Quarterly report pursuant to Section 13 or 15(d)

EMPLOYEE BENEFIT PLANS - Additional Information (Detail)

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EMPLOYEE BENEFIT PLANS - Additional Information (Detail)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2007
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Employees
Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan, assumptions used calculating benefit obligation, discount rate       0.32%
Defined benefit plan,actual return on plan assets       2.75%
Multiemployer Plans, Payment Term   44 years    
Number of employees under a national master agreement and various supplemental agreements with local unions affiliated with Teamsters | Employees     283,000  
Number of pilots under a collective bargaining agreement with the Independent Pilots Association | Employees     2,800  
Majority of ground mechanics not employed under agreements | Employees     3,100  
Multiemployer Plans, Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Multi-employer Plans, Withdrawal Obligation, Fair Value Disclosure   $ 851 $ 832  
Multiemployer Plans, Pension [Member] | Central States Pension Fund        
Defined Benefit Plan Disclosure [Line Items]        
Payment of withdrawal liability $ 6,100      
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)     1,600  
Pension liability   4,000    
Long term debt | Multiemployer Plans, Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Multiemployer Plans, Withdrawal Obligation, Present Value   850 852  
Other current liabilities | Multiemployer Plans, Pension [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Multiemployer Plans, Withdrawal Obligation, Present Value   7 $ 7  
Defined Benefit Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Estimated future employer contributions to defined benefit plan, current fiscal year   2,100    
Defined Benefit Pension Plan | U.S. Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Amount contributed to company- sponsored benefit plans   22    
U.S. Postretirement Medical Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Amount contributed to company- sponsored benefit plans   192    
Estimated future employer contributions to defined benefit plan, current fiscal year   52    
Pro Forma | Maximum [Member] | Multiemployer Plans, Pension [Member] | Central States Pension Fund        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease)   $ 2,400