Segment and Related Information (Geographic Data) (Details) - USD ($) $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015	[1]	Jun. 30, 2015	[1]	Mar. 31, 2015	[1]	Dec. 31, 2014		Sep. 30, 2014	[1]	Jun. 30, 2014	[1]	Mar. 31, 2014	[1]	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 1,780	[1]	$ 1,878		$ 1,781		$ 2,230		$ 1,946	[1]	$ 1,879		$ 1,849		$ 2,178		$ 7,669	$ 7,852	$ 7,263
Long-lived assets	31,056								28,832								31,056	28,832
US [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers																	5,259	5,231	4,860
Long-lived assets	18,569								16,890								18,569	16,890
UK [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers																	2,410	2,621	$ 2,403
Long-lived assets	$ 12,487								$ 11,942								$ 12,487	$ 11,942

[1] Quarterly results can vary depending on, among other things, weather . Accordingly, comparisons among quarters of a year may not be indicative of overall tr ends and changes in operations.