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Fair Value of Financial Instruments - Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | $ 1,017,607 | $ 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 | 48,765 | 46,830
Municipal debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | 93,074 | 91,257
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | 651,972 | 543,238
Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | 171,431 | 171,325
Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash, cash equivalents and short-term investments | 88,100 | 84,134
Total assets | 1,053,342 | 936,784
Warrant liability | 9,679 | 7,296
Total liabilities | 9,679 | 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 | 48,765 | 46,830
Recurring | Municipal debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | 93,074 | 91,257
Recurring | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | 651,972 | 543,238
Recurring | Asset-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of securities | 171,431 | 171,325
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 | 88,100 | 84,134
Total assets | 136,865 | 130,964
Warrant liability | 0 | 0
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 | 48,765 | 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 | 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 | 916,477 | 805,820
Warrant liability | 0 | 0
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 | 93,074 | 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 | 651,972 | 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 | 171,431 | 171,325
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
Warrant liability | 9,679 | 7,296
Total liabilities | 9,679 | 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

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