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	$ 111,736	$ 119,105	$ 325,741	$ 240,945
Less: investment expenses	(3,783)	(3,787)	(11,431)	(7,798)
Net investment income	107,953	115,318	314,310	233,147
Fixed maturities | Available-for-sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	83,198	75,271	246,853	168,655
Trading account assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	1,176	1,128	3,429	2,193
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	12,777	11,730	36,313	28,106
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	114	185	810	517
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	9,700	10,615	22,618	18,312
Other long-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 4,771	$ 20,176	$ 15,718	$ 23,162