Reinsurance (Schedule Of Ceded Credit Risk by Reinsurer) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 2,133,683	$ 1,932,291
Credit Risk Reinsurers, Subtotal, Excluding Residual Market Pools [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	1,814,515
Munich Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	243,021
Lloyds of London
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	201,092
Swiss Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	179,274
Alleghany Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	169,185
Partner Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	127,638
Hannover Re Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	95,486
Axis Capital
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	93,547
Berkshire Hathaway
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	82,882
Renaissance Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	79,954
Korean Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	64,464
Everest Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	55,431
Liberty Mutual
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	49,346
Arch Capital Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	27,116
Qatar Re GRP
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	22,477
Chubb Limited
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	15,199
Other Reinsurers Less than $20,000
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	308,404
Residual Market Pools
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 319,168