Derivative Instruments - Master netting agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets:
Cash Collateral	$ 0	$ 0
Natural gas swaps
Liabilities:
Gross Amounts of Recognized Assets (Liabilities)	(2,823)	(22,848)
Net Amounts of (Liabilities) Presented on the Balance Sheet	(2,823)	(22,848)
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	11,433	26,925
Gross Amounts offset on the Balance Sheets	$ (11,433)	(25,915)
Net Amounts of Assets Presented on the Balance Sheet		$ 1,010