Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Investment Income [Line Items]
Net investment income earned	$ 58,505	$ 45,553	$ 109,123	$ 88,784
Fixed Income Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	50,907	43,774	99,940	85,815
Equity Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	1,740	1,820	3,380	3,797
Short-Term Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	1,759	611	3,803	1,134
Other Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	7,494	2,094	8,154	3,657
Investment Expenses [Member]
Net Investment Income [Line Items]
Net investment income earned	$ (3,395)	$ (2,746)	$ (6,154)	$ (5,619)