Fair Value Measurements - Carrying Values and Estimated Fair Values of Other Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 1,163,138	$ 1,259,840
Junior Subordinated Notes	64,003	61,937
Total debt, net	1,227,141	1,321,777
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	1,096,214	1,355,657
Junior Subordinated Notes	64,003	61,937
Total debt, net	$ 1,160,217	$ 1,417,594