Income Taxes - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Federal net operating loss carryforwards	$ 429
Foreign deferred tax assets	224
Minimum taxable income needed to fully realize the U.S. federal net deferred tax assets	1,588
Deferred tax assets related to state net operating loss and tax credit carryforwards	166
Valuation allowance on deferred tax assets	173	$ 175
Approximate annual NOL utilization had an ownership change occurred	139
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Federal alternative minimum tax credit carryforwards	$ 19