Income Taxes Components of Income Tax Expense (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Current Income Tax Expense (Benefit)						$ 78	$ 5	$ (33)
Deferred income tax expense (benefit)						44	101	(28)
Income tax expense (benefit)	$ 56	$ 64	$ 45	$ 109	$ 165	$ 122	$ 106	$ (61)