Pension and other postretirement benefits - Pension Plan Asset Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Actual plan asset allocations	100.00%	100.00%
Fair value of assets	$ 303,367	$ 276,741
Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	303,367	276,741
Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	0	0
Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	$ 0	$ 0
U.S. Equities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Target allocation percentage of assets	29.00%
Actual plan asset allocations	29.00%	34.00%
Fair value of assets	$ 86,513	$ 93,773
U.S. Equities [Member] \| Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	86,513	93,773
U.S. Equities [Member] \| Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	0	0
U.S. Equities [Member] \| Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	$ 0	$ 0
International Equities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Target allocation percentage of assets	26.00%
Actual plan asset allocations	26.00%	22.00%
Fair value of assets	$ 79,120	$ 61,453
International Equities [Member] \| Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	79,120	61,453
International Equities [Member] \| Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	0	0
International Equities [Member] \| Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	$ 0	$ 0
Fixed Income Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Target allocation percentage of assets	45.00%
Actual plan asset allocations	45.00%	44.00%
Fair value of assets	$ 137,734	$ 121,515
Fixed Income Securities [Member] \| Level 1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	137,734	121,515
Fixed Income Securities [Member] \| Level 2 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	0	0
Fixed Income Securities [Member] \| Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of assets	$ 0	$ 0

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No definition available.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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