Accounting for Derivative Instruments and Hedging Activities (Offsetting) (Details) - Commodity Contract [Member] - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative Asset, Fair Value, Amount Offset Against Collateral [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 31,308	$ 49,019
Derivative Asset, Collateral, Obligation to Return Cash, Offset	0	0
Derivative Asset	31,308	49,019
Derivative, Collateral, Obligation to Return Securities	14,922	25,388
Derivative, Collateral, Obligation to Return Cash	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral	16,386	23,631
Derivative Liability, Fair Value, Amount Offset Against Collateral [Abstract]
Derivative Liability, Fair Value, Gross Liability	26,121	57,756
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0	0
Derivative Liability	26,121	57,756
Derivative, Collateral, Right to Reclaim Securities	14,922	25,388
Derivative, Collateral, Right to Reclaim Cash	(2,000)	0
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ 13,199	$ 32,368