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Employee Benefit Plans - Additional Information (Detail)
shares in Millions, $ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
shares
Pension Benefits  
Compensation and Retirement Disclosure [Abstract]  
Employer contribution to tax-qualified defined benefit pension plans | $ $ 113
RSUs  
Compensation and Retirement Disclosure [Abstract]  
Share-based compensation, option awards granted 1.7
Share-based compensation, vesting period 3 years
Performance Shares  
Compensation and Retirement Disclosure [Abstract]  
Share-based compensation, vesting period 3 years
Share-Based Compensation, vesting date Dec. 31, 2020
Number of days used to compute performance period average closing price of restricted stock units 20 days
Time Vested Restricted Stock Units RSU  
Compensation and Retirement Disclosure [Abstract]  
Share-based compensation, other than options, awards granted 1.0
Performance Based Restricted Stock Units RSU  
Compensation and Retirement Disclosure [Abstract]  
Share-based compensation, other than options, awards granted 0.7