Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Specialty Commercial Segment) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) claim	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	$ 5,953	$ 40,105
Incurred claims and claim adjustment expenses related to current year	250,075	248,203
Total incurred	256,028	288,308
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	232,345	181,277
Paid claims and allocated claim adjustment expenses related to current year	90,640	92,873
Total paid	322,985	274,150
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	305,531	372,488	$ 358,330
Specialty Commercial Segment Member
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	1,319,942
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	1,092,246
All outstanding liabilities before 2009, net of reinsurance	693
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	228,389
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2009 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	$ 62,509	61,761	61,361	$ 59,988	$ 59,635	$ 59,831	$ 59,471	$ 61,196	$ 62,679	$ 60,950
Cumulative number of reported claims | claim	5,389
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 61,397	61,202	60,785	59,149	57,641	56,791	53,135	45,757	34,411	$ 21,259
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2010 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	77,647	78,253	77,631	77,972	78,003	77,593	75,695	78,089	74,187
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 231
Cumulative number of reported claims | claim	5,071
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 77,075	76,906	75,787	74,513	73,398	68,625	58,139	45,234	$ 24,818
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	89,815	87,833	87,793	89,737	90,713	91,059	87,558	88,679
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 98
Cumulative number of reported claims | claim	5,852
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 87,200	85,845	84,936	83,787	80,004	71,697	53,509	$ 27,454
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2012 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	117,469	117,925	116,320	115,709	111,841	111,253	106,371
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (172)
Cumulative number of reported claims | claim	7,424
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 116,607	113,909	109,060	97,680	82,066	60,923	$ 37,655
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2013 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	151,471	150,177	143,983	137,230	135,114	140,546
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 336
Cumulative number of reported claims | claim	9,294
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 148,706	139,759	126,025	101,725	76,366	$ 40,475
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2014 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	152,025	144,728	138,842	133,464	144,996
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (800)
Cumulative number of reported claims | claim	10,242
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 146,290	123,649	99,521	73,631	$ 42,097
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized in 2015 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	171,315	162,616	146,610	147,304
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 854
Cumulative number of reported claims | claim	11,057
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 159,707	125,514	74,906	$ 39,515
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized in 2016 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	164,570	157,836	151,494
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 5,527
Cumulative number of reported claims | claim	11,750
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 143,685	84,616	$ 41,397
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized in 2017 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	162,193	170,622
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 17,247
Cumulative number of reported claims | claim	11,806
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 109,220	$ 45,477
Paid claims and allocated claim adjustment expenses related to current year	42,359
Specialty Commercial Segment Member | Loss Adjustment Expenses Recognized In 2018 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to current year	170,926
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 87,288
Cumulative number of reported claims | claim	9,702