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Income Taxes - Unrecognized tax benefit (expense) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]    
Balance, beginning of period $ 14 $ 14
Gross increases - tax positions in prior period 0 0
Gross decreases - tax positions in prior period 0 0
Balance, end of period $ 14 $ 14