Fair Value of Financial Instruments - Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Assets:
Available-for-sale securities	$ 90,763	$ 251,212
Convertible note hedges	171,880	132,521
Liabilities:
Note hedge warrants	149,458	113,237
Fair value transfers
Fair value transfer between measurement levels	0		$ 0
Convertible Note Hedge
Assets:
Convertible note hedges	91,900
Note Hedge Warrant Derivatives
Liabilities:
Note hedge warrants	70,800
Significant Unobservable Inputs (Level 3)
Liabilities:
Contingent consideration	86,198	77,660
Recurring basis
Assets:
Total assets measured at fair value	432,977	416,219
Liabilities:
Contingent consideration	86,198	77,660
Total liabilities	235,656	190,897
Recurring basis | Convertible Note Hedge
Assets:
Convertible note hedges	171,880	132,521
Recurring basis | Note Hedge Warrant Derivatives
Liabilities:
Note hedge warrants	149,458	113,237
Recurring basis | Money market funds
Assets:
Cash and cash equivalents	90,745	32,486
Recurring basis | U.S. Treasury securities
Assets:
Cash and cash equivalents	16,743
Available-for-sale securities	20,490	115,021
Recurring basis | U.S. government-sponsored securities
Assets:
Cash and cash equivalents	62,846
Available-for-sale securities	70,273	136,191
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured at fair value	127,978	147,507
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Cash and cash equivalents	90,745	32,486
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Assets:
Cash and cash equivalents	16,743
Available-for-sale securities	20,490	115,021
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured at fair value	133,119	136,191
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. government-sponsored securities
Assets:
Cash and cash equivalents	62,846
Available-for-sale securities	70,273	136,191
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets:
Total assets measured at fair value	171,880	132,521
Liabilities:
Contingent consideration	86,198	77,660
Total liabilities	235,656	190,897
Recurring basis | Significant Unobservable Inputs (Level 3) | Convertible Note Hedge
Assets:
Convertible note hedges	171,880	132,521
Recurring basis | Significant Unobservable Inputs (Level 3) | Note Hedge Warrant Derivatives
Liabilities:
Note hedge warrants	$ 149,458	$ 113,237