|
Significant accounting policies (Details 3) (USD $)
In Thousands, unless otherwise specified
|
12 Months Ended |
53 Months Ended |
|
|
Dec. 31, 2014
|
Dec. 31, 2013
|
Dec. 31, 2012
|
Dec. 31, 2014
|
Dec. 31, 2011
|
| Investments |
|
|
|
|
|
| Charges for other-than-temporary declines in fair value of short-term or long-term investments |
$ 0 |
$ 0 |
$ 0 |
$ 0 |
|
| Realized gains or losses on investments |
0 |
0 |
0 |
0 |
|
| Investments with a fair value that is significantly lower than the amortized cost basis |
0 |
0 |
|
0 |
|
| Cash and cash equivalents, Amortized cost |
33,901 |
18,889 |
10,096 |
33,901 |
20,954 |
| Cash and cash equivalents, Fair value |
33,901 |
18,889 |
|
33,901 |
|
| Amortized cost |
|
|
|
|
|
| Investments, Amortized Cost |
58,763 |
104,739 |
|
58,763 |
|
| Investments, Gross unrealized gains |
|
|
|
|
|
| Total cash, cash equivalents, and investments, Gross Unrealized Gains |
18 |
46 |
|
18 |
|
| Investments, Gross unrealized losses |
|
|
|
|
|
| Investments, Gross Unrealized Losses |
(7) |
(18) |
|
(7) |
|
| Fair value |
|
|
|
|
|
| Investments, Fair Value |
58,774 |
104,767 |
|
58,774 |
|
| Total cash, cash equivalents, and investments, Amortized Cost |
92,664 |
123,628 |
|
92,664 |
|
| Total cash, cash equivalents, and investments, Fair Value |
92,675 |
123,656 |
|
92,675 |
|
| Government-sponsored enterprise securities |
|
|
|
|
|
| Amortized cost |
|
|
|
|
|
| Due within 1 year |
3,700 |
30,652 |
|
3,700 |
|
| Due within 1 - 2 years |
|
4,001 |
|
|
|
| Investments, Gross unrealized gains |
|
|
|
|
|
| Due within 1 year |
|
12 |
|
|
|
| Due within 1 - 2 years |
|
2 |
|
|
|
| Fair value |
|
|
|
|
|
| Due within 1 year |
3,700 |
30,664 |
|
3,700 |
|
| Due within 1 - 2 years |
|
4,003 |
|
|
|
| Corporate bonds and commercial paper |
|
|
|
|
|
| Amortized cost |
|
|
|
|
|
| Due within 1 year |
55,063 |
51,735 |
|
55,063 |
|
| Due within 1 - 2 years |
|
18,351 |
|
|
|
| Investments, Gross unrealized gains |
|
|
|
|
|
| Due within 1 year |
18 |
30 |
|
18 |
|
| Due within 1 - 2 years |
|
2 |
|
|
|
| Investments, Gross unrealized losses |
|
|
|
|
|
| Due within 1 year |
(7) |
(6) |
|
(7) |
|
| Due within 1 - 2 years |
|
(12) |
|
|
|
| Fair value |
|
|
|
|
|
| Due within 1 year |
55,074 |
51,759 |
|
55,074 |
|
| Due within 1 - 2 years |
|
18,341 |
|
|
|
| Cash and cash equivalents |
|
|
|
|
|
| Investments |
|
|
|
|
|
| Cash and cash equivalents, Amortized cost |
33,901 |
18,889 |
|
33,901 |
|
| Cash and cash equivalents, Fair value |
$ 33,901 |
$ 18,889 |
|
$ 33,901 |
|