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Income Taxes - Valuation Allowance (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Valuation allowance    
Reduction of deferred tax assets due to issuance of Section 987 Regulations   $ 24
Net operating loss carryforwards    
Valuation allowance    
Release of valuation allowance $ 5 $ 127