Losses And Loss Adjustment Expense Reserves (Activity In The Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves at January 1	$ 1,290,248	$ 1,146,688
Reissuance recoverable	10,798	13,961	$ 13,161	$ 14,253
Net reserves at January 1	1,277,087	1,132,435
Incurred losses and loss adjustment expense related to:
Current year	1,772,203	1,696,350
Prior years claims and claim adjustment expense	18,347	69,134
Total incurred losses and loss adjustment expenses	1,790,550	1,765,484
Loss and loss adjustment expense payments related to:
Current year	1,099,656	1,065,948
Prior years	606,615	601,379
Total payments	1,706,271	1,667,327
Net reserves at period end	1,361,366	1,230,592
Gross reserves at period end	$ 1,372,164	$ 1,244,553