Taxation - Reconciliation of statutory tax rate and effective tax rate (Details) - PRC		12 Months Ended
Jan. 01, 2008	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the differences between statutory tax rate and the effective tax rate for China
PRC statutory tax rate (as a percent)	25.00%	25.00%	25.00%	25.00%
Effect on tax rates in different tax jurisdiction (as a percent)		0.08%	(0.78%)	(0.37%)
Effect on tax holiday (as a percent)		0.99%	0.34%	0.29%
Changes in valuation allowance (as a percent)		(16.91%)	(19.55%)	(12.86%)
Permanent book-tax differences-non-deductible expenses (as a percent)		(14.26%)	(5.95%)	(12.81%)
Effective tax rate (as a percent)		(5.10%)	(0.94%)	(0.75%)

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to income exempt from income tax because of a tax holiday.

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