Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 103,709	$ 107,958	$ 204,214	$ 214,005
Less: investment expenses	(3,549)	(3,850)	(7,343)	(7,648)
Net investment income	100,160	104,108	196,871	206,357
Equity securities, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	108	85	176	149
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	12,758	12,128	26,251	23,536
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	236	740	371	696
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	7,358	7,753	13,885	12,918
Other invested assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	4,880	4,176	8,510	10,947
Available-for-sale | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	77,885	82,013	153,936	163,655
Trading | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 484	$ 1,063	$ 1,085	$ 2,104