Financial Instruments (Details) - Measured on a recurring basis - USD ($)
$ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
| Fair Value Measurements |
|
|
| Adjusted Cost, assets |
$ 76,530
|
$ 85,918
|
| Unrealized Losses, available-for-sale securities |
(126)
|
(62)
|
| Fair Value, assets |
76,404
|
85,856
|
| Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
19,989
|
29,761
|
| Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
49,612
|
44,057
|
| Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
6,803
|
12,038
|
| Cash |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
14,739
|
24,367
|
| Cash | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
14,739
|
24,367
|
| Level 1 |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, assets |
15,267
|
14,454
|
| Unrealized Losses, available-for-sale securities |
(31)
|
(13)
|
| Fair Value, assets |
15,236
|
14,441
|
| Level 1 | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
5,250
|
5,394
|
| Level 1 | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
6,050
|
4,098
|
| Level 1 | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
3,936
|
4,949
|
| Level 1 | Money market funds |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
5,250
|
5,394
|
| Level 1 | Money market funds | Cash and Cash Equivalents |
|
|
| Fair Value Measurements |
|
|
| Fair value, cash and cash equivalents |
5,250
|
5,394
|
| Level 1 | U.S. government notes |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
10,017
|
9,060
|
| Unrealized Losses, available-for-sale securities |
(31)
|
(13)
|
| Fair Value, available-for-sale investments |
9,986
|
9,047
|
| Level 1 | U.S. government notes | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
6,050
|
4,098
|
| Level 1 | U.S. government notes | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
3,936
|
4,949
|
| Level 2 | Available-for-sale investments |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
46,524
|
47,097
|
| Unrealized Losses, available-for-sale securities |
(95)
|
(49)
|
| Fair Value, available-for-sale investments |
46,429
|
47,048
|
| Level 2 | Available-for-sale investments | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
43,562
|
39,959
|
| Level 2 | Available-for-sale investments | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
2,867
|
7,089
|
| Level 2 | Corporate bonds |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
26,497
|
24,943
|
| Unrealized Losses, available-for-sale securities |
(95)
|
(49)
|
| Fair Value, available-for-sale investments |
26,402
|
24,894
|
| Level 2 | Corporate bonds | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
23,535
|
17,805
|
| Level 2 | Corporate bonds | Long-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
2,867
|
7,089
|
| Level 2 | Commercial paper |
|
|
| Fair Value Measurements |
|
|
| Adjusted Cost, available-for-sale securities |
20,027
|
22,154
|
| Fair Value, available-for-sale investments |
20,027
|
22,154
|
| Level 2 | Commercial paper | Short-term investments |
|
|
| Fair Value Measurements |
|
|
| Fair Value, available-for-sale investments |
$ 20,027
|
$ 22,154
|