Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash accounts, Amount	$ 560,670	$ 568,587
Money market funds, Amount	324,823	312,914
Total	$ 885,493	$ 881,501	$ 866,188	$ 822,479
Cash accounts, % of Total	63.32%	64.50%
Money market funds, % of Total	36.68%	35.50%