Investments - Net Investment Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 1,146.8	$ 1,200.1
Less: Investment expenses	1.2	1.4
Net investment income	1,145.6	1,198.7
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	984.8	1,012.6
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3.8	2.6
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	115.3	118.2
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	28.0	29.9
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.8	0.9
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 14.1	$ 35.9