Financial Instruments Notional Amounts (Details) - Not Designated as Hedging Instrument [Member] - Commodity Contract [Member] - bbl bbl in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short [Member]
Derivative, Nonmonetary Notional Amount, Volume	12,979	33,694	23,228
Long [Member]
Derivative, Nonmonetary Notional Amount, Volume	13,430	33,819	22,946

Nominal volume used to calculate payments on a derivative instrument.

No definition available.

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountVolume
Namespace Prefix:	us-gaap_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration