Borrowings - Schedule of Long-term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Jun. 15, 2006
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 1,231,254	$ 1,327,412
Debt Instrument, Unamortized Premium	$ (2,640)	(3,413)
5 3/4% Senior Notes Maturing June 2019
Debt Instrument [Line Items]
Debt instrument stated interest rate (percent)	5.75%
8 3/4% Senior Notes Maturing March 2022
Debt Instrument [Line Items]
Debt instrument stated interest rate (percent)	8.75%
7 1/4% Senior Notes Maturing February 2023
Debt Instrument [Line Items]
Debt instrument stated interest rate (percent)	7.25%
6 3/4% Senior Notes Maturing March of 2025
Debt Instrument [Line Items]
Debt instrument stated interest rate (percent)	6.75%
5 7/8% Senior Notes Maturing October 2027
Debt Instrument [Line Items]
Debt instrument stated interest rate (percent)	5.88%
Senior Notes
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 1,163,138	1,259,840
Debt Instrument, Unamortized Premium	2,640	3,413
Unamortized debt issuance costs	(14,336)	(14,800)
Senior Notes | 5 3/4% Senior Notes Maturing June 2019
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 0	321,393
Debt instrument stated interest rate (percent)	5.75%
Senior Notes | 8 3/4% Senior Notes Maturing March 2022
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 500,000	500,000
Debt instrument stated interest rate (percent)	8.75%
Senior Notes | 7 1/4% Senior Notes Maturing February 2023
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 24,834	199,834
Debt instrument stated interest rate (percent)	7.25%
Senior Notes | 6 3/4% Senior Notes Maturing March of 2025
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 250,000	250,000
Debt instrument stated interest rate (percent)	6.75%		6.75%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 400,000	0
Debt instrument stated interest rate (percent)	5.875%
Junior Subordinated Notes (net of unamortized accretion of $36,770 and $38,837, respectively)
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 64,003	61,937
Debt instrument stated interest rate (percent)				7.987%
Other Secured Notes Payable
Debt Instrument [Line Items]
Total debt (net of premium/discount and debt issuance costs)	$ 4,113	$ 5,635