Fair Value Measurements - Carrying Values and Estimated Fair Values of Other Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 1,208,173	$ 1,207,526
Term Loan	53,080	52,669
Junior Subordinated Notes	60,387	59,870
Carrying values and estimated fair values of other financial assets and liabilities	1,321,640	1,320,065
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	1,282,912	1,253,614
Term Loan	53,080	52,669
Junior Subordinated Notes	60,387	59,870
Carrying values and estimated fair values of other financial assets and liabilities	$ 1,396,379	$ 1,366,153