Investments (Details 15) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real estate
Depreciation expense on invested real estate	$ 41.4	$ 41.1	$ 41.7
Accumulated depreciation on invested real estate	361.8	331.2
Summarized financial information of unconsolidated entities
Total assets, unconsolidated entities	38,213.8	31,130.6
Total liabilities, unconsolidated entities	31,305.9	25,257.0
Total equity, unconsolidated entities	6,907.9	5,873.6
Net investment in unconsolidated entities	928.3	804.0
Total revenues, unconsolidated entities	5,574.6	5,326.2	4,235.9
Total expenses, unconsolidated entities	4,849.8	4,812.3	4,228.2
Net income, unconsolidated entities	719.3	489.2	312.7
Our share of net income of unconsolidated entities	116.5	99.9	79.0
Other Types of Investments
Direct financing leases	507.5	443.1
Securities Posted as Collateral
Fixed maturity securities posted as collateral for a reinsurance arrangement, derivative credit support annex (collateral) agreements and obligation under funding agreements with the Federal Home Loan Bank of Des Moines	1,469.5
Commercial mortgage loans posted as collateral associated with obligation under funding agreements with the Federal Home Loan Bank of Des Moines	$ 1,683.2