Reserves for Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Casualty) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development	$ 58,288	$ 60,900
Incurred claims and claim adjustment expenses related to current year	332,863	301,265
Total incurred	412,851	362,165
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	185,407	192,335
Paid claims and allocated claim adjustment expenses related to current year	113,312	127,610
Total paid	320,419	319,945
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	440,183	347,751	$ 305,531
Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years										$ 2,897
E&S Casualty business unit [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	233,564
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	137,320
All outstanding liabilities before 2011, net of reinsurance	1,059
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	97,302
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		11,660	11,265	$ 11,407	$ 11,510	$ 11,855	$ 12,529	$ 12,942	$ 11,675	14,331
Incurred claims and claim adjustment expenses related to current year	11,542
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 103
Cumulative number of reported claims | claim	720
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 11,294	11,204	11,149	9,868	9,642	9,292	8,334	6,007	4,292	$ 2,340
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2012 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		12,944	11,852	12,390	13,330	12,230	13,098	11,301	13,020
Incurred claims and claim adjustment expenses related to current year	13,400
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (393)
Cumulative number of reported claims | claim	655
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 12,725	12,572	11,357	10,938	10,255	8,037	6,096	2,666	$ 1,337
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2013 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		13,330	12,205	12,874	12,792	12,384	12,002	13,379
Incurred claims and claim adjustment expenses related to current year	13,274
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (394)
Cumulative number of reported claims | claim	629
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 12,478	12,255	11,866	11,134	9,212	5,461	3,190	$ 1,331
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2014 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		13,167	11,676	11,663	12,034	14,007	15,590
Incurred claims and claim adjustment expenses related to current year	13,140
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (344)
Cumulative number of reported claims | claim	708
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 13,003	12,365	11,327	8,075	5,499	4,196	$ 1,829
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized in 2015 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		15,945	15,266	15,046	16,746	17,362
Incurred claims and claim adjustment expenses related to current year	16,931
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (298)
Cumulative number of reported claims | claim	741
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 15,729	16,158	13,553	8,258	4,133	$ 1,420
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized in 2016 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		18,625	16,927	16,513	16,039
Incurred claims and claim adjustment expenses related to current year	17,508
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (897)
Cumulative number of reported claims | claim	757
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 17,111	16,442	11,269	5,672	$ 1,753
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized in 2017 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		20,360	15,751	17,845
Incurred claims and claim adjustment expenses related to current year	18,023
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (781)
Cumulative number of reported claims | claim	1,208
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 16,264	11,268	5,884	$ 2,900
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2018 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		24,590	23,056
Incurred claims and claim adjustment expenses related to current year	23,856
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 766
Cumulative number of reported claims | claim	1,827
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 15,363	8,027	$ 2,708
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2019 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		34,610
Incurred claims and claim adjustment expenses related to current year	43,426
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 10,666
Cumulative number of reported claims | claim	2,693
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 17,177	$ 2,526
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2020 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to current year	62,464
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 40,788
Cumulative number of reported claims | claim	6,752
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 6,176