Reinsurance (Schedule Of Ceded Credit Risk by Reinsurer) (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Uncollectible reinsurance	$ (8,064)	$ (7,713)	$ (7,801)
Ceded reserve at end of year	3,187,730
Credit Risk Reinsurers, Subtotal, Excluding Residual Market Pools [Member]
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	3,015,037
Lloyds of London
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	347,927
Berkshire Hathaway
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	332,034
Munich Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	306,530
Partner Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	275,410
Hannover Re Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	191,264
Swiss Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	189,591
Lifson Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	180,724
Renaissance Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	163,973
Everest Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	155,847
Liberty Mutual
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	96,402
Axis Capital
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	81,538
Korean Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	59,884
Fairfax Financial
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	55,228
Axa Insurance
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	46,058
Arch Capital Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	45,663
Sompo Holdings Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	36,157
Helvetia Holdings Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	30,823
Markel Corp Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	30,216
Validus Holdings Group
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	24,548
TOA Re
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	22,945
Other Reinsurers Less than $20,000
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	342,275
Residual Market Pools
Ceded Credit Risk [Line Items]
Ceded reserve at end of year	$ 180,757