Summary of significant accounting policies - Inventory (Details)	Dec. 31, 2017
Accounting Policies [Abstract]
Percentage of FIFO inventory	21.00%
Percentage of weighted average cost inventory	13.00%
Percentage of LIFO inventory	66.00%

The percentage of weighted average cost inventory present at the reporting date when inventory is valued using different valuation methods.

No definition available.

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