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Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | $ 2,027,954 | $ 1,975,844 | $ 3,766,119 | $ 3,620,933
Direct premiums earned | 1,785,649 | 1,697,675 | 3,606,663 | 3,344,183
Assumed premiums written | 345,238 | 271,303 | 929,897 | 881,256
Assumed premiums earned | 372,850 | 344,720 | 713,273 | 648,490
Ceded premiums written | (916,797) | (941,509) | (1,593,143) | (1,719,458)
Ceded premiums earned | (798,325) | (842,934) | (1,629,053) | (1,589,235)
Net premiums written | 1,456,395 | 1,305,638 | 3,102,873 | 2,782,731
Net premiums earned | 1,360,174 | 1,199,461 | 2,690,883 | 2,403,438
Underwriting [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | 1,460,090 | 1,323,128 | 2,809,331 | 2,450,516
Direct premiums earned | 1,287,268 | 1,159,544 | 2,562,939 | 2,291,100
Assumed premiums written | 316,905 | 268,837 | 895,561 | 846,934
Assumed premiums earned | 346,262 | 331,842 | 672,962 | 619,217
Ceded premiums written | (319,772) | (286,978) | (599,161) | (515,610)
Ceded premiums earned | (272,600) | (292,374) | (542,138) | (507,540)
Net premiums written | 1,457,223 | 1,304,987 | 3,105,731 | 2,781,840
Net premiums earned | 1,360,930 | 1,199,012 | 2,693,763 | 2,402,777
Program services and other [Member] |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |
Direct premiums written | 567,864 | 652,716 | 956,788 | 1,170,417
Direct premiums earned | 498,381 | 538,131 | 1,043,724 | 1,053,083
Assumed premiums written | 28,333 | 2,466 | 34,336 | 34,322
Assumed premiums earned | 26,588 | 12,878 | 40,311 | 29,273
Ceded premiums written | (597,025) | (654,531) | (993,982) | (1,203,848)
Ceded premiums earned | (525,725) | (550,560) | (1,086,915) | (1,081,695)
Net premiums written | (828) | 651 | (2,858) | 891
Net premiums earned | $ (756) | $ 449 | $ (2,880) | $ 661

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