LONG-TERM DEBT: (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2015	Mar. 10, 2014
Long-term debt
Total long-term debt and capital leases		$ 208,949		$ 208,949		$ 280,087
Less current installments		32,205		32,205		32,232
Long-term debt, excluding current installments		$ 176,744		176,744		247,855
Existing debt repaid with proceeds of term loan				$ 71,138	$ 9,756
LIBOR rate (as a percent)		0.33%		0.33%
ITO
Long-term debt
Allocated interest expense		$ 100	$ 300	$ 400	$ 600
Interest rate swap | March 10, 2014 derivative agreement | Cash flow hedge
Long-term debt
Fixed interest rate payable on swap (as a percent)							0.98%
Notional amount of derivative							$ 50,000
Fair market value of the derivative		0		0
Unrealized loss of the derivative recorded in other comprehensive income (loss)				$ 300
LIBOR | Interest rate swap | March 10, 2014 derivative agreement
Long-term debt
Long-term debt variable interest rate description				LIBOR
Credit Agreement
Long-term debt
Outstanding letters of credit		2,100		$ 2,100
Credit Agreement | ITO
Long-term debt
Proceeds from sale utilized to repay outstanding indebtedness	$ 55,000
Allocated interest expense				400
Credit Agreement | ITO | Maximum
Long-term debt
Maximum amount of common stock that may be repurchased		100,000		100,000
Term loans
Long-term debt
Total long-term debt and capital leases		200,000		200,000		270,000
Aggregate amount of borrowing commitment		$ 300,000		$ 300,000
Weighted-average interest rate on long-term debt (as a percent)		2.50%		2.50%
Term loans | Quarterly installments payable through September 2017
Long-term debt
Required quarterly installment payments				$ 7,500
Term loans | Quarterly installments payable through June 2018
Long-term debt
Required quarterly installment payments				11,300
Term loans | Final payment due on October 9, 2018
Long-term debt
Final payment of long-term debt		$ 106,300		$ 106,300
Term loans | LIBOR
Long-term debt
Long-term debt basis spread on variable interest rate (as a percent)				2.00%
Revolving credit facility
Long-term debt
Aggregate amount of borrowing commitment		300,000		$ 300,000
Outstanding revolving credit borrowings				0		0
Revolving credit facility dividend restrictions amount, maximum		$ 30,000		$ 30,000
Revolving credit facility | LIBOR
Long-term debt
Long-term debt basis spread on variable interest rate (as a percent)				2.00%
Capital leases and installment payment obligations
Long-term debt
Total long-term debt and capital leases						63
Long-term debt stated interest rate percentage		4.00%		4.00%
Other debt and long-term liabilities
Long-term debt
Total long-term debt and capital leases		$ 8,949		$ 8,949		$ 10,024