Fair Value of Financial Instruments - Assets and Liabilities at Fair Value (Details) - USD ($)
$ in Thousands |
Dec. 31, 2019 |
Dec. 31, 2018 |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
$ 1,140,940
|
$ 911,490
|
| U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
48,929
|
46,830
|
| Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
190,216
|
91,257
|
| Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
684,881
|
543,238
|
| Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
172,642
|
171,325
|
| Recurring |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Cash, cash equivalents and short-term investments |
85,361
|
84,134
|
| Total assets |
1,182,029
|
936,784
|
| Total liabilities |
7,641
|
7,296
|
| Recurring | U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
48,929
|
46,830
|
| Recurring | Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
190,216
|
91,257
|
| Recurring | Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
684,881
|
543,238
|
| Recurring | Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
172,642
|
171,325
|
| Recurring | Warrant liability |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Warrant liability |
7,641
|
7,296
|
| Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Cash, cash equivalents and short-term investments |
85,361
|
84,134
|
| Total assets |
134,290
|
130,964
|
| Total liabilities |
0
|
0
|
| Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
48,929
|
46,830
|
| Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Warrant liability |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Warrant liability |
0
|
0
|
| Recurring | Significant Other Observable Inputs (Level 2) |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Cash, cash equivalents and short-term investments |
0
|
0
|
| Total assets |
1,047,739
|
805,820
|
| Total liabilities |
0
|
0
|
| Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
190,216
|
91,257
|
| Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
684,881
|
543,238
|
| Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
172,642
|
171,325
|
| Recurring | Significant Other Observable Inputs (Level 2) | Warrant liability |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Warrant liability |
0
|
0
|
| Recurring | Significant Unobservable Inputs (Level 3) |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Cash, cash equivalents and short-term investments |
0
|
0
|
| Total assets |
0
|
0
|
| Total liabilities |
7,641
|
7,296
|
| Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities and obligations of U.S. government agencies |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Significant Unobservable Inputs (Level 3) | Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Fair value of securities |
0
|
0
|
| Recurring | Significant Unobservable Inputs (Level 3) | Warrant liability |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Warrant liability |
$ 7,641
|
$ 7,296
|