Investments (excluding Consolidated Investment Entities) - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 1,192.1	$ 1,129.8	$ 3,518.4	$ 3,442.1
Less: Investment expenses	28.7	26.2	85.7	82.3
Net investment income	1,163.4	1,103.6	3,432.7	3,359.8
Net investment income	1,163.4	1,103.6	3,432.7	3,359.8
Restatement adjustment
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income		23.1		75.5
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1,004.4	983.6	3,005.0	2,963.0
Number of investments not producing income	14.2		14.2		$ 3.4
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	5.3	3.2	14.2	7.0
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	135.3	135.2	408.0	394.1
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	26.6	27.6	80.8	83.2
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.6	0.7	3.8	2.3
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 19.9	$ (20.5)	$ 6.6	$ (7.5)