Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Total assets	$ 385,824	$ 429,018
Liabilities:
Total liabilities	3,304	1,856
Common stock warrants
Liabilities:
Total liabilities	3,304	1,856
Money market funds
Assets:
Total assets	239,753	193,867
Corporate obligations
Assets:
Total assets	101,130	138,369
U.S. Treasury securities
Assets:
Total assets	36,691	83,819
Certificates of deposit
Assets:
Total assets	248	493
Mortgage and other asset backed securities
Assets:
Total assets	8,002	12,470
Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	239,753	193,867
Liabilities:
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Common stock warrants
Liabilities:
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Total assets	239,753	193,867
Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Total assets	0	0
Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Total assets	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Total assets	146,071	235,151
Liabilities:
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Common stock warrants
Liabilities:
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Total assets	0	0
Significant Other Observable Inputs (Level 2) | Corporate obligations
Assets:
Total assets	101,130	138,369
Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Total assets	36,691	83,819
Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Total assets	248	493
Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Total assets	8,002	12,470
Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Liabilities:
Total liabilities	3,304	1,856
Significant Unobservable Inputs (Level 3) | Common stock warrants
Liabilities:
Total liabilities	3,304	1,856
Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Total assets	0	0
Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Total assets	$ 0	$ 0