Debt - Outstanding Obligations (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018	May 14, 2018	Mar. 31, 2018	Jul. 31, 2017	Jun. 30, 2014
Gross Amount
Total	$ 513,546	$ 515,106		$ 354,792
Less current portion	195,213	197,748		143,750
Total long-term debt	318,333	317,358		211,042
Debt Discount and Issuance Costs
Total	(87,912)	(98,223)		(53,167)
Less current portion	(2,312)	(7,567)		(40,463)
Total long-term debt	(85,600)	(90,656)		(12,704)
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	425,634	416,883		301,625
Current maturities of long-term debt, net	192,901	190,181		103,287
Total long-term debt	232,733	226,702		198,338
2019 Convertible Notes
Long-term Debt, Current Maturities [Abstract]
Stated interest rate						2.125%
2024 Convertible Notes
Long-term Debt, Current Maturities [Abstract]
Stated interest rate					2.875%
Senior Notes | 2019 Convertible Notes
Gross Amount
Total	195,000	195,000		195,000
Debt Discount and Issuance Costs
Total	(2,312)	(7,567)		(12,704)
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	192,688	187,433		182,296
Senior Notes | 2019 Convertible Notes Embedded Derivative
Gross Amount
Total	0	2,552		16,042
Debt Discount and Issuance Costs
Total	0	0		0
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	0	2,552		16,042
Senior Notes | 2024 Convertible Notes
Gross Amount
Total	143,750	143,750		143,750
Debt Discount and Issuance Costs
Total	(35,217)	(37,892)		(40,463)
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	108,533	105,858		103,287
Senior Notes | 2025 Convertible Notes
Gross Amount
Total	172,500	172,500		0
Debt Discount and Issuance Costs
Total	(49,582)	(52,764)		0
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	122,918	119,736		0
Stated interest rate			2.375%
Unsecured Debt | 8.5% unsecured notes due 2024
Gross Amount
Total	1,191	1,304		0
Debt Discount and Issuance Costs
Total	0	0		0
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	$ 1,191	1,304		0
Stated interest rate	8.50%
Receivables Securitization Facility | CASHMAX secured borrowing facility
Gross Amount
Total	$ 1,105	0		0
Debt Discount and Issuance Costs
Total	(801)	0		0
Long-term Debt, Current Maturities [Abstract]
Carrying Amount	$ 304	$ 0		$ 0