Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 91,497	$ 108,564	$ 282,316	$ 304,523
Less: investment expenses	(4,975)	(4,821)	(15,138)	(16,709)
Net investment income	86,522	103,743	267,178	287,814
Fixed maturities | Available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	59,264	56,901	177,512	186,654
Equity securities | Available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0	0	0	1
Trading account assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	442	424	1,244	1,628
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	13,534	11,792	39,800	46,369
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	16,110	16,034	47,572	46,813
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	282	351	674	1,550
Other long-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 1,865	$ 23,062	$ 15,514	$ 21,508