Income Taxes (Unrecognized Tax Benefits) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		$ 160
Increase in Unrecognized Tax Benefits is Reasonably Possible		251
Unrecognized Tax Benefit Rollforward [Abstract]
Unrecognized Tax Benefits, Beginning Balance	$ 263	126	$ 66	$ 65
Additions for tax positions of prior years		152	83	6
Reductions for tax positions of prior years		(15)	(3)	(5)
Settlements		0	(20)	0
Statute of limitations		0	0	0
Unrecognized Tax Benefits, Ending Balance		$ 263	$ 126	$ 66
Subsequent Event [Member]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	$ 0