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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Fair Value Disclosures [Abstract]      
Estimated fair value of long-term debt $ 2,710 $ 2,490 $ 2,680
Long-term debt $ 2,900 $ 2,900 $ 2,900