Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 90,813	$ 91,497	$ 259,484	$ 282,316
Less: investment expenses	(4,627)	(4,975)	(13,734)	(15,138)
Net investment income	86,186	86,522	245,750	267,178
Equity securities, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	222	316	664	758
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	12,462	13,534	37,170	39,800
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	16,628	16,110	48,861	47,572
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	952	282	1,389	674
Other invested assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	5,169	1,865	7,027	15,514
Available-for-sale | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	55,109	59,264	163,564	177,512
Trading | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 271	$ 126	$ 809	$ 486