BASIS OF PRESENTATION, CONSOLIDATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)	3 Months Ended			6 Months Ended			12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) aircraft	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Apr. 01, 2019 USD ($)
Item Affected [Line Items]
Restricted cash	$ 38,910,000			$ 38,910,000	$ 0
Allowance for doubtful accounts receivable	800,000			800,000	1,600,000
Inventory allowance	18,700,000			18,700,000	19,400,000
Impairment of inventories	0	$ 9,276,000		0		$ 9,276,000
Short-term portion of contract acquisition and pre-operating costs	9,200,000			9,200,000	9,800,000
Amortization of other deferred charges	2,600,000	2,500,000		4,700,000		5,200,000
Long-term portion of contract acquisition and pre-operating costs	30,500,000			30,500,000	37,100,000
Accumulated Depreciation, Period Increase (Decrease)	1,400,000			2,800,000
Deferred OEM cost recovery	0			0	3,997,000
Right-of-use asset	331,743,000			331,743,000	0
Operating Lease, Liability	335,029,000			$ 335,029,000
Accounting Standards Update 2016-02 [Member]
Item Affected [Line Items]
Right-of-use asset								$ 281,000,000
Operating Lease, Liability								$ 285,300,000
OEM Cost recoveries
Item Affected [Line Items]
Accumulated Depreciation, Period Increase (Decrease)							$ (94,500,000)
Original Equipment Manufacturer, Recoveries Sought							136,000,000
Original Equipment Manufacturer, Amount Received			$ 11,000,000			11,000,000	125,000,000
Deferred OEM cost recovery							13,900,000
Number Of Aircraft Lease Return To Lessor | aircraft				4
OEM Cost recoveries | Rent Expense
Item Affected [Line Items]
Offset Amount, Expense		2,400,000		$ 6,000,000	2,000,000	5,900,000	$ 16,600,000
OEM Cost recoveries | Revenue
Item Affected [Line Items]
Former Gain Contingency, Recognized in Current Period		7,600,000				7,600,000
OEM Cost recoveries | Direct cost
Item Affected [Line Items]
Offset Amount, Expense					1,300,000
Former Gain Contingency, Recognized in Current Period		2,100,000				2,100,000
Affiliated entity
Item Affected [Line Items]
Allowance for doubtful accounts receivable	0			0	$ 0
H225 Aircraft
Item Affected [Line Items]
Impairment of inventories		8,900,000				8,900,000
Eastern Airways International Limited [Member]
Item Affected [Line Items]
Impairment of inventories		$ 300,000				$ 300,000
DIP Credit Agreement
Item Affected [Line Items]
Restricted cash	37,000,000			37,000,000
Norway Withholding Taxes [Member]
Item Affected [Line Items]
Restricted cash	$ 1,900,000			$ 1,900,000