Employee benefit obligations - Schedule of Asset Allocation by Category (Details)	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	100.00%
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	75.00%
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	16.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	3.00%
Others
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations	6.00%

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.

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=58740216&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=58740216&loc=d3e1928-114920

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant