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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