LONG TERM DEBT - Credit Agreement (Details) $ in Thousands								12 Months Ended			24 Months Ended
	Sep. 30, 2017	Jun. 23, 2017	Sep. 30, 2016	Jul. 15, 2016	Sep. 30, 2015	Sep. 30, 2014	Jun. 17, 2013 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) agreement
Long-Term Debt
Quarterly principal payments, as a percent to initial amount borrowed						1.25%
Final amount due in 2018								$ 155,749			$ 155,749
Federal income tax refunds								$ 34,100
Percentage tax refunds to be applied to reduce revolving commitments								50.00%
Credit Agreement
Long-Term Debt
Term of agreement							5 years
Maximum borrowing capacity							$ 425,000
Number of agreements entered into | agreement											7
Minimum commitment as a result of tax refunds								$ 108,000			$ 108,000
Maximum leverage ratio to use maximum credit facility								4.00
Interest rate margin (as a percent)								5.52%	2.43%	2.17%
Increase in interest rate									2.00%	2.00%
Credit Agreement | Last day of the fiscal quarter ending June 30, 2016
Long-Term Debt
Consolidated leverage ratio		5.00%
Interest coverage ratio		3.50
Credit Agreement | Last day of the fiscal quarter ending September 30, 2016
Long-Term Debt
Consolidated leverage ratio		5.75%
Credit Agreement | Last day of each fiscal quarter after September 30, 2016
Long-Term Debt
Consolidated leverage ratio		5.00%
Credit Agreement | LIBOR
Long-Term Debt
Interest rate margin (as a percent)								5.75%
Interest rate margin contingent on delivery of financial information and leverage ratio of 4.00 to 1.00								4.00%
Credit Agreement | Base rate
Long-Term Debt
Interest rate margin contingent on delivery of financial information and leverage ratio of 4.00 to 1.00								3.00%
Credit Agreement | Federal funds rate
Long-Term Debt
Interest rate margin (as a percent)								0.50%
Credit Agreement | One-month LIBOR
Long-Term Debt
Interest rate margin (as a percent)								4.75%
Fixed interest rate of debt								1.00%			1.00%
Revolving Credit Facility | Credit Agreement
Long-Term Debt
Maximum borrowing capacity							200,000
Available credit facility								$ 118,300			$ 118,300
Letters of credit outstanding amount								$ 6,100	$ 4,300		6,100
Unused commitment fee (as a percent)								0.375
Unused commitment fee								$ 300	200	$ 400
Balance available under the credit facility								94,900	$ 10,000		94,900
Revolving Credit Facility | Credit Agreement | From July 15, 2016 to September 30, 2016
Long-Term Debt
Reduction of the available credit facility depending on fiscal quarter results								13,900			13,900
Revolving Credit Facility | Credit Agreement | Three month period ending December 31, 2016
Long-Term Debt
Reduction of the available credit facility depending on fiscal quarter results								17,300			17,300
Revolving Credit Facility | Credit Agreement | Three month period ending March 31, 2017
Long-Term Debt
Reduction of the available credit facility depending on fiscal quarter results								10,700			10,700
Revolving Credit Facility | Credit Agreement | Nine month period ending December 31, 2017
Long-Term Debt
Reduction of the available credit facility depending on fiscal quarter results								20,700			20,700
Revolving Credit Facility | Credit Agreement | Three months period ending March 31, 2018
Long-Term Debt
Reduction of the available credit facility depending on fiscal quarter results								10,700			10,700
Revolving Credit Facility | Credit Agreement | Periods subsequent to March 31, 2018
Long-Term Debt
Available credit facility								20,700			20,700
Term Loan | Credit Agreement
Long-Term Debt
Maximum borrowing capacity							225,000
Available credit facility								225,000			225,000
Initial amount borrowed							$ 225,000
Quarterly principal payments, as a percent to initial amount borrowed	3.75%		2.50%		1.875%		0.625%
Final amount due in 2018								$ 143,400			$ 143,400
Interest rate in excess of applicable rate upon acceleration and default				2.00%