INCOME TAX: (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Income Tax Examination [Line Items]
Tax cuts and jobs act of 2017, change in tax rate, deferred tax liability, income tax (expense) benefit						$ 20,300
Income taxes (benefit)		$ (14,030)	$ 100	$ 6,334	$ 1,900	$ (19,994)	$ 7,099
Subsequent Event [Member]
Income Tax Examination [Line Items]
Calculated federal corporate income tax rate	31.50%