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Revenue - Other Contract Assets and Liabilities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Contract Liabilities    
Balance at beginning of period $ 294 $ 259
Unearned revenue from cash received during the period 505 411
Revenue recognized related to contract liability balance (394) (376)
Balance at end of period $ 405 $ 294