Fair Value Measurements - Carrying Value and Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Cash and cash equivalents, Carrying value	$ 2,454.9	$ 3,018.5
Restricted Cash and Cash Equivalents	20.0	21.6
Notes payable	4,225.6	4,283.3
Estimate of Fair Value Measurement [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Cash and cash equivalents, Fair value	2,454.9	3,018.5
Restricted cash, Fair value	20.0	21.6
Notes payable, Fair value	4,440.2	4,489.2
Fair Value, Inputs, Level 1 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Cash and cash equivalents, Fair value	2,454.9	3,018.5
Restricted cash, Fair value	20.0	21.6
Notes payable, Fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Cash and cash equivalents, Fair value	0.0	0.0
Restricted cash, Fair value	0.0	0.0
Notes payable, Fair value	3,389.9	3,285.5
Fair Value, Inputs, Level 3 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Cash and cash equivalents, Fair value	0.0	0.0
Restricted cash, Fair value	0.0	0.0
Notes payable, Fair value	$ 1,050.3	$ 1,203.7