Income Taxes - Schedule of Change in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes In Unrecognized Tax Benefits
Beginning balance of unrecognized tax benefits	$ 1	$ 426	$ 503
Additions based on tax positions related to current year	0	0	0
Additions (reductions) for tax positions of prior years	0	(5)	(76)
Settlements	0	(420)	(1)
Lapse of statute of limitations	0	0	(1)
Unrecognized deferred tax benefits from acquisitions	0	0	1
Ending balance of unrecognized tax benefits	1	1	426
Federal
Income Tax Contingency [Line Items]
Unrecognized tax benefits that would have impacted effective rate if recognized	0	0	422
State
Income Tax Contingency [Line Items]
Unrecognized tax benefits that would have impacted effective rate if recognized	$ 1	$ 1	$ 3