Derivative Financial Instruments (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Effect of derivatives in fair value hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	$ (135.2)	$ (135.8)	$ 365.0
Amount of gain (loss) recognized in net income on related hedged item	132.2	139.5	(309.3)
Fair Value Hedges
Effect of derivatives in fair value hedging relationships and the related hedged items on the consolidated statements of operations
Gain (loss) on periodic settlements on interest rate and foreign exchange contracts in fair value hedge of fixed maturities, available-for-sale reported in net investment income	(158.9)	(161.9)	(143.5)
Gain (loss) on periodic settlements on interest rate and foreign exchange contracts in fair value hedge of investment-type insurance contracts reported in benefits, claims and settlement expenses	44.0	76.3	106.2
Fair Value Hedges | Interest rate contracts | Fixed maturities, available-for-sale
Effect of derivatives in fair value hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	(108.5)	(100.2)	308.6
Amount of gain (loss) recognized in net income on related hedged item	105.4	106.4	(264.0)
Fair Value Hedges | Interest rate contracts | Investment-type insurance contracts
Effect of derivatives in fair value hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	(2.2)	(19.2)	(30.8)
Amount of gain (loss) recognized in net income on related hedged item	2.4	20.6	46.9
Fair Value Hedges | Foreign exchange contracts | Fixed maturities, available-for-sale
Effect of derivatives in fair value hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	1.1	6.9	4.8
Amount of gain (loss) recognized in net income on related hedged item	(1.3)	(5.6)	(6.0)
Fair Value Hedges | Foreign exchange contracts | Investment-type insurance contracts
Effect of derivatives in fair value hedging relationships and the related hedged items on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	(25.6)	(23.3)	82.4
Amount of gain (loss) recognized in net income on related hedged item	$ 25.7	$ 18.1	$ (86.2)