LONG-TERM DEBT - LINE OF CREDIT (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) credit_facility	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity	$ 475,000,000
Number of credit facilities | credit_facility	3
Credit Facility 1 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, asset restrictions, unrestricted cash and marketable securities	$ 500,000,000
Credit Facility 1 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity			$ 250,000,000	$ 100,000,000
Line of Credit Facility, Expiration Date	Jun. 30, 2021
Number of credit facilities | credit_facility	1
Credit Facility 2 [Member] | Secured by other [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity				$ 150,000,000	$ 100,000,000
Line of Credit Facility, Expiration Date	Mar. 31, 2022
Credit Facility 3 [Member] | Secured by aircraft [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity		$ 75,000,000	$ 52,000,000
Line of Credit Facility, Expiration Date	Sep. 30, 2018
Credit Facilities 1 and 2 [Member]
Line of Credit Facility [Line Items]
Number of credit facilities, no borrowing | credit_facility	2