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Goodwill - Summary of Changes in the Carrying Amount of Goodwill by Segment (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Line Items]      
Accumulated goodwill impairment $ 21,000.0 $ 18,000.0 $ 900.0