Pension and Other Postretirement Benefits - Plan assets and type of fair value measurement (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Pension and Other Postretirement Benefits
Fair Value of plan Assets	$ 206,429	$ 201,497
Level 2
Pension and Other Postretirement Benefits
Fair Value of plan Assets	1,628	1,520
Fixed income
Pension and Other Postretirement Benefits
Fair Value of plan Assets	1,628	1,520
Fixed income \| Level 2
Pension and Other Postretirement Benefits
Fair Value of plan Assets	1,628	1,520
Commingled funds
Pension and Other Postretirement Benefits
Fair Value of plan Assets	$ 204,801	$ 199,977

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

Reference 2: 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=65897934&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6749337

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6520858

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant