Investments - Cash Flows from Consolidated Investing Activities by Security (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Gain (Loss) on Investments [Line Items]
Document Period End Date	Mar. 31, 2014
Proceeds from investments sold, matured or repaid	$ 2,863.6	$ 6,016.3
Cost of investments acquired	(4,032.9)	(5,916.2)
Available-for-sale Securities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	2,675.4	5,769.7
Cost of investments acquired	(3,815.7)	(5,839.0)
Trading (acquired for holding) [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid		91.8
Cost of investments acquired	(40.5)	(0.9)
Derivative and other [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	188.2	154.8
Cost of investments acquired	$ (176.7)	$ (76.3)