Summary of Significant Accounting Policies - Equity-Based Compensation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016
New Accounting Pronouncement, Early Adoption [Line Items]
Income tax benefits related to excess tax benefits for share-based awards	$ 10.3
Decrease to deferred income tax liabilities	$ (1,357.0)	$ (1,212.4)
Retained earnings
New Accounting Pronouncement, Early Adoption [Line Items]
Increase to retained earnings		4.9
ASU 2016-09 \| New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncement, Early Adoption [Line Items]
Decrease to deferred income tax liabilities		4.9
ASU 2016-09 \| New Accounting Pronouncement, Early Adoption, Effect \| Retained earnings
New Accounting Pronouncement, Early Adoption [Line Items]
Increase to retained earnings		$ 4.9

Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount

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- Definition

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

+ References

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- Definition

Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences with jurisdictional netting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84176650&loc=d3e31931-109318

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