Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Casualty) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development	$ 60,900	$ 5,953
Incurred claims and claim adjustment expenses related to current year	301,265	250,075
Total incurred	362,165	256,028
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	192,335	232,345
Paid claims and allocated claim adjustment expenses related to current year	127,610	90,640
Total paid	319,945	322,985
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	347,751	305,531	$ 372,488
E&S Casualty business unit [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	174,831
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	112,163
All outstanding liabilities before 2010, net of reinsurance	860
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	63,528
E&S Casualty business unit [Member] | Loss Adjustment Expenses Recognized In 2010 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development		11,368	11,928	$ 11,830	$ 12,329	$ 12,921	$ 12,413	$ 11,848	$ 13,241	$ 14,928
Incurred claims and claim adjustment expenses related to current year	9,600
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 88
Cumulative number of reported claims | claim	836
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 9,345	11,138	10,991	10,557	10,407	10,023	8,817	7,175	5,096	$ 3,035
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 net prior year development		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	11,660
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 144
Cumulative number of reported claims | claim	714
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		11,852	12,390	13,330	12,230	13,098	11,301	13,020
Incurred claims and claim adjustment expenses related to current year	12,944
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (318)
Cumulative number of reported claims | claim	637
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		12,205	12,874	12,792	12,384	12,002	13,379
Incurred claims and claim adjustment expenses related to current year	13,330
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (190)
Cumulative number of reported claims | claim	618
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		11,676	11,663	12,034	14,007	15,590
Incurred claims and claim adjustment expenses related to current year	13,167
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (49)
Cumulative number of reported claims | claim	704
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		15,266	15,046	16,746	17,362
Incurred claims and claim adjustment expenses related to current year	15,945
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 247
Cumulative number of reported claims | claim	726
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		16,927	16,513	16,039
Incurred claims and claim adjustment expenses related to current year	18,625
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 240
Cumulative number of reported claims | claim	738
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		15,751	17,845
Incurred claims and claim adjustment expenses related to current year	20,360
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,900
Cumulative number of reported claims | claim	1,123
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		23,056
Incurred claims and claim adjustment expenses related to current year	24,590
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,930
Cumulative number of reported claims | claim	1,739
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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]
Incurred claims and claim adjustment expenses related to current year	34,610
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 26,946
Cumulative number of reported claims | claim	2,277
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 2,526