Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Specialty Commercial Segment) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) claim	Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	$ 40,105	$ 7,608	$ (6,953)
Incurred claims and claim adjustment expenses related to current year	248,203	246,080	237,102
Total incurred	288,308	253,688	230,149
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	181,277	150,378	122,122
Paid claims and allocated claim adjustment expenses related to current year	92,873	93,067	83,132
Total paid	274,150	243,445	205,254
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	372,488	358,330	348,087	$ 323,192
Specialty Commercial Segment [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	1,188,023
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	912,971
All outstanding liabilities before 2008, net of reinsurance	384
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	275,436
Specialty Commercial Segment [Member] | Loss Adjustment Expenses Recognized In 2008 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	56,272	55,864	55,457	55,425	$ 55,157	$ 56,579	$ 57,923	$ 58,143	$ 56,150	$ 55,617
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (534)
Cumulative number of reported claims | claim	7,373
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 56,094	55,372	54,909	54,080	53,577	53,723	51,280	44,903	37,803	$ 24,134
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	61,761	61,361	59,988	59,635	59,831	59,471	61,196	62,679	60,950
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 60
Cumulative number of reported claims | claim	5,459
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	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	$ 900
Cumulative number of reported claims | claim	5,001
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	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	$ 1,102
Cumulative number of reported claims | claim	5,790
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	117,925	116,320	115,709	111,841	111,253	106,371
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,094
Cumulative number of reported claims | claim	7,339
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	150,177	143,983	137,230	135,114	140,546
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,742
Cumulative number of reported claims | claim	9,210
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	144,728	138,842	133,464	144,996
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,260
Cumulative number of reported claims | claim	10,076
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	162,616	146,610	147,304
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 8,213
Cumulative number of reported claims | claim	10,826
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	157,836	151,494
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 21,557
Cumulative number of reported claims | claim	11,358
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 current year	170,622
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 64,261
Cumulative number of reported claims | claim	10,227
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 45,477