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Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Premiums Written:                      
Direct                 $ 2,733,459 $ 2,577,259 $ 2,403,519
Assumed                 26,685 28,779 23,848
Ceded premiums written                 (389,503) (368,750) (357,463)
Net                 2,370,641 2,237,288 2,069,904
Premiums Earned:                      
Direct                 2,647,488 2,484,715 2,330,267
Assumed                 25,831 28,214 23,209
Ceded premiums earned                 (382,292) (363,357) (363,567)
Total Net Premiums Earned $ 590,088 $ 572,055 $ 568,030 $ 560,854 $ 552,753 $ 542,429 $ 531,932 $ 522,458 2,291,027 2,149,572 1,989,909
Loss and loss expense incurred:                      
Direct                 1,570,678 1,560,356 1,274,872
Assumed                 17,588 22,708 16,996
Ceded loss and loss expense                 (243,192) (348,267) (143,327)
Net                 $ 1,345,074 $ 1,234,797 $ 1,148,541