Investments - Summary of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 827,189	$ 762,336
Gross Unrealized Gains	12,337	98
Gross Unrealized Losses	(1,012)	(1,652)
Estimated Fair Value	838,514	760,782
Due in 1 year or less	496,505
Due in 1 to 2 years	319,795
Total	816,300
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,075	56,839
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	68,075	56,839
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	656,402	628,054
Gross Unrealized Gains	123	91
Gross Unrealized Losses	(878)	(1,590)
Estimated Fair Value	655,647	626,555
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,389	35,355
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	36,389	35,355
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	56,323	42,088
Gross Unrealized Gains	0	7
Gross Unrealized Losses	(134)	(62)
Estimated Fair Value	56,189	$ 42,033
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,000
Gross Unrealized Gains	12,214
Gross Unrealized Losses	0
Estimated Fair Value	$ 22,214

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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