Financial Instruments and Risk Management - Fair Value of the Partnership's Senior Notes (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Bank Debt and Senior Notes [Abstract]
Fair value of Senior Notes	$ 1,083,609	$ 1,125,763
5.5% Senior Notes due June 1, 2024 [Member]
Bank Debt and Senior Notes [Abstract]
Fair value of Senior Notes	509,250	527,888
5.75% Senior Notes due March 1, 2025 [Member]
Bank Debt and Senior Notes [Abstract]
Fair value of Senior Notes	241,250	248,750
5.875% Senior Notes due March 1, 2027 [Member]
Bank Debt and Senior Notes [Abstract]
Fair value of Senior Notes	$ 333,109	$ 349,125