Derivative Instruments - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Gross amounts of commodity derivative contracts recognized at fair value	$ 812	$ 1,272
Cash collateral	8,799	9,366
Total gross amount recognized	9,611	10,638
Gross amounts offset	(8,001)	(3,511)
Net amount reported in the Consolidated Balance Sheets	1,610	7,127
Liabilities
Gross amounts of commodity derivative contracts recognized at fair value	8,001	3,511
Cash collateral	0	0
Total gross amount recognized	8,001	3,511
Gross amounts offset	(8,001)	(3,511)
Net amount reported in the Consolidated Balance Sheets	$ 0	$ 0