Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Statement of Financial Position [Abstract]
Investments, at fair value	$ 997.3	$ 589.7
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	497,599,517	499,303,269
Common stock, shares outstanding	497,599,517	499,303,269