Income Taxes - Income from continuing operations before income tax (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income before income taxes:
Income from continuing operations before income taxes	$ 1,765	$ 1,538	$ 1,380
Current:
Total current	468	404	455
Deferred tax expense (benefit):
Total deferred	(134)	(14)	(95)
Total income taxes expenses	334	390	360
U.K. [Member]
Income before income taxes:
Income from continuing operations before income taxes	347	96	36
Current:
Total current	1	(18)	(10)
Deferred tax expense (benefit):
Total deferred	38	43	46
U.S. [Member]
Income before income taxes:
Income from continuing operations before income taxes	(55)	349	468
Current:
Total current	156	111	170
Deferred tax expense (benefit):
Total deferred	(133)	(48)	(83)
U.S. state and local [Member]
Current:
Total current	75	52	57
Deferred tax expense (benefit):
Total deferred	(24)	10	(10)
Other [Member]
Income before income taxes:
Income from continuing operations before income taxes	1,473	1,093	876
Current:
Total current	236	259	238
Deferred tax expense (benefit):
Total deferred	$ (15)	$ (19)	$ (48)