Reinsurance (Schedule Of Ceded Credit Risk by Reinsurer) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 1,783,200	$ 1,743,980
Credit Risk Reinsurers, Subtotal, Excluding Residual Market Pools [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	1,371,940
Munich Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	156,368
Lloyds of London
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	152,934
Alleghany Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	152,468
Swiss Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	129,369
Partner Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	87,491
Axis Capital
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	82,803
Hannover Re Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	64,011
Berkshire Hathaway [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	56,892
Everest Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	50,387
Korean Re [Domain]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	44,072
Chubb Ltd. [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	30,977
Renaissance Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	27,095
Liberty Mutual [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	22,629
Arch Capital Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	21,310
Other Reinsurers
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	293,134
Residual Market Pools
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 411,260