Borrowings - Debt Redemption (Details) - Senior Notes	6 Months Ended
	Mar. 31, 2018	Sep. 30, 2017
5 3/4% Senior Notes Maturing June 2019
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	5.75%
7 1/2% Senior Notes Maturing September 2021
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	7.50%	7.50%
7 1/2% Senior Notes Maturing September 2021 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	105.625%
7 1/2% Senior Notes Maturing September 2021 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	103.75%
7 1/2% Senior Notes Maturing September 2021 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	101.875%
7 1/2% Senior Notes Maturing September 2021 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
8 3/4% Senior Notes Maturing March 2022
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	8.75%	8.75%
8 3/4% Senior Notes Maturing March 2022 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
8 3/4% Senior Notes Maturing March 2022 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	104.375%
8 3/4% Senior Notes Maturing March 2022 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	102.188%
8 3/4% Senior Notes Maturing March 2022 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
7 1/4% Senior Notes Maturing February 2023
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	7.25%	7.25%
7 1/4% Senior Notes Maturing February 2023 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
7 1/4% Senior Notes Maturing February 2023 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	103.625%
7 1/4% Senior Notes Maturing February 2023 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	102.41%
7 1/4% Senior Notes Maturing February 2023 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	101.208%
6 3/4% Senior Notes Maturing March of 2025
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	6.75%	6.75%
6 3/4% Senior Notes Maturing March of 2025 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
6 3/4% Senior Notes Maturing March of 2025 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	105.063%
6 3/4% Senior Notes Maturing March of 2025 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	103.375%
6 3/4% Senior Notes Maturing March of 2025 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	101.688%
6 3/4% Senior Notes Maturing March of 2025 | Debt Instrument, Redemption, Period Five
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
5 7/8% Senior Notes Maturing October 2027
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	5.875%	0.00%
5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	102.938%
5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	101.958%
5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.979%
5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Five
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%