BASIS OF PRESENTATION, CONSOLIDATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($)	3 Months Ended				6 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2019	Mar. 31, 2018
Item Affected [Line Items]
Allowance for doubtful accounts receivable			$ 600,000		$ 600,000			$ 3,300,000
Allowance for Doubtful Accounts Receivable, Write-offs			2,300,000		2,300,000
Inventory allowance			22,400,000		22,400,000			26,000,000
Impairment of inventories			9,276,000	$ 0	9,276,000	$ 1,192,000
Short-term portion of contract acquisition and pre-operating costs			9,900,000		9,900,000			10,800,000
Amortization of other deferred charges			2,500,000	2,800,000	5,200,000	5,700,000
Goodwill			18,778,000		18,778,000			19,907,000
Accumulated goodwill impairment			50,861,000		50,861,000			50,861,000
Impairment of intangible assets			3,005,000	0	3,005,000	0
Long-term portion of contract acquisition and pre-operating costs			42,200,000		42,200,000			50,600,000
Deferred OEM cost recovery			3,997,000		3,997,000			8,082,000
Impairment of property and equipment			104,939,000	0	104,939,000	0
Impairment charges on aircraft held for sale			0	8,183,000	0	9,747,000
Other accrued liabilities			52,735,000		52,735,000			65,978,000
Progress payments for aircraft			0	1,000,000	0	2,300,000
Accounting Standards Update 2016-16 [Member] | Retained Earnings
Item Affected [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			(1,700,000)		(1,700,000)
Accounting Standards Update 2017-07 [Member]
Item Affected [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results				100,000		100,000
Bristow Academy [Member]
Item Affected [Line Items]
Impairment charges on aircraft held for sale				$ 6,500,000		6,500,000
OEM Cost recoveries
Item Affected [Line Items]
Original Equipment Manufacturer, Recoveries Sought								136,000,000
Original Equipment Manufacturer, Amount Received					11,000,000			125,000,000
Accumulated Depreciation, Period Increase (Decrease)								(94,500,000)
Deferred OEM cost recovery			8,000,000		8,000,000			13,900,000
OEM Cost recoveries | Rent Expense
Item Affected [Line Items]
Offset Amount, Expense			2,400,000		5,900,000			16,600,000
Offset Amount, Expense, Remainder of Fiscal Year			2,000,000		2,000,000
Offset Amount, Expense, Rolling Year Two			4,000,000		4,000,000
Offset Amount, Expense, Rolling Year Three			2,000,000		2,000,000
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]
Former Gain Contingency, Recognized in Current Period			2,100,000		2,100,000
OEM Cost recoveries | Direct cost | Scenario, Forecast [Member]
Item Affected [Line Items]
Effect on Future Earnings, Offset Amount	$ (300,000)	$ (1,000,000)					$ (1,300,000)
Affiliated entity
Item Affected [Line Items]
Allowance for doubtful accounts receivable			0		0			$ 0
Training Aircraft [Member]
Item Affected [Line Items]
Impairment of inventories						$ 1,200,000
H225 Aircraft [Member]
Item Affected [Line Items]
Impairment of inventories			8,900,000		8,900,000
Impairment of property and equipment			87,500,000		87,500,000
H225 Aircraft [Member] | Depreciation Expense
Item Affected [Line Items]
Offset Amount, Expense, Remainder of Fiscal Year			3,000,000		3,000,000
Offset Amount, Expense, Rolling Year Two			5,900,000		5,900,000
Offset Amount, Expense, Rolling Year Three			1,900,000		1,900,000
Offset Amount, Expense, Rolling Year Four And Thereafter			(10,300,000)		(10,300,000)
Oil and Gas Properties [Member]
Item Affected [Line Items]
Impairment of property and equipment			0		0
Eastern Airways [Member]
Item Affected [Line Items]
Impairment of inventories			300,000		300,000
Impairment of intangible assets			3,000,000		3,005,000
Impairment of property and equipment			$ 17,500,000		$ 17,500,000