Summary of Significant Accounting Policies - Equity and Cost Method Investments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Accounting Policies [Abstract]
Equity method investments	$ 59.1	$ 25.9	$ 16.2
Schedule of Cost-method Investments [Line Items]
Cost method investments	61.3	70.1
Private Equity Partnership That Invests in Renewable Energy Companies
Schedule of Cost-method Investments [Line Items]
Cost method investments	$ 7.0	$ 18.0