Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits, beginning of year	$ 462	$ 412	$ 410
Gross decreases to tax positions in prior periods	(7)		(10)
Gross increases to tax positions in prior periods		6
Gross increases due to current period business acquisitions	0	10	0
Gross increases to current period tax positions	59	34	12
Unrecognized tax benefits, end of year	$ 514	$ 462	$ 412