DEBT - Narrative (Details)		1 Months Ended			6 Months Ended
	Jul. 17, 2017 USD ($) tranche Months Days Loan	Oct. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Apr. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Proceeds from borrowings					$ 338,018,000	$ 195,954,000
Payment of contingent consideration					0	$ 10,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Proceeds from borrowings					107,800,000
Repayment of debt			$ 103,000,000		157,800,000
Maximum borrowing capacity			400,000,000		400,000,000
Letters of credit outstanding, amount			18,900,000		18,900,000
Term Loan
Debt Instrument [Line Items]
Repayment of debt			93,700,000		111,200,000
Face amount of debt			350,000,000		$ 350,000,000
Other Debt | Other Debt
Debt Instrument [Line Items]
Contingent consideration, period, payment					3 years
Payment of contingent consideration				$ 16,000,000
Secured Debt | PK Air Debt
Debt Instrument [Line Items]
Face amount of debt	$ 230,000,000
Number of term loans | Loan	24
Number of tranches | tranche	2
Payment starts date - number of months following borrowing date	7 months
Final payment amount as a percentage of face amount	53.00%
Terms of debt instruments	70 months
Secured Debt | One-Month LIBOR | PK Air Debt
Debt Instrument [Line Items]
Basis spread on variable rate	5.00%
Variable In Determining Interest Rate, Number of Days | Days	2
Fix rate option, number of 30-day months on a notional interest rate swap | Months	12
Term Loan Credit Facility
Debt Instrument [Line Items]
Repayment of debt			17,000,000		$ 17,000,000
Face amount of debt			$ 200,000,000		$ 200,000,000
Subsequent Event | Term Loan Credit Facility
Debt Instrument [Line Items]
Repayment of debt		$ 28,900,000