Fair Value Measurements - Financial Instrument at Fair Value Schedule (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Available-for-sale debt securities	$ 157,676	$ 216,169
Corporate obligations
Assets:
Available-for-sale debt securities	39,218	124,880
Certificates of deposit
Assets:
Available-for-sale debt securities	247	493
Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	4,999	12,469
Recurring
Assets:
Total assets	358,171	429,018
Liabilities:
Total liabilities		1,856
Recurring | Warrants to purchase common stock
Liabilities:
Total liabilities		1,856
Recurring | Money market funds
Assets:
Available-for-sale debt securities	186,379	193,867
Recurring | Corporate obligations
Assets:
Available-for-sale debt securities	43,336	138,369
Recurring | U.S. Treasury securities
Assets:
Available-for-sale debt securities	123,210	83,819
Recurring | Certificates of deposit
Assets:
Available-for-sale debt securities	247	493
Recurring | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	4,999	12,470
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1)
Assets:
Total assets	186,379	193,867
Liabilities:
Total liabilities		0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Warrants to purchase common stock
Liabilities:
Total liabilities		0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Money market funds
Assets:
Available-for-sale debt securities	186,379	193,867
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Corporate obligations
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Certificates of deposit
Assets:
Available-for-sale debt securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Items (Level 1) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	171,792	235,151
Liabilities:
Total liabilities		0
Recurring | Significant Other Observable Inputs (Level 2) | Warrants to purchase common stock
Liabilities:
Total liabilities		0
Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Corporate obligations
Assets:
Available-for-sale debt securities	43,336	138,369
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	123,210	83,819
Recurring | Significant Other Observable Inputs (Level 2) | Certificates of deposit
Assets:
Available-for-sale debt securities	247	493
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	4,999	12,470
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0	0
Liabilities:
Total liabilities		1,856
Recurring | Significant Unobservable Inputs (Level 3) | Warrants to purchase common stock
Liabilities:
Total liabilities		1,856
Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate obligations
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Certificates of deposit
Assets:
Available-for-sale debt securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage and other asset backed securities
Assets:
Available-for-sale debt securities	$ 0	$ 0