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Summary of significant accounting policies, Part 4 (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Capitalized interest [Abstract] |  |  |
Interest costs incurred | $ 7.0 | $ 8.3 | $ 7.8
Interest costs capitalized | $ 2.2 | $ 2.2 | $ 2.9
Earnings Per Share [Abstract] |  |  |
Interest rate, stated percentage | 2.875% |  |
Shares issued to extinguish convertible notes | 8,508,088 |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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