Investments - Net Investment Income (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 340,200,000	$ 355,700,000	$ 1,055,700,000	$ 1,042,600,000
Less: investment expense	12,000,000	11,000,000	36,400,000	34,000,000
Net investment income	328,200,000	344,700,000	1,019,300,000	1,008,600,000
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	292,200,000	308,200,000	900,500,000	916,200,000
Investments in fixed maturities not producing income	0		0		0
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1,700,000	2,600,000	1,600,000	6,700,000
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	39,000,000	35,000,000	116,500,000	106,100,000
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3,200,000	3,300,000	9,600,000	9,900,000
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	200,000	300,000	700,000	800,000
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 3,900,000	$ 6,300,000	$ 26,800,000	$ 2,900,000