Segment Information (Geographical)(Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	$ 4,373	$ 3,170	$ 4,635
Long-Lived Assets	18,512	17,658
UNITED STATES
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	3,086	2,400	3,804
Long-Lived Assets	15,971	14,272
EQUATORIAL GUINEA
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	530	444	444
Long-Lived Assets	1,582	1,794
LIBYAN ARAB JAMAHIRIYA
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	431	54	0
UNITED KINGDOM
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	289	263	380
Other International [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenues	37	9	$ 7
Long-Lived Assets	$ 959	$ 1,592