Investments (Details 10) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 D	Dec. 31, 2010
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 10,829.3	$ 11,246.2
Commercial mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	9,461.4	9,689.6
Commercial mortgage loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	5,991.1	5,106.5
Commercial mortgage loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	2,351.1	2,885.6
Commercial mortgage loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	420.1	764.6
Commercial mortgage loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	699.1	932.9
Brick and mortar
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	8,891.8	9,073.0
Brick and mortar | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	5,682.5	4,781.8
Brick and mortar | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	2,112.3	2,636.1
Brick and mortar | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	403.7	726.1
Brick and mortar | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	693.3	929.0
Credit tenant loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	569.6	616.6
Credit tenant loans | A- and above
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	308.6	324.7
Credit tenant loans | BBB+ thru BBB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	238.8	249.5
Credit tenant loans | BB+ thru BB-
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	16.4	38.5
Credit tenant loans | B+ and below
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	5.8	3.9
Residential mortgage loans
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,367.9	1,556.6
Residential mortgage loans | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	1,331.5	1,516.6
Residential mortgage loans | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	36.4	40.0
Mortgage loans, Days delinquent to be considered nonperforming (in days)	90
Home equity
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	611.0	719.3
Home equity | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	597.8	705.0
Home equity | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	13.2	14.3
First liens
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	756.9	837.3
First liens | Performing
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	733.7	811.6
First liens | Nonperforming
Mortgage loan credit quality disclosures
Mortgage loans, Total amortized cost	$ 23.2	$ 25.7