Contingencies and Commitments Commitments - Drilling Contracts Table (Details) $ in Millions	Dec. 31, 2017 USD ($)
Drilling Rig Leases [Member]
Other Commitments [Line Items]
Drilling Contracts Contractual Obligation, Due in Next Fiscal Year	$ 23

Amount of contractual obligation due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

Name:	us-gaap_ContractualObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant