Fair Value Measurements - Carrying Values and Estimated Fair Values of Other Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 1,259,840	$ 1,207,526
Term Loan		52,669
Junior Subordinated Notes	61,937	59,870
Total debt, net	1,321,777	1,320,065
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	1,355,657	1,253,614
Term Loan		52,669
Junior Subordinated Notes	61,937	59,870
Total debt, net	$ 1,417,594	$ 1,366,153