Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	$ 760,782	$ 811,612
Total	926,409	1,078,246
Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	56,839	32,394
Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	626,555	615,034
Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	35,355	29,595
US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	42,033	134,589
Cash equivalents [Member] | Money market funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents	165,627	263,515
Cash equivalents [Member] | Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents		2,000
Cash equivalents [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents		1,119
Short-term investments [Member] | Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	56,839	32,394
Short-term investments [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	388,429	303,567
Short-term investments [Member] | Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	35,355	23,736
Short-term investments [Member] | US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	17,501	29,248
Other long-term investments [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	238,125	311,467
Other long-term investments [Member] | Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments		5,859
Other long-term investments [Member] | US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	24,533	105,341
Level 1 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total	165,627	263,515
Level 1 [Member] | Cash equivalents [Member] | Money market funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents	165,627	263,515
Level 1 [Member] | Cash equivalents [Member] | Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents		0
Level 1 [Member] | Cash equivalents [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents		0
Level 1 [Member] | Short-term investments [Member] | Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	0	0
Level 1 [Member] | Short-term investments [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments		0
Level 1 [Member] | Short-term investments [Member] | Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	0	0
Level 1 [Member] | Short-term investments [Member] | US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	0	0
Level 1 [Member] | Other long-term investments [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	0	0
Level 1 [Member] | Other long-term investments [Member] | Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments		0
Level 1 [Member] | Other long-term investments [Member] | US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	0	0
Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total	760,782	814,731
Level 2 [Member] | Cash equivalents [Member] | Money market funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents	0	0
Level 2 [Member] | Cash equivalents [Member] | Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents		2,000
Level 2 [Member] | Cash equivalents [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Cash equivalents		1,119
Level 2 [Member] | Short-term investments [Member] | Commercial paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	56,839	32,394
Level 2 [Member] | Short-term investments [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	388,429	303,567
Level 2 [Member] | Short-term investments [Member] | Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	35,355	23,736
Level 2 [Member] | Short-term investments [Member] | US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	17,501	29,248
Level 2 [Member] | Other long-term investments [Member] | Corporate notes and bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	238,125	311,467
Level 2 [Member] | Other long-term investments [Member] | Certificates of deposit [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments		5,859
Level 2 [Member] | Other long-term investments [Member] | US government and agency securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Investments	$ 24,533	$ 105,341