Reserves for Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Commercial Auto Liability) (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 Auto business units [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	1,136,532
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	918,550
All outstanding liabilities before 2011, net of reinsurance	1,017
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	218,999
Commercial Auto business units [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		57,499	55,775	$ 53,691	$ 53,587	$ 55,259	$ 55,853	$ 55,934	$ 52,099	49,933
Incurred claims and claim adjustment expenses related to current year	57,502
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 52
Cumulative number of reported claims | claim	3,299
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 57,450	53,275	53,276	53,203	52,928	52,261	49,793	44,227	27,773	$ 8,288
Commercial Auto business units [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		76,333	75,657	75,464	73,153	71,515	68,225	69,628	60,844
Incurred claims and claim adjustment expenses related to current year	75,907
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 4
Cumulative number of reported claims | claim	3,645
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 75,893	75,039	75,190	72,907	69,026	59,883	46,510	30,046	$ 12,859
Commercial Auto business units [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		103,571	103,379	102,031	96,974	90,726	86,902	93,692
Incurred claims and claim adjustment expenses related to current year	105,448
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (132)
Cumulative number of reported claims | claim	4,694
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 105,194	106,894	101,146	93,554	83,184	63,255	40,670	$ 13,333
Commercial Auto business units [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		116,373	113,357	106,138	99,280	93,187	102,053
Incurred claims and claim adjustment expenses related to current year	120,417
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (1,657)
Cumulative number of reported claims | claim	5,258
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 121,683	112,617	107,912	88,823	67,410	43,078	$ 17,145
Commercial Auto business units [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		135,774	133,574	125,161	106,608	106,133
Incurred claims and claim adjustment expenses related to current year	141,197
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 443
Cumulative number of reported claims | claim	5,919
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 139,605	141,678	123,668	95,056	48,239	$ 18,108
Commercial Auto business units [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		137,690	121,714	115,044	111,913
Incurred claims and claim adjustment expenses related to current year	140,794
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (47)
Cumulative number of reported claims | claim	6,118
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 133,918	129,761	106,707	53,398	$ 19,788
Commercial Auto business units [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		148,563	119,583	125,315
Incurred claims and claim adjustment expenses related to current year	154,993
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (437)
Cumulative number of reported claims | claim	6,265
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 143,791	133,880	77,884	$ 22,578
Commercial Auto business units [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		118,334	119,070
Incurred claims and claim adjustment expenses related to current year	110,052
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 6,965
Cumulative number of reported claims | claim	5,257
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 86,894	49,912	$ 26,101
Commercial Auto business units [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		118,351
Incurred claims and claim adjustment expenses related to current year	128,652
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 46,179
Cumulative number of reported claims | claim	3,886
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 39,632	$ 16,812
Commercial Auto business units [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	101,568
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 65,968
Cumulative number of reported claims | claim	2,645
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 14,491