Financial Instruments (Details) - Measured on a recurring basis - USD ($)
$ in Thousands |
Jun. 30, 2018 |
Dec. 31, 2017 |
| Fair Value Measurements |
|
|
| Adjusted Cost, assets |
$ 83,199
|
$ 85,918
|
| Unrealized Gains, available-for-sale securities |
1
|
|
| Unrealized Losses, available-for-sale securities |
(97)
|
(62)
|
| Fair Value, assets |
83,103
|
85,856
|
| Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
27,229
|
29,761
|
| Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
53,317
|
44,057
|
| Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
2,557
|
12,038
|
| Cash |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
21,145
|
24,367
|
| Cash | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
21,145
|
24,367
|
| Level 1 |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, assets |
14,022
|
14,454
|
| Unrealized Losses, available-for-sale securities |
(32)
|
(13)
|
| Fair Value, assets |
13,990
|
14,441
|
| Level 1 | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
5,085
|
5,394
|
| Level 1 | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
7,430
|
4,098
|
| Level 1 | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
1,475
|
4,949
|
| Level 1 | Money market funds |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
5,085
|
5,394
|
| Level 1 | Money market funds | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
5,085
|
5,394
|
| Level 1 | U.S. government notes |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
8,937
|
9,060
|
| Unrealized Losses, available-for-sale securities |
(32)
|
(13)
|
| Fair Value, available-for-sale investments |
8,905
|
9,047
|
| Level 1 | U.S. government notes | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
7,430
|
4,098
|
| Level 1 | U.S. government notes | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
1,475
|
4,949
|
| Level 2 | Available-for-sale investments |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
48,032
|
47,097
|
| Unrealized Gains, available-for-sale securities |
1
|
|
| Unrealized Losses, available-for-sale securities |
(65)
|
(49)
|
| Fair Value, available-for-sale investments |
47,968
|
47,048
|
| Level 2 | Available-for-sale investments | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
999
|
|
| Level 2 | Available-for-sale investments | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
45,887
|
39,959
|
| Level 2 | Available-for-sale investments | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
1,082
|
7,089
|
| Level 2 | Corporate bonds |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
26,951
|
24,943
|
| Unrealized Gains, available-for-sale securities |
1
|
|
| Unrealized Losses, available-for-sale securities |
(65)
|
(49)
|
| Fair Value, available-for-sale investments |
26,887
|
24,894
|
| Level 2 | Corporate bonds | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
25,805
|
17,805
|
| Level 2 | Corporate bonds | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
1,082
|
7,089
|
| Level 2 | Commercial paper |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
21,081
|
22,154
|
| Fair Value, available-for-sale investments |
21,081
|
22,154
|
| Level 2 | Commercial paper | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
999
|
|
| Level 2 | Commercial paper | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
$ 20,082
|
$ 22,154
|