Borrowings - Debt Redemption (Details)	3 Months Ended
	Dec. 31, 2019
Senior Notes | 6 3/4% Senior Notes Maturing March of 2025
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	6.75%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	5.875%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029
Debt Instrument [Line Items]
Stated interest rate on debt instrument (percent)	7.25%
Senior Notes | 6 3/4% Senior Notes Maturing March of 2025
Debt Instrument [Line Items]
Percentage of principal amount redeemed	35.00%
Stated interest rate on debt instrument (percent)	6.75%
Redemption price (as a percentage)	106.75%
Percentage of principal amount outstanding after redemption	65.00%
Senior Notes | 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%
Senior Notes | 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%
Senior Notes | 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%
Senior Notes | 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%
Senior Notes | 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%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027
Debt Instrument [Line Items]
Percentage of principal amount redeemed	35.00%
Stated interest rate on debt instrument (percent)	5.88%
Redemption price (as a percentage)	105.875%
Percentage of principal amount outstanding after redemption	65.00%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	102.938%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	101.958%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.979%
Senior Notes | 5 7/8% Senior Notes Maturing October 2027 | Debt Instrument, Redemption, Period Five
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029
Debt Instrument [Line Items]
Percentage of principal amount redeemed	35.00%
Stated interest rate on debt instrument (percent)	7.25%
Redemption price (as a percentage)	107.25%
Percentage of principal amount outstanding after redemption	65.00%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029 | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Redemption price (as a percentage)	103.325%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029 | Debt Instrument, Redemption, Period Three
Debt Instrument [Line Items]
Redemption price (as a percentage)	102.417%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029 | Debt Instrument, Redemption, Period Four
Debt Instrument [Line Items]
Redemption price (as a percentage)	101.208%
Senior Notes | 7 1/4% Senior Notes Maturing October 2029 | Debt Instrument, Redemption, Period Five
Debt Instrument [Line Items]
Redemption price (as a percentage)	100.00%