Fair Value Measurements - Carrying Values and Estimated Fair Values of Other Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 1,062,664	$ 1,111,085
Junior Subordinated Notes	68,137	66,070
Total debt, net	1,130,801	1,177,155
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	1,098,117	1,115,011
Junior Subordinated Notes	68,137	66,070
Total debt, net	$ 1,166,254	$ 1,181,081