Loss And Loss Adjustment Expense Reserves (Activity In The Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves at January 1	$ 1,510,613	$ 1,290,248
Less reinsurance recoverables on unpaid losses	46,756	11,726	$ 64,001	$ 13,161
Net reserves at January 1	1,446,612	1,277,087
Incurred losses and loss adjustment expenses related to:
Current year	1,173,862	1,181,108
Prior years	63,919	14,152
Total incurred losses and loss adjustment expenses	1,237,781	1,195,260
Loss and loss adjustment expense payments related to:
Current year	640,496	685,137
Prior years	540,803	471,973
Total payments	1,181,299	1,157,110
Net reserves at June 30	1,503,094	1,315,237
Gross reserves at June 30	$ 1,549,850	$ 1,326,963