Reinsurance (Schedule Of Ceded Credit Risk by Reinsurer) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 1,743,980	$ 1,532,829
Credit Risk Reinsurers, Subtotal, Excluding Residual Market Pools [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	1,177,930
Validus [Domain]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	22,871
Korean Re [Domain]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	28,654
Alleghany Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	150,604
Munich Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	130,623
Swiss Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	120,906
Lloyds of London
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	118,607
Axis Capital
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	72,600
Partner Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	74,948
Hannover Re Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	52,472
Everest Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	53,482
Berkshire Hathaway [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	49,340
Chubb Ltd. [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	35,304
Arch Capital Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	21,359
Other Reinsurers
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	246,160
Residual Market Pools
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 566,050