Fair Value Measurements - Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Assets, fair value	$ 1,020,595	$ 188,764
Liabilities:
Liabilities, fair value	12,731	13,961
Transferred over Time | Phase 3 Clinical Trial
Liabilities:
Contract asset	10,000
Maximum | Transferred over Time | Development, Regulatory, & Commercial Milestones and Tiered Royalties
Liabilities:
Contract asset	375,000
Maximum | Transferred over Time | Milestone Payments and Royalties
Liabilities:
Contract asset	529,000
Current portion of contingent liabilities - Crystal
Liabilities:
Liabilities, fair value	7,401	8,401
Current contingent liabilities-CyDex
Liabilities:
Liabilities, fair value	514	1,589
Long-term contingent liabilities-Metabasis
Liabilities:
Liabilities, fair value	4,816	3,971
Short-term investments
Assets:
Assets, fair value	996,311	181,041
Note Receivable - Viking Therapeutics
Assets:
Assets, fair value	0	3,877
Investment in Warrants
Assets:
Assets, fair value	24,284	3,846
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Assets, fair value	131,150	5,742
Liabilities:
Liabilities, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Current portion of contingent liabilities - Crystal
Liabilities:
Liabilities, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Current contingent liabilities-CyDex
Liabilities:
Liabilities, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term contingent liabilities-Metabasis
Liabilities:
Liabilities, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments
Assets:
Assets, fair value	106,866	1,896
Quoted Prices in Active Markets for Identical Assets (Level 1) | Note Receivable - Viking Therapeutics
Assets:
Assets, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investment in Warrants
Assets:
Assets, fair value	24,284	3,846
Significant Other Observable Inputs (Level 2)
Assets:
Assets, fair value	889,445	179,145
Liabilities:
Liabilities, fair value	4,816	3,971
Significant Other Observable Inputs (Level 2) | Current portion of contingent liabilities - Crystal
Liabilities:
Liabilities, fair value	0	0
Significant Other Observable Inputs (Level 2) | Current contingent liabilities-CyDex
Liabilities:
Liabilities, fair value	0	0
Significant Other Observable Inputs (Level 2) | Long-term contingent liabilities-Metabasis
Liabilities:
Liabilities, fair value	4,816	3,971
Significant Other Observable Inputs (Level 2) | Short-term investments
Assets:
Assets, fair value	889,445	179,145
Significant Other Observable Inputs (Level 2) | Note Receivable - Viking Therapeutics
Assets:
Assets, fair value	0	0
Significant Other Observable Inputs (Level 2) | Investment in Warrants
Assets:
Assets, fair value	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Assets, fair value	0	3,877
Liabilities:
Liabilities, fair value	7,915	9,990
Significant Unobservable Inputs (Level 3) | Current portion of contingent liabilities - Crystal
Liabilities:
Liabilities, fair value	7,401	8,401
Significant Unobservable Inputs (Level 3) | Current contingent liabilities-CyDex
Liabilities:
Liabilities, fair value	514	1,589
Significant Unobservable Inputs (Level 3) | Long-term contingent liabilities-Metabasis
Liabilities:
Liabilities, fair value	0	0
Significant Unobservable Inputs (Level 3) | Short-term investments
Assets:
Assets, fair value	0	0
Significant Unobservable Inputs (Level 3) | Note Receivable - Viking Therapeutics
Assets:
Assets, fair value	0	3,877
Significant Unobservable Inputs (Level 3) | Investment in Warrants
Assets:
Assets, fair value	$ 0	$ 0