Derivative Instruments - Master netting agreements (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Natural gas swaps
Liabilities:
Gross Amounts of Recognized Assets (Liabilities)	$ (25,919)	$ (22,848)
Net Amounts of (Liabilities) Presented on the Balance Sheet	(25,919)	(22,848)
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	22,057	26,925
Gross Amounts offset on the Balance Sheets	(21,962)	(25,915)
Net Amounts of Assets Presented on the Balance Sheet	$ 95	$ 1,010