Fair Value Measurements - Financial Assets and Liabilities Subject to Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial instruments carried at fair value (asset position):
Marketable securities	$ 1,243,469	$ 1,648,745
Financial instruments carried at fair value (liability position):
Long-term contingent consideration	(176,500)	0
Recurring basis
Financial instruments carried at fair value (asset position):
Total financial assets	1,535,278	1,836,605
Financial instruments carried at fair value (liability position):
Total financial liabilities	(183,854)	(448)
Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Total financial assets	1,065,569	1,410,155
Financial instruments carried at fair value (liability position):
Total financial liabilities	0	0
Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Total financial assets	469,709	426,450
Financial instruments carried at fair value (liability position):
Total financial liabilities	(7,354)	(448)
Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Total financial assets	0	0
Financial instruments carried at fair value (liability position):
Total financial liabilities	(176,500)	0
Corporate equity securities | Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	282,084	167,323
Corporate equity securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	261,797	153,733
Corporate equity securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	20,287	13,590
Corporate equity securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
U.S. Treasury securities
Financial instruments carried at fair value (asset position):
Cash equivalents		5,966
U.S. Treasury securities | Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	12,733	6,026
U.S. Treasury securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	12,733	6,026
U.S. Treasury securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
U.S. Treasury securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents		7,123
Government-sponsored enterprise securities | Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities		10,704
Government-sponsored enterprise securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Marketable securities		10,704
Government-sponsored enterprise securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Marketable securities		0
Government-sponsored enterprise securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Marketable securities		0
Corporate debt securities | Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	301,799	233,665
Corporate debt securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Corporate debt securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	301,799	233,665
Corporate debt securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents	29,472	58,268
Commercial paper | Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	102,356	100,390
Commercial paper | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Commercial paper | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	102,356	100,390
Commercial paper | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Money market funds | Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents	791,039	1,226,603
Money market funds | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents	791,039	1,226,603
Money market funds | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Money market funds | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Corporate debt securities | Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents	6,070
Corporate debt securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents	0
Corporate debt securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents	6,070
Corporate debt securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents	0
U.S. Treasury securities | Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents		5,966
U.S. Treasury securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents		5,966
U.S. Treasury securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents		0
U.S. Treasury securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents		0
Government-sponsored enterprise securities | Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents		7,123
Government-sponsored enterprise securities | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents		7,123
Government-sponsored enterprise securities | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents		0
Government-sponsored enterprise securities | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents		0
Commercial paper | Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents	29,472	58,268
Commercial paper | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Commercial paper | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents	29,472	58,268
Commercial paper | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Other assets | Foreign currency forward contracts | Recurring basis
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts		1,514
Other assets | Foreign currency forward contracts | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts		0
Other assets | Foreign currency forward contracts | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts		1,514
Other assets | Foreign currency forward contracts | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts		0
Prepaid expenses and other current assets | Foreign currency forward contracts | Recurring basis
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	9,725	19,023
Prepaid expenses and other current assets | Foreign currency forward contracts | Recurring basis | Level 1
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0	0
Prepaid expenses and other current assets | Foreign currency forward contracts | Recurring basis | Level 2
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	9,725	19,023
Prepaid expenses and other current assets | Foreign currency forward contracts | Recurring basis | Level 3
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0	0
Other current liabilities | Recurring basis
Financial instruments carried at fair value (liability position):
Long-term contingent consideration	(176,500)
Other current liabilities | Recurring basis | Level 1
Financial instruments carried at fair value (liability position):
Long-term contingent consideration	0
Other current liabilities | Recurring basis | Level 2
Financial instruments carried at fair value (liability position):
Long-term contingent consideration	0
Other current liabilities | Recurring basis | Level 3
Financial instruments carried at fair value (liability position):
Long-term contingent consideration	(176,500)
Other current liabilities | Foreign currency forward contracts | Recurring basis
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	(5,533)	(340)
Other current liabilities | Foreign currency forward contracts | Recurring basis | Level 1
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	0	0
Other current liabilities | Foreign currency forward contracts | Recurring basis | Level 2
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	(5,533)	(340)
Other current liabilities | Foreign currency forward contracts | Recurring basis | Level 3
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	0	0
Other long-term liabilities | Foreign currency forward contracts | Recurring basis
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	(1,821)	(108)
Other long-term liabilities | Foreign currency forward contracts | Recurring basis | Level 1
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	0	0
Other long-term liabilities | Foreign currency forward contracts | Recurring basis | Level 2
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	(1,821)	(108)
Other long-term liabilities | Foreign currency forward contracts | Recurring basis | Level 3
Financial instruments carried at fair value (liability position):
Foreign currency forward contracts	$ 0	$ 0