Stock-based compensation - RSU, DSU, and PSU Activity (Details) - $ / shares	8 Months Ended	12 Months Ended
Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	May 15, 2016
RSUs and DSUs
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares
Nonvested beginning balance (shares)	167,000	149,000	149,000	167,000	0
Granted (shares)	190,000	61,000
Vested (shares)	(23,000)	(75,000)
Forfeited (shares)	0	(4,000)
Nonvested ending balance (shares)	167,000	149,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value
Weighted average grant date fair value (per share), beginning balance		$ 28.08
Weighted average grant date fair value (per share)		28.08	$ 39.67	$ 28.08	$ 0.00
Weighted average grant date fair value (per share) granted	$ 28.08	57.21
Weighted average grant date fair value (per share) vested	27.90	28.47
Weighted average grant date fair value (per share) forfeited	$ 0.00	31.00
Weighted average grant date fair value (per share), ending balance		$ 28.08	$ 39.67	$ 28.08	$ 0.00
Deferred Stock Units (DSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares
Nonvested beginning balance (shares)	0	8,000	8,000	0
Nonvested ending balance (shares)	0	8,000
Performance Shares (PSUs)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares
Nonvested beginning balance (shares)	127,000	184,000	184,000	127,000	0
Granted (shares)	127,000	66,000
Vested (shares)	0	0
Forfeited (shares)	0	(9,000)
Nonvested ending balance (shares)	127,000	184,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value
Weighted average grant date fair value (per share), beginning balance		$ 28.06
Weighted average grant date fair value (per share)		28.06	$ 37.10	$ 28.06	$ 0.00
Weighted average grant date fair value (per share) granted	$ 28.06	53.11
Weighted average grant date fair value (per share) vested	0.00	0.00
Weighted average grant date fair value (per share) forfeited	$ 0.00	27.90
Weighted average grant date fair value (per share), ending balance		$ 28.06	$ 37.10	$ 28.06	$ 0.00

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Per share weighted average intrinsic value of equity-based compensation awards forfeited. Excludes stock and unit options.

+ 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)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&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=96867065&loc=d3e5070-113901

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Per share weighted average intrinsic value of equity-based compensation awards granted. Excludes stock and unit options.

+ 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=96867065&loc=d3e5070-113901

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- Definition

Per share weighted average intrinsic value of equity-based compensation awards not vested. Excludes stock and unit options.

+ 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)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

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.

+ 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)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

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

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Per share weighted average intrinsic value of equity-based compensation awards vested. Excludes stock and unit options.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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