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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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LEASES LEASES - NARRATIVE (Details) $ in Millions		3 Months Ended
LEASES LEASES - NARRATIVE (Details) $ in Millions	Jan. 01, 2019 USD ($)	Mar. 31, 2019 USD ($) aircraft	Mar. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Operating Leases, Rent Expense			$ 153
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 3
Lessee Operating Lease Lease Not Yet Commenced Amount		$ 104
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
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		61
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
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
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		2
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
Aircraft [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost		$ 1
Airport Facilities [Member]
Lessee, Lease, Description [Line Items]
Variable Lease, Cost		84
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
Maximum [Member] \| Aircraft [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract		11 years
Maximum [Member] \| CPA Aircraft [Member]
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Term of Contract		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