Derivative instruments: - Fair value of derivative instruments not designated as hedging (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Interest rate options
Assets:
Fair value of assets	$ 0	$ 1,010,000
Natural gas swaps
Assets:
Fair value of assets	15,090,000
Liabilities:
Fair Value of liabilities	22,848,000	22,848,000
Not designated as hedges | Interest rate options | Other deferred charges
Assets:
Fair value of assets		1,010,000
Not designated as hedges | Natural gas swaps | Other deferred charges
Assets:
Fair value of assets	3,289,000
Not designated as hedges | Natural gas swaps | Other current assets
Assets:
Fair value of assets	13,833,000
Not designated as hedges | Natural gas swaps | Other current liabilities
Liabilities:
Fair Value of liabilities	54,000	$ 22,848,000
Not designated as hedges | Natural gas swaps | Other deferred credits
Liabilities:
Fair Value of liabilities	$ 1,977,000