Derivatives (Schedule of Outstanding Derivatives Entered Into to Hedge Inventory or Fixed Price Purchase Commitments) (Details) bbl in Thousands	9 Months Ended
	Sep. 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)	50
Weighted average contract price per bbl | $ / bbl	69.14
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)	315
Weighted average contract price per bbl | $ / bbl	68.70
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Future | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	54
Weighted average contract price per bbl | $ / bbl	3.01
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)	255
Weighted average contract price per bbl | $ / bbl	65.90
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Future | Diesel Futures
Derivative [Line Items]
Contract volumes (1,000 bbls)	15
Weighted average contract price per bbl | $ / bbl	2.29
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Future | NYM RBOB Gas Futures
Derivative [Line Items]
Contract volumes (1,000 bbls)	22
Weighted average contract price per bbl | $ / bbl	2.03
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Swap | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	502
Weighted average contract price per bbl | $ / bbl	0.65
Not qualifying or not designated as hedges under accounting rules | Sell (Short) Contracts | Options | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	50
Weighted average premium received | $ / bbl	1.34
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | Crude Oil
Derivative [Line Items]
Contract volumes (1,000 bbls)	221
Weighted average contract price per bbl | $ / bbl	68.15
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | Natural Gas
Derivative [Line Items]
Contract volumes (1,000 bbls)	507
Weighted average contract price per bbl | $ / bbl	2.82
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)	0
Weighted average contract price per bbl | $ / bbl	0.00
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | Diesel Futures
Derivative [Line Items]
Contract volumes (1,000 bbls)	5
Weighted average contract price per bbl | $ / bbl	2.23
Not qualifying or not designated as hedges under accounting rules | Buy (Long) Contracts | Future | NYM RBOB Gas Futures
Derivative [Line Items]
Contract volumes (1,000 bbls)	12
Weighted average contract price per bbl | $ / bbl	2.00
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	0.31