Income Taxes - Income from continuing operations before income tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income before income taxes:
Income from continuing operations before income taxes	$ 685	$ 1,401	$ 1,428
Current:
Total current	268	248	341
Deferred tax expense (benefit):
Total deferred	(18)	(100)	(166)
Total income taxes expenses	250	148	175
U.K.
Income before income taxes:
Income from continuing operations before income taxes	(420)	(201)	144
Current:
Total current	1	(54)	42
Deferred tax expense (benefit):
Total deferred	(5)	59	(39)
U.S. federal
Income before income taxes:
Income from continuing operations before income taxes	(765)	(329)	(283)
Current:
Total current	48	88	22
Deferred tax expense (benefit):
Total deferred	12	(110)	(94)
U.S. state and local
Current:
Total current	18	7	32
Deferred tax expense (benefit):
Total deferred	(35)	(9)	(4)
Other
Income before income taxes:
Income from continuing operations before income taxes	1,870	1,931	1,567
Current:
Total current	201	207	245
Deferred tax expense (benefit):
Total deferred	$ 10	$ (40)	$ (29)