Income Taxes - Income from continuing operations before income tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income before income taxes:
Income before income taxes	$ 1,246	$ 685	$ 1,401
Current:
Total current tax expense	371	268	248
Deferred tax expense (benefit):
Total deferred tax benefit	(225)	(18)	(100)
Total income tax expense	146	250	148
U.K.
Income before income taxes:
Income before income taxes	(240)	(420)	(201)
Current:
Total current tax expense	21	1	(54)
Deferred tax expense (benefit):
Total deferred tax benefit	19	(5)	59
U.S. federal
Income before income taxes:
Income before income taxes	(601)	(765)	(329)
Current:
Total current tax expense	101	48	88
Deferred tax expense (benefit):
Total deferred tax benefit	(165)	12	(110)
U.S. state and local
Current:
Total current tax expense	35	18	7
Deferred tax expense (benefit):
Total deferred tax benefit	(56)	(35)	(9)
Other
Income before income taxes:
Income before income taxes	2,087	1,870	1,931
Current:
Total current tax expense	214	201	207
Deferred tax expense (benefit):
Total deferred tax benefit	$ (23)	$ 10	$ (40)