Taxation - Reconciliation of unrecognized tax benefits (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of the year	$ 30,000us-gaap_UnrecognizedTaxBenefits	$ 0us-gaap_UnrecognizedTaxBenefits
Increases for tax positions taken during the year	12,182	9,591
Increases for tax positions taken in prior years	15,304	20,409
Decreases for tax positions taken during the year	0	0
Decreases for tax positions taken in prior years	(20,296)	0
Decreases for settlement with taxing authorities	0	0
Decreases for lapse of the applicable statute of limitations	0	0
Unrecognized tax benefits, end of the year	$ 37,190us-gaap_UnrecognizedTaxBenefits	$ 30,000us-gaap_UnrecognizedTaxBenefits

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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

-Glossary Unrecognized Tax Benefit

-URI http://asc.fasb.org/extlink&oid=6527854

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15A

-Subparagraph (a)

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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period 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 15A

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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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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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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Amount of increase or decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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