Income taxes - Reconciliation of Unrecognized Income Tax Benefits (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		$ 200	$ 200	$ 0
Unrecognized Tax Benefits, Period Increase (Decrease)	$ 19,700
Unrecognized Tax Benefits	$ 16,558	16,558	9,898	4,439
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		19,948	6,401	5,280
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions		108	259	179
Unrecognized Tax Benefits		36,273	16,558	9,898
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions		$ (341)	$ 0	$ 0

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- Definition

Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
-URI http://asc.fasb.org/extlink&oid=6527854

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- Definition

Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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- Definition

Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319

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- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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The net amount of all increases and decreases in unrecognized tax benefits for the period.

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