Summary of Significant Accounting Policies - Income taxes (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Summary of Significant Accounting Policies
Foreign earnings	$ 0	$ 0

Amount of undistributed earnings of foreign subsidiaries intended to be permanently reinvested outside the country of domicile.

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Name:	us-gaap_UndistributedEarningsOfForeignSubsidiaries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant