Income Taxes Reconciliation of the beginning and ending amount of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Gross unrecognized tax benefits, beginning of period	$ 263	$ 224	$ 92
Current year tax positions	356	37	0
Prior year tax positions, gross increases	40	24	55
Acquired reserves	0	0	89
Prior year tax positions, gross decreases	(33)	(4)	(2)
Settlements	(24)	(6)	(1)
Statute of limitations lapses	(4)	(12)	(9)
Gross unrecognized tax benefits, end of period	$ 598	$ 263	$ 224