Derivative instruments: Fair value of derivative instruments not designated as hedging (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Interest rate options
Assets:
Fair value of assets	$ 1,010,000	$ 4,371,000
Natural gas swaps
Liabilities:
Fair Value of liabilities	22,848,000	18,914,000
Not designated as hedges | Interest rate options
Assets:
Fair value of assets	1,010,000	4,371,000
Not designated as hedges | Natural gas swaps
Liabilities:
Fair Value of liabilities	$ 22,848,000	$ 18,914,000