Financial Instruments (Details) - Measured on a recurring basis - USD ($)
$ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
| Fair Value Measurements |
|
|
| Adjusted Cost, assets |
$ 71,813
|
$ 93,239
|
| Fair Value, assets |
71,818
|
93,189
|
| Unrealized Gains, assets |
10
|
1
|
| Unrealized Losses, assets |
(5)
|
(51)
|
| Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, assets |
19,828
|
40,395
|
| Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, assets |
48,789
|
52,794
|
| Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, assets |
3,201
|
|
| Cash |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
14,782
|
30,501
|
| Adjusted Cost, assets |
14,782
|
30,501
|
| Cash | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
14,782
|
30,501
|
| Level 1 |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, assets |
20,129
|
17,323
|
| Fair Value, assets |
20,130
|
17,309
|
| Unrealized Gains, assets |
1
|
|
| Unrealized Losses, assets |
|
(14)
|
| Level 1 | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, assets |
5,046
|
6,903
|
| Level 1 | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, assets |
15,084
|
10,406
|
| Level 1 | Money market funds |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
5,046
|
6,903
|
| Adjusted Cost, assets |
5,046
|
6,903
|
| Level 1 | Money market funds | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
5,046
|
6,903
|
| Level 1 | U.S. government notes |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
15,083
|
10,420
|
| Unrealized Gains, available-for-sale securities |
1
|
|
| Unrealized Losses, available-for-sale securities |
|
(14)
|
| Fair Value, available-for-sale investments |
15,084
|
10,406
|
| Level 1 | U.S. government notes | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
15,084
|
10,406
|
| Level 2 |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
36,902
|
45,415
|
| Unrealized Gains, available-for-sale securities |
9
|
1
|
| Unrealized Losses, available-for-sale securities |
(5)
|
(37)
|
| Fair Value, available-for-sale investments |
36,906
|
45,379
|
| Level 2 | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
|
2,991
|
| Level 2 | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
33,705
|
42,388
|
| Level 2 | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
3,201
|
|
| Level 2 | Corporate bonds |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
22,098
|
21,612
|
| Unrealized Gains, available-for-sale securities |
9
|
1
|
| Unrealized Losses, available-for-sale securities |
(5)
|
(37)
|
| Fair Value, available-for-sale investments |
22,102
|
21,576
|
| Level 2 | Corporate bonds | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
18,901
|
21,576
|
| Level 2 | Corporate bonds | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
3,201
|
|
| Level 2 | Commercial paper |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
14,804
|
23,803
|
| Fair Value, available-for-sale investments |
14,804
|
23,803
|
| Level 2 | Commercial paper | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
|
2,991
|
| Level 2 | Commercial paper | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
$ 14,804
|
$ 20,812
|