Fair Value of Financial Instruments - Assets and Liabilities at Fair Value (Details) - USD ($)
$ in Thousands |
Sep. 30, 2016 |
Dec. 31, 2015 |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
$ 641,572
|
$ 559,235
|
| Warrant liability |
1,654
|
1,467
|
| 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 |
57,877
|
84,482
|
| Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
40,933
|
20,079
|
| Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
346,329
|
333,327
|
| Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
122,069
|
100,793
|
| Fair Value, Measurements, Recurring |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Cash, cash equivalents and short-term investments |
118,886
|
77,872
|
| Total assets |
686,094
|
616,553
|
| Warrant liability |
1,654
|
1,467
|
| Total liabilities |
1,654
|
1,467
|
| 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 |
57,877
|
84,482
|
| Fair Value, Measurements, Recurring | Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
40,933
|
20,079
|
| Fair Value, Measurements, Recurring | Corporate debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
346,329
|
333,327
|
| Fair Value, Measurements, Recurring | Asset-backed securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
122,069
|
100,793
|
| 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, cash equivalents and short-term investments |
118,886
|
77,872
|
| Total assets |
164,284
|
143,057
|
| 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 |
45,398
|
65,185
|
| Fair Value, Measurements, 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] |
|
|
| 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, cash equivalents and short-term investments |
0
|
0
|
| Total assets |
521,810
|
473,496
|
| 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 |
12,479
|
19,297
|
| Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Available-for-sale securities |
40,933
|
20,079
|
| 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 |
346,329
|
333,327
|
| 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 |
122,069
|
100,793
|
| Fair Value, Measurements, 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 |
1,654
|
1,467
|
| Total liabilities |
1,654
|
1,467
|
| 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 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) | 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
|