Derivative instruments: - Master netting agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Natural gas swaps
Assets:
Gross Amounts of Recognized Assets (Liabilities)	$ 15,090
Net Amounts of Assets Presented on the Balance Sheet	15,090
Liabilities:
Gross Amounts of Recognized Assets (Liabilities)	(15,090)	$ (22,848)
Net Amounts of (Liabilities) Presented on the Balance Sheet	(15,090)	(22,848)
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	5,788	26,925
Gross Amounts offset on the Balance Sheets	$ (5,788)	(25,915)
Net Amounts of Assets Presented on the Balance Sheet		$ 1,010