Share-based compensation - Non-vested Restricted Shares - Additional Information (Details) - Non-Vested Restricted Shares - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized share-based compensation	$ 15,833
Independent Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted	50,000	50,000
Management
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares granted	45,000	121,000
Management \| Each yearly anniversary from the date of the agreement
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting percentage	20.00%	20.00%

Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Net number of non-option equity instruments granted to participants.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

Percentage of vesting of award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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