,590
Cash and Cash Equivalents	193,451	394,590	$ 527,266	$ 147,154
Cash [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	62,187	167,871
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	62,187	167,871
Cash and Cash Equivalents	62,187	167,871
Money Market Funds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Adjusted Cost	131,264	226,719
Cash, Cash Equivalents and Available-for-Sale Securities, Fair Value	131,264	226,719
Cash and Cash Equivalents	$ 131,264	$ 226,719