Fair Value Measurements - Financial Assets and Liabilities Subject to Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial instruments carried at fair value (liability positions):
Long-term contingent consideration	$ (175,000)	$ 0
Recurring basis
Financial instruments carried at fair value (asset positions):
Total financial assets	2,173,085	1,836,605
Financial instruments carried at fair value (liability positions):
Total financial liabilities	(175,095)	(448)
Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Total financial assets	1,730,298	1,410,155
Financial instruments carried at fair value (liability positions):
Total financial liabilities	0	0
Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Total financial assets	442,787	426,450
Financial instruments carried at fair value (liability positions):
Total financial liabilities	(95)	(448)
Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Total financial assets	0	0
Financial instruments carried at fair value (liability positions):
Total financial liabilities	(175,000)	0
Other current liabilities | Recurring basis
Financial instruments carried at fair value (liability positions):
Long-term contingent consideration	(175,000)
Other current liabilities | Recurring basis | Level 1
Financial instruments carried at fair value (liability positions):
Long-term contingent consideration	0
Other current liabilities | Recurring basis | Level 2
Financial instruments carried at fair value (liability positions):
Long-term contingent consideration	0
Other current liabilities | Recurring basis | Level 3
Financial instruments carried at fair value (liability positions):
Long-term contingent consideration	(175,000)
Money market funds | Recurring basis
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	1,476,254	1,226,603
Money market funds | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	1,476,254	1,226,603
Money market funds | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Money market funds | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
U.S. Treasury securities | Recurring basis
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	5,893	5,966
Marketable securities		6,026
U.S. Treasury securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	5,893	5,966
Marketable securities		6,026
U.S. Treasury securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Marketable securities		0
U.S. Treasury securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Marketable securities		0
Government-sponsored enterprise securities | Recurring basis
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	13,370	7,123
Marketable securities	14,217	10,704
Government-sponsored enterprise securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	13,370	7,123
Marketable securities	14,217	10,704
Government-sponsored enterprise securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Marketable securities	0	0
Government-sponsored enterprise securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Marketable securities	0	0
Commercial paper | Recurring basis
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	50,119	58,268
Marketable securities	82,938	100,390
Commercial paper | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Marketable securities	0	0
Commercial paper | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	50,119	58,268
Marketable securities	82,938	100,390
Commercial paper | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Cash and cash equivalents	0	0
Marketable securities	0	0
Corporate equity securities | Recurring basis
Financial instruments carried at fair value (asset positions):
Marketable securities	236,184	167,323
Corporate equity securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Marketable securities	220,564	153,733
Corporate equity securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Marketable securities	15,620	13,590
Corporate equity securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Marketable securities	0	0
Corporate debt securities | Recurring basis
Financial instruments carried at fair value (asset positions):
Marketable securities	265,051	233,665
Corporate debt securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Marketable securities	0	0
Corporate debt securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Marketable securities	265,051	233,665
Corporate debt securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Marketable securities	0	0
Foreign currency forward contracts | Other current liabilities | Recurring basis
Financial instruments carried at fair value (liability positions):
Derivative liability current	(85)	(340)
Foreign currency forward contracts | Other current liabilities | Recurring basis | Level 1
Financial instruments carried at fair value (liability positions):
Derivative liability current	0	0
Foreign currency forward contracts | Other current liabilities | Recurring basis | Level 2
Financial instruments carried at fair value (liability positions):
Derivative liability current	(85)	(340)
Foreign currency forward contracts | Other current liabilities | Recurring basis | Level 3
Financial instruments carried at fair value (liability positions):
Derivative liability current	0	0
Foreign currency forward contracts | Other long-term liabilities | Recurring basis
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	(10)	(108)
Foreign currency forward contracts | Other long-term liabilities | Recurring basis | Level 1
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	0	0
Foreign currency forward contracts | Other long-term liabilities | Recurring basis | Level 2
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	(10)	(108)
Foreign currency forward contracts | Other long-term liabilities | Recurring basis | Level 3
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	0	0
Foreign currency forward contracts | Prepaid expenses and other current assets | Recurring basis
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	27,253	19,023
Foreign currency forward contracts | Prepaid expenses and other current assets | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0	0
Foreign currency forward contracts | Prepaid expenses and other current assets | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	27,253	19,023
Foreign currency forward contracts | Prepaid expenses and other current assets | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0	0
Foreign currency forward contracts | Other assets | Recurring basis
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	1,806
Total financial assets		1,514
Foreign currency forward contracts | Other assets | Recurring basis | Level 1
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0
Total financial assets		0
Foreign currency forward contracts | Other assets | Recurring basis | Level 2
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	1,806
Total financial assets		1,514
Foreign currency forward contracts | Other assets | Recurring basis | Level 3
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	$ 0
Total financial assets		$ 0