Debt - Carrying Amount and Fair Value of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Publicly Traded Debt | Carrying Amount
Debt Instrument [Line Items]
Debt	$ 8,734,479	$ 9,445,569
Publicly Traded Debt | Fair Value
Debt Instrument [Line Items]
Debt	8,512,513	9,734,890
Non-Publicly Traded Debt | Carrying Amount
Debt Instrument [Line Items]
Debt	286,913	1,339,678
Non-Publicly Traded Debt | Fair Value
Debt Instrument [Line Items]
Debt	$ 276,440	$ 1,290,634