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Stock-Based Compensation - Annual Activity Of Stock Units and Cash Units (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Weighted Average Remaining Contractual Term 1 year    
Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Number of Shares Outstanding 1,160    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Number of RSUs - Balance at beginning of year 878    
Number of RSUs - Granted 914    
Number of RSUs - Vested (470)    
Number of RSUs - Forfeited/expired (162)    
Number of RSUs - Balance at end of year   878  
Weighted Average Grant Price, Balance at beginning of year $ 34.83    
Weighted Average Grant Price - Vested 37.12    
Weighted Average Grant Price - Forfeited/expired 33.07    
Weighted Average Grant Price - Granted 35.32 $ 25.92 $ 54.70
Weighted Average Grant Price, Balance at end of year   $ 34.83  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Weighted Average Grant Date Fair Value $ 34.54    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Weighted Average Remaining Contractual Term 10 months 24 days    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Expected to Vest $ 51    
Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms 1 year 5 months    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Number of Shares Outstanding 97    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Number of RSUs - Balance at beginning of year 923    
Number of RSUs - Granted 572    
Number of RSUs - Vested (146)    
Number of RSUs - Forfeited/expired (355)    
Number of RSUs - Balance at end of year 994 923  
Weighted Average Grant Price, Balance at beginning of year $ 34.11    
Weighted Average Grant Price - Vested 36.55    
Weighted Average Grant Price - Forfeited/expired 37.82    
Weighted Average Grant Price - Granted 35.21 $ 23.33 $ 55.51
Weighted Average Grant Price, Balance at end of year 33.06 $ 34.11  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Weighted Average Grant Date Fair Value $ 36.64    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest, Weighted Average Remaining Contractual Term 2 years 2 months    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding $ 44    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Expected to Vest $ 4