Losses And Loss Adjustment Expense Reserves (Activity In The Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 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	11,726	14,055	$ 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	1,181,108	1,127,203
Prior years claims and claim adjustment expense	14,152	61,965
Total incurred losses and loss adjustment expenses	1,195,260	1,189,168
Loss and loss adjustment expense payments related to:
Current year	685,137	650,741
Prior years	471,973	474,124
Total payments	1,157,110	1,124,865
Net reserves at year-end	1,315,237	1,196,738
Gross reserves at year-end	$ 1,326,963	$ 1,210,793