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
Net Investment Income [Line Items]
Net investment income earned	$ 68,185	$ 55,826	$ 158,596	$ 164,949
Fixed Income Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	51,285	50,749	152,617	150,689
Commercial Mortgage Loans [Member]
Net Investment Income [Line Items]
Net investment income earned	245	0	463	0
Equity Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	1,948	1,885	5,523	5,265
Short-Term Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	244	1,410	1,830	5,213
Other Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	18,671	5,267	9,167	13,421
Investment Expenses [Member]
Net Investment Income [Line Items]
Net investment income earned	(4,208)	(3,485)	(11,004)	(9,639)
Alternative Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	$ 18,700	$ 5,200	$ 8,900	$ 13,000