Income tax expense - Valuation allowance (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Income tax expense
Balance as of January 1	¥ 16,145	¥ 15,415
Addition	12,532	730
Balance as of December 31	¥ 3,613	¥ 16,145

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-2

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