Losses And Loss Adjustment Expense Reserves (Activity In The Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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
Reissuance recoverable	52,794	12,404	$ 64,001	$ 13,161
Net reserves at January 1	1,446,612	1,277,087
Incurred losses and loss adjustment expense related to:
Current year	589,582	602,541
Prior years claims and claim adjustment expense	42,652	4,124
Total incurred losses and loss adjustment expenses	632,234	606,665
Loss and loss adjustment expense payments related to:
Current year	254,841	275,890
Prior years	345,826	303,333
Total payments	600,667	579,223
Net reserves at period end	1,478,179	1,304,529
Gross reserves at period end	$ 1,530,973	$ 1,316,933