Derivative Instruments - (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Gross amounts of commodity derivative contracts recognized at fair value	$ 4,644	$ 14,901
Cash collateral	5,638	2,870
Total gross amount recognized	10,282	17,771
Gross amounts offset	(185)	(4,833)
Net amount reported in the Consolidated Balance Sheets	10,097	12,938
Liabilities
Gross amounts of commodity derivative contracts recognized at fair value	185	205
Cash collateral	0	4,628
Total gross amount recognized	185	4,833
Gross amounts offset	(185)	(4,833)
Net amount reported in the Consolidated Balance Sheets	$ 0	$ 0