Fair Value Measurement - Additional Information (Detail) - Fair Value, Inputs, Level 2 [Member] $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($)
Senior Notes Due in 2023 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Debt instrument maturity year	2023
Senior Notes Due in 2025 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Debt instrument maturity year	2025
Carrying Value [Member] | Senior Notes Due in 2023 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	$ 300.0
Carrying Value [Member] | Senior Notes Due in 2025 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	1,300.0
Fair Value [Member] | Senior Notes Due in 2023 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	312.4
Fair Value [Member] | Senior Notes Due in 2025 [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	$ 1,290.0