Financial Instruments and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Recurring basis
Assets
Total assets measured at estimated fair value	$ 395,187	$ 306,741
Recurring basis | U.S. government securities
Assets
Total assets measured at estimated fair value	13,999	53,834
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	48,964	18,915
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	285,309	233,992
Recurring basis | Equity Investment | Common Stock | Armata
Assets
Total assets measured at estimated fair value	27,003
Recurring basis | Equity Investment | Warrants | Armata
Assets
Total assets measured at estimated fair value	19,912
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	312,312	233,992
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. government securities
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Money market funds
Assets
Total assets measured at estimated fair value	285,309	233,992
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Equity Investment | Common Stock | Armata
Assets
Total assets measured at estimated fair value	27,003
Recurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Equity Investment | Warrants | Armata
Assets
Total assets measured at estimated fair value	0
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	82,875	72,749
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Assets
Total assets measured at estimated fair value	13,999	53,834
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	48,964	18,915
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Other Observable Inputs, Level 2 | Equity Investment | Common Stock | Armata
Assets
Total assets measured at estimated fair value	0
Recurring basis | Significant Other Observable Inputs, Level 2 | Equity Investment | Warrants | Armata
Assets
Total assets measured at estimated fair value	19,912
Recurring basis | Significant Unobservable Inputs, Level 3
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government securities
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Equity Investment | Common Stock | Armata
Assets
Total assets measured at estimated fair value	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Equity Investment | Warrants | Armata
Assets
Total assets measured at estimated fair value	0
Nonrecurring basis | Debt
Debt
2023 Notes	216,886	243,394
2025 Notes	191,538	208,976
Total fair value of liabilities	408,424	452,370
Nonrecurring basis | Quoted Price in Active Markets for Identical Assets, Level 1 | Debt
Debt
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Debt
Debt
2023 Notes	216,886	243,394
2025 Notes	191,538	208,976
Total fair value of liabilities	408,424	452,370
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Debt
Debt
2023 Notes	0	0
2025 Notes	0	0
Total fair value of liabilities	$ 0	$ 0