Fair Value of Financial Instruments - Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | $ 314,455 | $ 336,501 |
Warrant liability | 1,899 | 3,372 |
U.S. Treasury securities and obligations of U.S. government agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 68,223 | 68,345 |
Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 10,984 | 11,963 |
Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 177,343 | 199,785 |
Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 57,905 | 56,408 |
Fair Value, Measurements, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents | 132,791 | 103,021 |
Total assets | 447,246 | 439,522 |
Warrant liability | 1,899 | 3,372 |
Total liabilities | 1,899 | 3,372 |
Fair Value, Measurements, 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] | | |
Available-for-sale Securities | 68,223 | 68,345 |
Fair Value, Measurements, Recurring | Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 10,984 | 11,963 |
Fair Value, Measurements, Recurring | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 177,343 | 199,785 |
Fair Value, Measurements, Recurring | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 57,905 | 56,408 |
Fair Value, Measurements, 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 and cash equivalents | 132,791 | 103,021 |
Total assets | 174,195 | 142,197 |
Warrant liability | 0 | 0 |
Total liabilities | 0 | 0 |
Fair Value, Measurements, 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] | | |
Available-for-sale Securities | 41,404 | 39,176 |
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 0 | 0 |
Fair Value, Measurements, 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] | | |
Available-for-sale Securities | 0 | 0 |
Fair Value, Measurements, 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] | | |
Available-for-sale Securities | 0 | 0 |
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents | 0 | 0 |
Total assets | 273,051 | 297,325 |
Warrant liability | 0 | 0 |
Total liabilities | 0 | 0 |
Fair Value, Measurements, 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] | | |
Available-for-sale Securities | 26,819 | 29,169 |
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 10,984 | 11,963 |
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 177,343 | 199,785 |
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 57,905 | 56,408 |
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents | 0 | 0 |
Total assets | 0 | 0 |
Warrant liability | 1,899 | 3,372 |
Total liabilities | 1,899 | 3,372 |
Fair Value, Measurements, 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] | | |
Available-for-sale Securities | 0 | 0 |
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 0 | 0 |
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | 0 | 0 |
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available-for-sale Securities | $ 0 | $ 0 |