Derivative instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Assets:
Fair value of assets	$ 858,000	$ 15,090,000
Liabilities:
Fair value of liabilities	858,000	15,090,000
Not designated as hedges | Other current assets
Assets:
Fair value of assets	3,182,000	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	482,000	54,000
Not designated as hedges | Other deferred credits
Liabilities:
Fair value of liabilities	$ 3,558,000	$ 1,977,000