Fair Value of Financial Instruments - Assets and Liabilities at Fair Value (Details) (USD $) | Sep. 30, 2014 | Dec. 31, 2013 |
In Thousands, unless otherwise specified |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | $308,564 | $409,088 |
Warrant liability | 3,312 | 6,371 |
Fair Value, Measurements, Recurring [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Cash and cash equivalents | 137,485 | 55,929 |
Total Assets | 446,049 | 465,017 |
Warrant liability | 3,312 | 6,371 |
Total Liabilities | 3,312 | 6,371 |
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Cash and cash equivalents | 137,485 | 55,929 |
Total Assets | 176,232 | 105,413 |
Warrant liability | 0 | 0 |
Total Liabilities | 0 | 0 |
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Cash and cash equivalents | 0 | 0 |
Total Assets | 269,817 | 359,604 |
Warrant liability | 0 | 0 |
Total Liabilities | 0 | 0 |
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | ' | ' |
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 | 3,312 | 6,371 |
Total Liabilities | 3,312 | 6,371 |
U.S. Treasury securities and obligations of U.S. government agencies [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 83,674 | 106,606 |
U.S. Treasury securities and obligations of U.S. government agencies [Member] | Fair Value, Measurements, Recurring [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 83,674 | 106,606 |
U.S. Treasury securities and obligations of U.S. government agencies [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 38,747 | 49,484 |
U.S. Treasury securities and obligations of U.S. government agencies [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 44,927 | 57,122 |
U.S. Treasury securities and obligations of U.S. government agencies [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 0 | 0 |
Municipal bonds [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 11,979 | 11,933 |
Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 11,979 | 11,933 |
Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 0 | 0 |
Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 11,979 | 11,933 |
Municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 0 | 0 |
Corporate debt securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 156,072 | 217,257 |
Corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 156,072 | 217,257 |
Corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 0 | 0 |
Corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 156,072 | 217,257 |
Corporate debt securities [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 0 | 0 |
Asset-backed securities [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 56,839 | 73,292 |
Asset-backed securities [Member] | Fair Value, Measurements, Recurring [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 56,839 | 73,292 |
Asset-backed securities [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 0 | 0 |
Asset-backed securities [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | 56,839 | 73,292 |
Asset-backed securities [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | ' | ' |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ' | ' |
Available-for-sale Securities | $0 | $0 |