Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 154,699	$ 107,047	$ 421,093	$ 311,261
Less: investment expenses	(5,332)	(3,883)	(14,685)	(11,226)
Net investment income	149,367	103,164	406,408	300,035
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	67	72	213	248
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	12,761	12,567	37,661	38,818
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	153	175	430	546
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	31,807	5,103	62,313	18,988
Other invested assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	7,083	5,573	23,690	14,083
Available-for-sale | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	100,249	81,551	289,289	235,487
Trading | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 2,579	$ 2,006	$ 7,497	$ 3,091