Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Level 1 \| Variable Interest Entity, Primary Beneficiary \| BioAxone Biosciences, Inc
Financial instruments carried at fair value (asset position):
Cash and cash equivalents	$ 1,500
Recurring basis
Financial instruments carried at fair value (asset position):
Total financial assets	1,108,253	$ 546,765
Total financial liabilities	(14,508)	(144)
Recurring basis \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents:	614,951	280,560
Recurring basis \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents:	12,678
Marketable securities:	2,303	15,508
Recurring basis \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents:	57,357
Marketable securities:	80,263	59,404
Recurring basis \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities:	74,821	64,560
Recurring basis \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities:	265,867	111,140
Recurring basis \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets:	13	14,407
Recurring basis \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets:		1,186
Recurring basis \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, current portion:	(13,642)	(144)
Recurring basis \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion:	(866)
Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Total financial assets	704,753	360,628
Total financial liabilities	0	0
Recurring basis \| Level 1 \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents:	614,951	280,560
Recurring basis \| Level 1 \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents:	12,678
Marketable securities:	2,303	15,508
Recurring basis \| Level 1 \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents:	0
Marketable securities:	0	0
Recurring basis \| Level 1 \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities:	74,821	64,560
Recurring basis \| Level 1 \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities:	0	0
Recurring basis \| Level 1 \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets:	0	0
Recurring basis \| Level 1 \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets:		0
Recurring basis \| Level 1 \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, current portion:	0	0
Recurring basis \| Level 1 \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion:	0
Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Total financial assets	403,500	186,137
Total financial liabilities	(14,508)	(144)
Recurring basis \| Level 2 \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents:	0	0
Recurring basis \| Level 2 \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents:	0
Marketable securities:	0	0
Recurring basis \| Level 2 \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents:	57,357
Marketable securities:	80,263	59,404
Recurring basis \| Level 2 \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities:	0	0
Recurring basis \| Level 2 \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities:	265,867	111,140
Recurring basis \| Level 2 \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets:	13	14,407
Recurring basis \| Level 2 \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets:		1,186
Recurring basis \| Level 2 \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, current portion:	(13,642)	(144)
Recurring basis \| Level 2 \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion:	(866)
Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Total financial assets	0	0
Total financial liabilities	0	0
Recurring basis \| Level 3 \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents:	0	0
Recurring basis \| Level 3 \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents:	0
Marketable securities:	0	0
Recurring basis \| Level 3 \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents:	0
Marketable securities:	0	0
Recurring basis \| Level 3 \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities:	0	0
Recurring basis \| Level 3 \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities:	0	0
Recurring basis \| Level 3 \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets:	0	0
Recurring basis \| Level 3 \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets:		0
Recurring basis \| Level 3 \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, current portion:	0	$ 0
Recurring basis \| Level 3 \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion:	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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