INCOME TAXES (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate, percent					24.50%
Tax Cuts And Jobs Act Of 2017, income tax benefit	$ (967)		$ (58,500)
Forecasted effective tax rate		(5.80%)	17.20%
Forecasted tax credits, net of deferred tax					$ 23,200
Excess tax benefits associated with vesting of share-based awards		$ 1,300	$ 2,800
Actual effective tax rate since tax effects of awards are treated as discrete item		(6.60%)	(7.90%)
Deferred tax assets, operating loss carryforwards, domestic, refundable and receivables		$ 23,000			23,000
Deferred tax assets, operating loss carryforwards, domestic		24,500
ITC carryforward		109,900			121,100
Valuation allowance		4,000			4,000
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		136,800			136,800
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 385,400			$ 578,800
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate, percent				21.00%
Forecasted tax credits, net of deferred tax				$ 45,200