Derivative Instruments (Details 5) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Natural gas swaps
Liabilities:
Gross Amounts of Recognized Assets (Liabilities)	$ (15,942)	$ (18,914)
Net Amounts of (Liabilities) Presented on the Balance Sheet	(15,942)	(18,914)
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	41,038	53,603
Gross Amounts offset on the Balance Sheets	(36,323)	(49,232)
Net Amounts of Assets Presented on the Balance Sheet	$ 4,715	$ 4,371

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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