Fair Value of Financial Instruments - Other Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Fair value of financial instruments
Principal Amount	$ 891,562	$ 875,562
Level 2 | Carrying reported amount fair value disclosure
Fair value of financial instruments
Principal Amount	891,562	875,562
Level 2 | Estimate of fair value measurement
Fair value of financial instruments
Fair Value	878,399	860,627
Term loan A facility | Level 2 | Carrying reported amount fair value disclosure
Fair value of financial instruments
Principal Amount	370,000	380,000
Term loan A facility | Level 2 | Estimate of fair value measurement
Fair value of financial instruments
Fair Value	367,410	376,960
Term loan B facility | Level 2 | Carrying reported amount fair value disclosure
Fair value of financial instruments
Principal Amount	440,562	440,562
Term loan B facility | Level 2 | Estimate of fair value measurement
Fair value of financial instruments
Fair Value	429,989	428,667
Revolving facility | Level 2 | Carrying reported amount fair value disclosure
Fair value of financial instruments
Principal Amount	81,000	55,000
Revolving facility | Level 2 | Estimate of fair value measurement
Fair value of financial instruments
Fair Value	$ 81,000	$ 55,000