Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development – Commercial Auto Liability) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development	$ 60,900	$ 5,953
Incurred claims and claim adjustment expenses related to current year	301,265	250,075
Total incurred	362,165	256,028
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	192,335	232,345
Paid claims and allocated claim adjustment expenses related to current year	127,610	90,640
Total paid	319,945	322,985
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	347,751	305,531	$ 372,488
Commercial Auto business units [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	1,056,890
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	866,994
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	189,896
Commercial Auto business units [Member] | Loss Adjustment Expenses Recognized In 2010 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable net prior year development		46,944	46,805	$ 46,637	$ 46,557	$ 46,097	$ 46,338	$ 44,597	$ 44,052	$ 38,276
Incurred claims and claim adjustment expenses related to current year	$ 44,402
Cumulative number of reported claims | claim	2,656
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 47,125	46,604	46,607	46,487	45,544	44,818	41,298	32,807	23,464	$ 7,476
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 net prior year development		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	57,499
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 46
Cumulative number of reported claims | claim	3,328
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		75,657	75,464	73,153	71,515	68,225	69,628	60,844
Incurred claims and claim adjustment expenses related to current year	76,333
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 45
Cumulative number of reported claims | claim	4,932
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		103,379	102,031	96,974	90,726	86,902	93,692
Incurred claims and claim adjustment expenses related to current year	103,571
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (417)
Cumulative number of reported claims | claim	6,564
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		113,357	106,138	99,280	93,187	102,053
Incurred claims and claim adjustment expenses related to current year	116,373
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (1,289)
Cumulative number of reported claims | claim	7,308
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		133,574	125,161	106,608	106,133
Incurred claims and claim adjustment expenses related to current year	135,774
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 1,093
Cumulative number of reported claims | claim	8,167
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		121,714	115,044	111,913
Incurred claims and claim adjustment expenses related to current year	137,690
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 2,352
Cumulative number of reported claims | claim	8,771
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		119,583	125,315
Incurred claims and claim adjustment expenses related to current year	148,563
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 12,280
Cumulative number of reported claims | claim	8,586
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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 net prior year development		119,070
Incurred claims and claim adjustment expenses related to current year	118,334
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 34,175
Cumulative number of reported claims | claim	7,605
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 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]
Incurred claims and claim adjustment expenses related to current year	118,351
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 48,275
Cumulative number of reported claims | claim	6,293
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 16,812