Fair Value Measurements - Carrying Values and Estimated Fair Values of Other Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 983,827	$ 1,062,664
Junior Subordinated Notes	70,203	68,137
Total debt, net	1,054,030	1,130,801
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	1,046,965	1,098,117
Junior Subordinated Notes	70,203	68,137
Total debt, net	$ 1,117,168	$ 1,166,254