Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash accounts, Amount	$ 528,134	$ 488,504
Money market funds, Amount	391,437	333,975
Total	$ 919,571	$ 822,479	$ 837,916	$ 784,168
Cash accounts, % of Total	57.43%	59.39%
Money market funds, % of Total	42.57%	40.61%