Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 151,356	$ 154,699	$ 445,997	$ 421,093
Less: investment expenses	(6,687)	(5,332)	(19,865)	(14,685)
Net investment income	144,669	149,367	426,132	406,408
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	63	67	194	213
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	13,898	12,761	42,517	37,661
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	182	153	538	430
Other invested assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	15,102	7,083	20,968	23,690
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	4,601	31,807	48,569	62,313
Available-for-sale | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	113,538	100,249	323,737	289,289
Trading | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 3,972	$ 2,579	$ 9,474	$ 7,497