Cash Equivalents and Investments (Summary of Cash and Available-For-Sale Securities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019		Dec. 31, 2018		Sep. 30, 2018	[2]	Dec. 31, 2017	[3]
Schedule of Available-For-Sale Securities [Line Items]
Cash and cash equivalents	$ 294,270	[1],[2]	$ 394,590	[3]	$ 400,984		$ 527,266
Cash [Member]
Schedule of Available-For-Sale Securities [Line Items]
Cash and cash equivalents	64,039		167,871
Money Market Funds [Member]
Schedule of Available-For-Sale Securities [Line Items]
Cash and cash equivalents	$ 230,231		$ 226,719

[1] unaudited

[2] unaudited

[3] Note 1