Investments - Cash Flows from Consolidated Investing Activities by Security (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	$ 5,609.2	$ 9,432.2	$ 6,206.7
Cost of investments acquired	(7,221.4)	(8,940.8)	(5,972.7)
Available-for-sale Securities
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid		8,986.9	5,833.4
Cost of investments acquired	(6,741.2)	(8,757.5)	(5,640.1)
Held-to-maturity Securities
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid			109.6
Cost of investments acquired			(68.7)
Trading (acquired for holding)
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	54.9	92.9	106.1
Cost of investments acquired	(99.7)	(20.8)	(122.3)
Derivative and other
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	470.2	352.4	157.6
Cost of investments acquired	(380.5)	(162.5)	(141.6)
Net Investment Income
Gain (Loss) on Investments [Line Items]
Available For Sale Securities On Fixed Maturity Under Commitment Agreement	5,033.4	8,986.9	4,603.0
Net Investment Income | Available-for-sale Securities
Gain (Loss) on Investments [Line Items]
Proceeds from investments sold, matured or repaid	$ 5,084.1