Investments - Real Estate and Other Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Real estate
Depreciation expense on invested real estate	$ 49.3	$ 46.5	$ 44.4
Accumulated depreciation on invested real estate	409.3	388.0
Summarized financial information of unconsolidated entities
Total assets, unconsolidated entities	101,099.0	60,190.2
Total liabilities, unconsolidated entities	52,839.3	47,468.3
Total equity, unconsolidated entities	48,259.7	12,721.9
Net investment in unconsolidated entities	1,098.3	1,159.4
Total revenues, unconsolidated entities	13,171.0	6,297.0	3,002.9
Net income, unconsolidated entities	4,866.0	1,152.7	738.6
Our share of net income of unconsolidated entities	165.3	148.4	$ 133.4
Other Types of Investments
Direct financing leases	819.3	$ 790.8
Securities Posted as Collateral
Fixed maturity securities posted as collateral for a reinsurance arrangement, derivative credit support annex (collateral) agreements, Futures Commission Merchant agreements and a lending arrangement	935.7
Commercial mortgage loans and home equity mortgages posted as collateral associated with obligation under funding agreements with the Federal Home Loan Bank of Des Moines	2,705.5
Securities posted as collateral eligible to be sold or repledged	$ 295.2