LEASES LEASES - NARRATIVE (Details) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 01, 2019 USD ($)	Jun. 30, 2019 USD ($) aircraft	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) aircraft	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)
Lessee, Lease, Description [Line Items]
Operating Leases, Rent Expense			$ 137		$ 290
Cumulative Effect of New Accounting Principle in Period of Adoption						$ 3
Lessee Operating Lease Lease Not Yet Commenced Amount		$ 52		$ 52
Sublease Income		$ 3		$ 6
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		9		9
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
Property Available for Operating Lease [Member] | Airbus [Member]
Lessee, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Number of Optional Additional Units | aircraft		1		1
Property Available for Operating Lease [Member] | E175 [Member]
Lessee, Lease, Description [Line Items]
Property Subject to or Available for Operating Lease, Number of Optional Additional Units | aircraft		3		3
Aircraft [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost		$ 1		$ 2
Airport Facilities [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost		$ 61		$ 145
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