Reinsurance (Schedule Of Ceded Credit Risk by Reinsurer) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Ceded Credit Risk [Line Items]
Uncollectible reinsurance	$ (8,404)	$ (8,064)	$ (7,713)
Ceded reserve at end of year	3,534,527
Credit Risk Reinsurers, Subtotal, Excluding Residual Market Pools [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	3,408,138
Lloyds of London
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	402,210
Lifson Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	335,382
Partner Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	314,471
Munich Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	310,985
Berkshire Hathaway
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	307,878
Hannover Re Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	231,172
Renaissance Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	217,008
Swiss Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	176,377
Everest Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	145,155
Liberty Mutual
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	117,556
Axis Capital
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	86,680
Fairfax Financial
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	64,300
Korean Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	57,530
Arch Capital Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	54,175
Sompo Holdings Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	47,520
Axa Insurance
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	45,680
TOA Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	37,140
Nationwide Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	32,411
Markel Corp Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	31,827
Helvetia Holdings Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	31,429
Chubb Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	24,459
MS & AD Insurance Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	20,144
Other Reinsurers Less than $20,000
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	316,649
Residual Market Pools
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 134,793