Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 165,211	$ 137,372	$ 294,641	$ 266,394
Less: investment expenses	(6,785)	(5,400)	(13,178)	(9,353)
Net investment income	158,426	131,972	281,463	257,041
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	64	73	131	146
Commercial mortgage and other loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	15,437	13,168	28,619	24,900
Policy loans
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	229	218	356	277
Other invested assets
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	18,579	7,765	5,866	16,607
Short-term investments and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	18,516	16,766	43,968	30,506
Available-for-sale | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	109,465	96,829	210,199	189,040
Trading | Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 2,921	$ 2,553	$ 5,502	$ 4,918