| Schedule of liability and equity components of convertible debt [Table Text Block] |
September 30, 2017 and December 31, 2016:
| | | | | | | | | | | | | | September 30, 2017 | | 0.75% Notes | | 1.25% Notes | | Total | Liability component: | | | | | | Principal | $ | 230,000 |
| | $ | 199,276 |
| | $ | 429,276 |
| Less: debt discount and issuance costs, net of amortization | (35,148 | ) | | (12,979 | ) | | (48,127 | ) | Net carrying amount | $ | 194,852 |
| | $ | 186,297 |
| | $ | 381,149 |
| | | | | | | Equity component (1) | $ | 54,049 |
| | $ | 40,851 |
| | $ | 94,900 |
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| | | | | | | | | | | | | | December 31, 2016 | | 0.75% Notes | | 1.25% Notes | | Total | Liability component: | | | | | | Principal | $ | 230,000 |
| | $ | 201,250 |
| | $ | 431,250 |
| Less: debt discount and issuance costs, net of amortization | (43,896 | ) | | (20,813 | ) | | (64,709 | ) | Net carrying amount | $ | 186,104 |
| | $ | 180,437 |
| | $ | 366,541 |
| | | | | | | Equity component (1) | $ | 54,049 |
| | $ | 43,293 |
| | $ | 97,342 |
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(1) Recorded on the accompanying Condensed Consolidated Balance Sheets as additional paid-in capital, net of the $2,877 issuance costs.
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| Schedule of convertible note interest and accretion [Table Text Block] |
For the three and nine months ended September 30, 2017 and 2016, the Company incurred the following interest expense related to the Notes:
| | | | | | | | | | | | | | | | | | Three Months Ended September 30, | | Nine Months Ended September 30, | | 2017 | | 2016 | | 2017 | | 2016 | Interest expense related to contractual interest coupon | $ | 1,053 |
| | $ | 1,060 |
| | $ | 3,173 |
| | $ | 3,180 |
| Amortization of debt discount and issuance costs | 5,603 |
| | 5,248 |
| | 16,491 |
| | 15,516 |
| | $ | 6,656 |
| | $ | 6,308 |
| | $ | 19,664 |
| | $ | 18,696 |
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