Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions	$ 5,000,000	$ 0	$ 5,000,000	$ 0
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Amount	11,000,000	0	13,000,000	0
Income Tax Examination, Penalties and Interest Expense	0	0	0	0
Income Tax Examination, Interest Accrued	0	$ 0	0	$ 0
Tax Credit Carryforward, Valuation Allowance	0		0
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	789,000,000		789,000,000
Income Tax Examination, Liability (Refund) Adjustment from Settlement with Taxing Authority	4,000,000		4,000,000
Income Taxes Receivable, Sequestration Fee Payable	52,000,000		52,000,000
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 0		$ 3,000,000