Investments - Cash Flows from Consolidated Investing Activities by Security (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	$ 982.2	$ 1,820.3
Cost of investments acquired	(1,526.0)	(2,728.2)
Available-for-sale Securities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid		1,705.9
Cost of investments acquired	(1,371.8)	(2,598.5)
Trading Securities [Member]
Gain (Loss) on Investments [Line Items]
Cost of investments acquired	0	(4.4)
Derivative and other [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	128.1	114.4
Cost of investments acquired	(154.2)	(125.3)
Investment Income [Member] | Available-for-sale Securities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	$ 854.1