Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits [Roll Forward]
Beginning Balance	$ 162	$ 115	$ 3
Additions based on tax positions related to the current year	0	113	113
Additions for tax positions of prior years	13	1	0
Reductions for tax positions for prior years	(5)	(92)	(1)
Acquisition of OpCo	0	67	0
Settlements		1	0	$ 0
Effect of changes in federal tax law	0	(42)	0
Ending Balance	$ 169	$ 162	$ 115	$ 3