Derivative Instruments - (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Gross amounts of commodity derivative contracts recognized at fair value	$ 1,633	$ 11,843
Cash collateral	9,256	3,755
Total gross amount recognized	10,889	15,598
Gross amounts offset	(4,749)	(1,799)
Net amount reported in the Consolidated Balance Sheets	6,140	13,799
Liabilities
Gross amounts of commodity derivative contracts recognized at fair value	4,749	1,799
Cash collateral	0	0
Total gross amount recognized	4,749	1,799
Gross amounts offset	(4,749)	(1,799)
Net amount reported in the Consolidated Balance Sheets	$ 0	$ 0