Derivative instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Assets:
Fair value of assets	$ 807,000	$ 15,090,000
Liabilities:
Fair value of liabilities	807,000	15,090,000
Not designated as hedges | Other current assets
Assets:
Fair value of assets	3,302,000	13,833,000
Not designated as hedges | Other deferred charges
Assets:
Fair value of assets		3,289,000
Not designated as hedges | Other current liabilities
Liabilities:
Fair value of liabilities		54,000
Not designated as hedges | Other deferred credits
Liabilities:
Fair value of liabilities	$ 4,109,000	$ 1,977,000