Fair Value of Financial Instruments - Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Assets:
Available-for-sale securities	$ 136,936	$ 178,107
Convertible note hedges	137,302	86,466
Liabilities:
Note hedge warrants	(120,121)	(75,328)
Contingent Consideration	(96,300)
Fair value transfers
Fair value transfer between measurement levels	0		$ 0
Convertible Note Hedge
Assets:
Convertible note hedges	91,900
Note Hedge Warrants
Liabilities:
Note hedge warrants	(70,800)
Significant Unobservable Inputs (Level 3)
Liabilities:
Contingent Consideration	(96,260)
Recurring basis
Assets:
Total assets measured at fair value	439,764	522,816
Liabilities:
Contingent Consideration	(96,260)
Total liabilities measured at fair value	(216,381)	(75,328)
Recurring basis | Convertible Note Hedge
Assets:
Convertible note hedges	137,302	86,466
Recurring basis | Note Hedge Warrants
Liabilities:
Note hedge warrants	(120,121)	(75,328)
Recurring basis | Money market funds
Assets:
Cash and cash equivalents	163,521	254,903
Recurring basis | U.S. Treasury securities
Assets:
Available-for-sale securities	40,404	50,091
Recurring basis | U.S. government-sponsored securities
Assets:
Cash and cash equivalents	2,005	3,340
Available-for-sale securities	96,532	128,016
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Total assets measured at fair value	203,925	304,994
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Assets:
Cash and cash equivalents	163,521	254,903
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Assets:
Available-for-sale securities	40,404	50,091
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Total assets measured at fair value	98,537	131,356
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. government-sponsored securities
Assets:
Cash and cash equivalents	2,005	3,340
Available-for-sale securities	96,532	128,016
Recurring basis | Significant Unobservable Inputs (Level 3)
Assets:
Total assets measured at fair value	137,302	86,466
Liabilities:
Contingent Consideration	(96,260)
Total liabilities measured at fair value	(216,381)	(75,328)
Recurring basis | Significant Unobservable Inputs (Level 3) | Convertible Note Hedge
Assets:
Convertible note hedges	137,302	86,466
Recurring basis | Significant Unobservable Inputs (Level 3) | Note Hedge Warrants
Liabilities:
Note hedge warrants	$ (120,121)	$ (75,328)