Derivative instruments - Fair value of derivative instruments not designated as hedging (Details) - Natural gas swaps - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Assets:
Fair value of assets	$ 4,968,000	$ 15,090,000
Not designated as hedges | Other current assets
Assets:
Fair value of assets	8,917,000	13,833,000
Not designated as hedges | Other deferred charges
Assets:
Fair value of assets	144,000	3,289,000
Not designated as hedges | Other current liabilities
Liabilities:
Fair Value of liabilities	271,000	54,000
Not designated as hedges | Other deferred credits
Liabilities:
Fair Value of liabilities	$ 3,822,000	$ 1,977,000