Fair value measurements (Nonrecurring Fair Value Measurements) (Details) - Significant Unobservable Inputs (Level 3) - Nonrecurring Fair Value Measurements	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Nonrecurring Fair Value Measurements
Annual inflation rate	2.30%	2.42%
Credit-adjusted risk-free interest rate		20.00%
Minimum
Nonrecurring Fair Value Measurements
Credit-adjusted risk-free interest rate	5.13%
Maximum
Nonrecurring Fair Value Measurements
Credit-adjusted risk-free interest rate	7.63%

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Balance Type:	na
Period Type:	duration