Investments (excluding Consolidated Investment Entities) - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 1,129.8	$ 1,166.0	$ 3,442.1	$ 3,434.6
Less: Investment expenses	3.1	2.4	6.8	4.5
Net investment income	1,126.7	1,163.6	3,435.3	3,430.1
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	983.6	1,006.8	2,963.0	2,983.9
Number of investments not producing income	3.6		3.6		$ 0.1
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.2	2.3	7.0	9.7
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	135.2	124.2	394.1	358.5
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	27.6	28.5	83.2	84.6
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.7	0.9	2.3	2.4
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ (20.5)	$ 3.3	$ (7.5)	$ (4.5)