Investments - Cash Flows from Consolidated Investing Activities by Security (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Gain (Loss) on Investments [Line Items]
Document Period End Date	Jun. 30, 2014
Proceeds from investments sold, matured or repaid	$ 4,754.5	$ 7,396.3
Cost of investments acquired	(5,929.7)	(7,271.4)
Available-for-sale Securities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid		7,052.1
Cost of investments acquired	(5,594.5)	(7,148.7)
Trading (acquired for holding) [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	54.9	91.8
Cost of investments acquired	(67.8)	(10.2)
Derivative and other [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	297.5	252.4
Cost of investments acquired	(267.4)	(112.5)
Net Investment Income [Member] | Available-for-sale Securities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	$ 4,402.1