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LEASES LEASES - NARRATIVE (Details) $ in Millions		3 Months Ended		9 Months Ended
LEASES LEASES - NARRATIVE (Details) $ in Millions	Jan. 01, 2019 USD ($)	Sep. 30, 2019 USD ($) aircraft	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) aircraft	Sep. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Operating Leases, Rent Expense			$ 165		$ 455
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 3
Sublease Income		$ 3		$ 9
Property Subject to Operating Lease [Member] \| B-737 [Member]
Lessee, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Number of Units \| aircraft		10		10
Property Subject to Operating Lease [Member] \| Airbus [Member]
Lessee, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Number of Units \| aircraft		62		62
Property Subject to Operating Lease [Member] \| Q400 [Member]
Lessee, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Number of Units \| aircraft		8		8
Property Subject to Operating Lease [Member] \| E175 [Member]
Lessee, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Number of Units \| aircraft		32		32
Aircraft [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost		$ 1		$ 4
Airport Facilities [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost		$ 86		231
Corporate real estate [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost				$ 8
Retained Earnings [Member]
Lessee, Lease, Description [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 3
Minimum [Member] \| CPA Aircraft [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract		8 years		8 years
Maximum [Member] \| Aircraft [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract		12 years		12 years
Maximum [Member] \| CPA Aircraft [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract		11 years		11 years
Accounting Standards Update 2016-02 [Member]
Lessee, Lease, Description [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 150