Derivatives (Schedule of Outstanding Derivatives Entered Into to Hedge Inventory or Fixed Price Purchase Commitments) (Details) bbl in Thousands	6 Months Ended
	Jun. 30, 2018 bbl $ / bbl $ / units
Designated as hedges under accounting rules | Sell (Short) Contracts | Future | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	217
Weighted average contract price per bbl | $ / bbl	65.89
Designated as hedges under accounting rules | Buy (Long) Contracts | Future | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	0
Weighted average contract price per bbl | $ / bbl	0.00
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Future | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	261
Weighted average contract price per bbl | $ / bbl	67.17
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Future | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	51
Weighted average contract price per bbl | $ / bbl	2.99
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Future | Fuel Oil Futures
Derivative [Line Items]
Contract volumes (1,000 bbls)	143
Weighted average contract price per bbl | $ / bbl	64.87
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Swap | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	367
Weighted average contract price per bbl | $ / bbl	0.70
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Options | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	35
Weighted average premium received | $ / bbl	1.35
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	197
Weighted average contract price per bbl | $ / bbl	69.95
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	368
Weighted average contract price per bbl | $ / bbl	2.81
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | Fuel Oil Futures
Derivative [Line Items]
Contract volumes (1,000 bbls)	40
Weighted average contract price per bbl | $ / bbl	63.84
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Swap | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	0
Weighted average contract price per bbl | $ / units	0.00
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Options | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	10
Weighted average premium received | $ / bbl	0.22