Reserves for Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Commercial Accounts) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development	$ 58,288	$ 60,900
Incurred claims and claim adjustment expenses related to current year	332,863	301,265
Total incurred	412,851	362,165
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	185,407	192,335
Paid claims and allocated claim adjustment expenses related to current year	113,312	127,610
Total paid	320,419	319,945
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	440,183	347,751	$ 305,531
Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years										$ 2,897
Commercial Accounts business unit [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	409,365
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	351,328
All outstanding liabilities before 2011, net of reinsurance	1,566
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	59,603
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2011 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		38,585	38,585	$ 40,100	$ 40,745	$ 41,081	$ 41,721	$ 45,723	$ 46,540	49,375
Incurred claims and claim adjustment expenses related to current year	38,492
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 145
Cumulative number of reported claims | claim	2,915
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 38,278	38,228	38,311	38,253	37,214	36,302	35,333	33,701	30,811	$ 22,002
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2012 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		38,669	38,669	40,350	41,451	42,632	44,625	48,085	47,194
Incurred claims and claim adjustment expenses related to current year	39,462
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 228
Cumulative number of reported claims | claim	2,711
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 38,518	38,211	38,044	37,808	35,943	34,597	32,378	30,096	$ 22,264
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2013 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		40,211	39,567	42,822	43,917	46,990	47,385	46,413
Incurred claims and claim adjustment expenses related to current year	40,141
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 216
Cumulative number of reported claims | claim	2,804
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 39,560	39,329	38,892	37,947	36,225	33,927	29,586	$ 19,386
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2014 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		42,463	43,673	43,806	43,806	46,470	46,280
Incurred claims and claim adjustment expenses related to current year	42,503
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 411
Cumulative number of reported claims | claim	2,741
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 41,658	41,162	39,185	36,775	33,544	31,150	$ 21,322
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized in 2015 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		39,287	38,385	41,429	42,580	40,966
Incurred claims and claim adjustment expenses related to current year	39,206
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 931
Cumulative number of reported claims | claim	2,576
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 37,135	35,611	35,238	30,974	28,501	$ 16,557
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized in 2016 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		43,111	41,983	43,449	43,327
Incurred claims and claim adjustment expenses related to current year	45,267
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,662
Cumulative number of reported claims | claim	2,539
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 40,828	39,417	35,035	29,456	$ 19,776
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized in 2017 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		43,579	42,704	40,943
Incurred claims and claim adjustment expenses related to current year	44,732
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,971
Cumulative number of reported claims | claim	2,733
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 41,631	36,650	28,813	$ 16,644
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2018 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		41,290	42,898
Incurred claims and claim adjustment expenses related to current year	44,590
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 4,879
Cumulative number of reported claims | claim	2,591
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 33,546	29,381	$ 19,233
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2019 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development		37,984
Incurred claims and claim adjustment expenses related to current year	34,616
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 4,149
Cumulative number of reported claims | claim	2,689
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 26,233	$ 17,490
Commercial Accounts business unit [Member] | Loss Adjustment Expenses Recognized In 2020 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to current year	40,356
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 19,332
Cumulative number of reported claims | claim	2,574
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 13,942