Fair Value of Financial Instruments - Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Assets:
Available-for-sale securities	$ 162,020	$ 251,212
Convertible note hedges	150,509	132,521
Liabilities:
Note hedge warrants	133,424	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	79,273	77,660
Recurring basis
Assets:
Total assets measured at fair value	440,409	416,219
Liabilities:
Contingent consideration	79,273	77,660
Total liabilities	212,697	190,897
Recurring basis | Convertible Note Hedge
Assets:
Convertible note hedges	150,509	132,521
Recurring basis | Note Hedge Warrant Derivatives
Liabilities:
Note hedge warrants	133,424	113,237
Recurring basis | Money market funds
Assets:
Cash and cash equivalents	86,428	32,486
Recurring basis | U.S. Treasury securities
Assets:
Cash and cash equivalents	21,473
Available-for-sale securities	68,980	115,021
Recurring basis | U.S. government-sponsored securities
Assets:
Cash and cash equivalents	19,979
Available-for-sale securities	93,040	136,191
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured at fair value	176,881	147,507
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Cash and cash equivalents	86,428	32,486
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Assets:
Cash and cash equivalents	21,473
Available-for-sale securities	68,980	115,021
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured at fair value	113,019	136,191
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. government-sponsored securities
Assets:
Cash and cash equivalents	19,979
Available-for-sale securities	93,040	136,191
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets:
Total assets measured at fair value	150,509	132,521
Liabilities:
Contingent consideration	79,273	77,660
Total liabilities	212,697	190,897
Recurring basis | Significant Unobservable Inputs (Level 3) | Convertible Note Hedge
Assets:
Convertible note hedges	150,509	132,521
Recurring basis | Significant Unobservable Inputs (Level 3) | Note Hedge Warrant Derivatives
Liabilities:
Note hedge warrants	$ 133,424	$ 113,237