Fair Value Measurements - Carrying Values and Estimated Fair Values of Other Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 1,178,792	$ 1,177,155
Carrying Amount | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	1,111,688	1,111,085
Carrying Amount | Junior Subordinated Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	67,104	66,070
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	950,530	1,181,081
Fair Value | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	883,426	1,115,011
Fair Value | Junior Subordinated Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 67,104	$ 66,070