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Share-Based Compensation - Liability-Classified Awards (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Value Creation Plan [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Liability paid     $ 134,000,000
Performance-Based Unit Awards [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Liability paid $ 10,000,000 $ 6,000,000 $ 9,000,000
Outstanding liability $ 26,000,000    
Number of shares of common stock represented by each performance unit 1    
Total unrecognized compensation cost $ 40,000,000    
Unrecognized compensation cost, weighted-average remaining service period 2 years 1 month 22 days    
Performance-Based Unit Awards [Member] | Maximum [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Vesting period 3 years