Investments (Net Investment Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement [Line Items]
Net investment income earned	$ 57,594	$ 55,826	$ 58,505	$ 50,618	$ 54,109	$ 52,443	$ 45,553	$ 43,231	$ 222,543	$ 195,336	$ 161,882
Fixed Income Securities [Member]
Statement [Line Items]
Net investment income earned									203,255	178,104	153,230
Equity Securities [Member]
Statement [Line Items]
Net investment income earned									6,996	7,764	6,442
Short-term Investments [Member]
Statement [Line Items]
Net investment income earned									6,653	3,472	1,526
Other Investment Portfolio [Member]
Statement [Line Items]
Net investment income earned									18,778	17,799	12,871
Investment Expenses [Member]
Statement [Line Items]
Net investment income earned									$ (13,139)	$ (11,803)	$ (12,187)