Income Taxes - Income from continuing operations before income tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income before income taxes:
Income from continuing operations before income taxes	$ 1,669	$ 1,689	$ 1,765
Current:
Total current	263	490	468
Deferred tax expense (benefit):
Total deferred	(24)	(223)	(134)
Total income taxes expenses	239	267	334
UK
Income before income taxes:
Income from continuing operations before income taxes	(202)	149	347
Current:
Total current	(54)	43	1
Deferred tax expense (benefit):
Total deferred	59	(39)	38
U.S. federal
Income before income taxes:
Income from continuing operations before income taxes	(104)	(51)	(55)
Current:
Total current	94	137	156
Deferred tax expense (benefit):
Total deferred	(47)	(140)	(133)
U.S. state and local
Current:
Total current	0	54	75
Deferred tax expense (benefit):
Total deferred	6	(14)	(24)
Other
Income before income taxes:
Income from continuing operations before income taxes	1,975	1,591	1,473
Current:
Total current	223	256	236
Deferred tax expense (benefit):
Total deferred	$ (42)	$ (30)	$ (15)