Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Standard 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
Standard Commercial Segment [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	458,755
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	382,460
All outstanding liabilities before 2007, net of reinsurance	1,824
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	78,119
Standard 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	38,294	39,195	40,001	40,387	$ 41,597	$ 42,758	$ 45,556	$ 46,762	$ 47,557	$ 49,452
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 701
Cumulative number of reported claims | claim	3,256
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 37,002	36,859	36,276	35,360	34,762	34,056	32,523	29,058	25,624	$ 17,182
Standard 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	42,168	43,107	43,851	44,159	45,970	46,778	46,772	45,674	44,719
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,082
Cumulative number of reported claims | claim	2,635
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 40,835	40,629	40,575	40,389	38,316	35,818	32,075	28,313	$ 15,242
Standard 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	42,880	42,175	42,426	43,459	43,164	44,847	45,235	45,263
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,265
Cumulative number of reported claims | claim	2,914
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 40,937	39,706	39,115	37,166	33,428	30,957	28,342	$ 21,302
Standard 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	46,841	47,423	47,266	49,268	55,156	56,489	60,236
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,038
Cumulative number of reported claims | claim	4,370
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 43,680	42,441	41,140	40,301	38,909	35,119	$ 24,899
Standard 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	42,986	44,272	45,990	48,222	52,554	51,998
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,505
Cumulative number of reported claims | claim	3,219
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 40,408	38,526	37,191	34,789	32,203	$ 23,445
Standard 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	52,076	53,174	56,703	57,528	55,482
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 4,317
Cumulative number of reported claims | claim	3,920
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 46,756	44,475	41,809	36,411	$ 23,123
Standard 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	53,882	53,568	55,808	55,488
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 5,602
Cumulative number of reported claims | claim	3,555
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 45,779	41,514	37,122	$ 24,255
Standard 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	50,053	49,857	49,571
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 8,768
Cumulative number of reported claims | claim	3,165
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 38,302	34,245	$ 19,085
Standard 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	48,182	46,880
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 11,673
Cumulative number of reported claims | claim	2,779
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 32,006	$ 21,508
Standard 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	41,393
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 18,099
Cumulative number of reported claims | claim	2,330
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 16,755