Quarterly Financial Information (Unaudited) (Amortization Of Stock-Based Compensation) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Stock compensation	$ 1,948	$ 8,722	$ 2,824	$ 3,820	$ 2,488	$ 4,907	$ 8,234	$ 30,866	$ 17,314	$ 46,495	$ 46,983
Cost of revenues
Quarterly Financial Information [Line Items]
Stock compensation	320	384	433	629	133	656	2,409	1,643	1,766	4,841	5,886
Selling and marketing
Quarterly Financial Information [Line Items]
Stock compensation	808	1,461	1,532	1,446	516	2,012	1,934	6,505	5,247	10,967	11,502
Research and development
Quarterly Financial Information [Line Items]
Stock compensation	462	537	450	821	682	910	1,494	2,816	2,270	5,902	5,193
General and administrative
Quarterly Financial Information [Line Items]
Stock compensation	$ 358	$ 6,340	$ 409	$ 924	$ 1,157	$ 1,329	$ 2,397	$ 19,902	$ 8,031	$ 24,785	$ 24,402