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Investments and Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Financial Instruments (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, carrying amount $ 13,445 $ 13,268
Synergy Term Loan, carrying amount 478 0
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 13,450 13,787
Synergy Term Loan, fair value 422 0
Fair Value | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 0 0
Synergy Term Loan, fair value 0 0
Fair Value | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 9,525 10,115
Synergy Term Loan, fair value 0 0
Fair Value | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 3,925 3,672
Synergy Term Loan, fair value $ 422 $ 0