Deferred compensation (2017 Management Incentive Plan) (Parenthetical) (Details) - Performance Shares - 2017 Management Incentive Plan	Dec. 31, 2017 shares
2018 and 2019 Performance Conditions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested and outstanding restricted stock awards	269,473
2017 Conditions
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unvested and outstanding restricted stock awards	89,955

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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