Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Statement of Financial Position [Abstract]
Investments, at fair value	$ 619.4	$ 551.6
Preferred stock, par value	$ 1	$ 1
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$ 0.1	$ 0.1
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	504,120,477	519,122,574
Common stock, shares outstanding	504,120,477	519,122,574