Investments - Net Investment Income (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 364,200,000	$ 356,300,000
Less: investment expense	12,700,000	12,700,000
Net investment income	351,500,000	343,600,000
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investments, Nonaccrual Status	3,300,000		0
Gross investment income	305,400,000	300,800,000
Equity securities, available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1,600,000	2,100,000
Mortgage loans on real estate
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	45,500,000	40,100,000
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	3,200,000	3,300,000
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	200,000	100,000
Other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 8,300,000	$ 9,900,000