Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Specialty Commercial Segment) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) claim	Dec. 31, 2015 USD ($)	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 ($)	Dec. 31, 2007 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	$ 7,608	$ (6,953)	$ (5,203)
Incurred claims and claim adjustment expenses related to current year	246,080	237,102	215,258
Total incurred	253,688	230,149	210,055
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	150,378	122,122	123,100
Paid claims and allocated claim adjustment expenses related to current year	93,067	83,132	76,231
Total paid	243,445	205,254	199,331
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	358,330	348,087	323,192	$ 312,468
Specialty Commercial Segment [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	1,029,442
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	776,814
All outstanding liabilities before 2007, net of reinsurance	(26)
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	252,602
Specialty Commercial Segment [Member] | Loss Adjustment Expenses Recognized In 2007 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to prior years	49,544	49,290	49,056	49,096	$ 49,654	$ 50,158	$ 52,673	$ 51,813	$ 52,141	$ 51,458
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 68
Cumulative number of reported claims | claim	6,095
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 49,025	48,860	48,783	48,612	48,612	47,453	44,861	41,344	33,504	$ 18,851
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	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	$ (102)
Cumulative number of reported claims | claim	7,367
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	61,361	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	$ 87
Cumulative number of reported claims | claim	5,456
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	77,631	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	$ 1,227
Cumulative number of reported claims | claim	4,999
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	87,793	89,737	90,713	91,059	87,558	88,679
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,641
Cumulative number of reported claims | claim	5,782
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	116,320	115,709	111,841	111,253	106,371
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,020
Cumulative number of reported claims | claim	7,337
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	143,983	137,230	135,114	140,546
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,127
Cumulative number of reported claims | claim	9,188
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	138,842	133,464	144,996
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 6,547
Cumulative number of reported claims | claim	10,049
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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	146,610	147,304
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 23,447
Cumulative number of reported claims | claim	10,707
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 current year	151,494
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 52,283
Cumulative number of reported claims | claim	10,424
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 41,397