Share-based compensation plans - Grants in period and fair value assumptions for share-based compensation plans (Details) - 2009 share award and incentive plan	3 Months Ended
Mar. 31, 2017 $ / shares shares
Vesting March 17, 2018 \| Restricted shares without performance criteria
Share-based compensation plans
Granted deferred shares (in shares) \| shares	34,450
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
Vesting March 17, 2019 \| Restricted shares without performance criteria
Share-based compensation plans
Granted deferred shares (in shares) \| shares	34,450
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
Vesting March 17, 2020 \| Restricted shares without performance criteria
Share-based compensation plans
Granted deferred shares (in shares) \| shares	117,231
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
Vesting March 17, 2020 \| Deferred shares with performance criteria
Share-based compensation plans
Granted deferred shares (in shares) \| shares	228,500
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01
Vesting March 17, 2021 \| Restricted shares without performance criteria
Share-based compensation plans
Granted deferred shares (in shares) \| shares	34,450
Exercise/Purchase price (in dollars per share) \| $ / shares	$ 0.01

The per share price that the individual must pay to acquire shares under the deferred compensation arrangement.

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)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&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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