Fair value measurements - Summary of Level 3 financial instruments (Details) - Level 3 $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($)
Fair value measurements
Fair value	$ 25
Residential land
Fair value measurements
Fair value	$ 25
Residential loan \| Fair Value of Property or Collateral [Member]
Fair value measurements
Appraised value, selling cost	7.00%

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant