Investments (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Major categories of net investment income
Gross investment income (loss)	$ 3,458.8	$ 3,586.0	$ 3,512.3
Investment expenses	(83.0)	(89.5)	(111.5)
Net investment income (loss)	3,375.8	3,496.5	3,400.8
Mortgage loans
Major categories of net investment income
Gross investment income (loss)	649.2	673.3	688.9
Real estate
Major categories of net investment income
Gross investment income (loss)	74.2	57.5	35.9
Policy loans
Major categories of net investment income
Gross investment income (loss)	58.2	60.9	62.0
Cash and cash equivalents
Major categories of net investment income
Gross investment income (loss)	8.5	7.2	13.0
Derivatives
Major categories of net investment income
Gross investment income (loss)	(196.1)	(174.4)	(128.3)
Other investment types
Major categories of net investment income
Gross investment income (loss)	189.0	152.6	104.3
Available-for-sale | Fixed maturities
Major categories of net investment income
Gross investment income (loss)	2,596.2	2,702.1	2,679.3
Available-for-sale | Equity securities
Major categories of net investment income
Gross investment income (loss)	10.5	11.4	16.8
Trading | Fixed maturities
Major categories of net investment income
Gross investment income (loss)	64.7	92.6	37.9
Trading | Equity securities
Major categories of net investment income
Gross investment income (loss)	$ 4.4	$ 2.8	$ 2.5