Income Taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income Tax Examination, Penalties and Interest Expense		$ 0	$ 0
Income Tax Examination, Interest Accrued	$ 0	0	$ 0
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	793	793
Income Taxes Receivable, Sequestration Fee Payable	53	53
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 0	$ 3