Fair Value (Tables)

6 Months Ended

Mar. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair Value Assets and Liabilities Measured on Recurring Basis

Assets and liabilities recorded at fair value on a recurring basis consisted of the following (in millions):         

	As of March 31, 2017								As of September 30, 2016
			Fair Value Measurements								Fair Value Measurements
	Total		Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
Assets
Money market funds	$	494.6	$	494.6	$	—	$	—	$	408.7	$	408.7	$	—	$	—
Auction rate security	—		—		—		—		2.3		—		—		2.3
Total	$	494.6	$	494.6	$	—	$	—	$	411.0	$	408.7	$	—	$	2.3
Liabilities
Contingent consideration liability recorded for business combinations	$	15.7	$	—	$	—	$	15.7	$	7.9	$	—	$	—	$	7.9
Total	$	15.7	$	—	$	—	$	15.7	$	7.9	$	—	$	—	$	7.9

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation

The following table summarizes changes to the fair value of the Level 3 assets (in millions):

	Auction rate security
Balance as of September 30, 2016	$	2.3
Decreases in Level 3 assets	(2.3		)
Balance as of March 31, 2017	$	—

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation

The following table summarizes changes to the fair value of the Level 3 liabilities (in millions):

	Contingent consideration
Balance as of September 30, 2016	$	7.9
Increases to Level 3 liabilities	10.7
Decreases to Level 3 liabilities	(2.9		)
Balance as of March 31, 2017	$	15.7