Number of previously issued common shares repurchased by the issuing entity and held in treasury.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.30)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

Name:	us-gaap_TreasuryStockCommonShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Treasury stock (Details) - shares	Oct. 16, 2017	Oct. 17, 2016	Oct. 16, 2015	Jun. 30, 2019	Dec. 31, 2018
Treasury stock
Shares of treasury stock held				33,801	33,801
Restricted stock
Treasury stock
Shares surrendered by holder to cover taxes associated with vesting	14,895	9,709	9,197