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Pension and other postretirement benefits - PBGC settlement (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Pension contributions	$ 0	$ 0	$ 34,595,000
PBGC Agreement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Required pension contributions above minimum					$ 17,400,000
Pension contributions	0	$ 0	$ 1,100,000	$ 0	$ 6,700,000
Pension contributions above minimum remaining	$ 9,600,000

X

- Definition

Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, this amount represents the remaining additional contributions (above any minimum required contributions) to our defined benefit pension plans.

+ References

No definition available.

+ Details

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X

- Definition

Pursuant to the terms of the settlement agreement with the Pension Benefit Guaranty Corporation, we will make additional contributions (above any minimum required contributions) to our defined benefit pension plans.

+ References

No definition available.

+ Details

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X

- 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.

+ Details

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X

- Definition

Amount of payment for pension benefits. Excludes other postretirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

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X

- Details

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