Investments - Net Investment Income (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 355,300,000	$ 323,600,000	$ 715,500,000	$ 686,900,000
Less: investment expense	12,400,000	11,700,000	24,400,000	23,000,000
Net investment income	342,900,000	311,900,000	691,100,000	663,900,000
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	300,700,000	303,700,000	608,300,000	608,000,000
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	(1,000,000)	3,000,000	(100,000)	4,100,000
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	40,900,000	37,500,000	77,500,000	71,100,000
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3,200,000	3,300,000	6,400,000	6,600,000
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	300,000	300,000	500,000	500,000
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 11,200,000	$ (24,200,000)	$ 22,900,000	$ (3,400,000)