Reserves for Losses and Loss Adjustment Expenses (Incurred and Paid Claims Development - Commercial Auto Liability) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total unfavorable (favorable) net prior year development	$ 6,040	$ 58,288
Incurred claims and claim adjustment expenses related to current year	269,204	326,919
Total incurred	275,244	406,907
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	177,571	185,407
Paid claims and allocated claim adjustment expenses related to current year	101,475	113,312
Total paid	279,046	320,419
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	428,766	432,568	$ 346,080
Loss Adjustment Expenses Recognized In 2012 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years										$ 1,181
Commercial Auto business units [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Total incurred	1,127,961
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Total paid	938,823
All outstanding liabilities before 2012, net of reinsurance	1,069
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	190,207
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		75,907	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	75,910
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (25)
Cumulative number of reported claims | claim	3,646
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 75,763	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		105,448	103,571	103,379	102,031	96,974	90,726	86,902	93,692
Incurred claims and claim adjustment expenses related to current year	105,402
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (147)
Cumulative number of reported claims | claim	4,695
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 105,194	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		120,417	116,373	113,357	106,138	99,280	93,187	102,053
Incurred claims and claim adjustment expenses related to current year	120,705
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (1,742)
Cumulative number of reported claims | claim	5,259
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 121,964	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		141,197	135,774	133,574	125,161	106,608	106,133
Incurred claims and claim adjustment expenses related to current year	141,222
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 376
Cumulative number of reported claims | claim	5,923
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 139,681	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		140,794	137,690	121,714	115,044	111,913
Incurred claims and claim adjustment expenses related to current year	141,739
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (217)
Cumulative number of reported claims | claim	6,126
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 135,247	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		154,993	148,563	119,583	125,315
Incurred claims and claim adjustment expenses related to current year	156,528
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ (1,301)
Cumulative number of reported claims | claim	6,276
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 141,545	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		110,052	118,334	119,070
Incurred claims and claim adjustment expenses related to current year	114,281
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 9,449
Cumulative number of reported claims | claim	5,299
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 101,502	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		128,652	118,351
Incurred claims and claim adjustment expenses related to current year	125,815
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 17,576
Cumulative number of reported claims | claim	3,972
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 78,955	61,332	$ 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]
Total unfavorable (favorable) net prior year development		101,568
Incurred claims and claim adjustment expenses related to current year	100,430
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 29,443
Cumulative number of reported claims | claim	3,064
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to prior years	$ 37,308	$ 14,491
Commercial Auto business units [Member] | Loss Adjustment Expenses Recognized In 2021 [Member]
Liability For Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Incurred claims and claim adjustment expenses related to current year	45,927
Total of incurred-but-not-reported liabilities plus expected development on reported claims	$ 32,756
Cumulative number of reported claims | claim	2,075
Liability For Unpaid Claims and Claims Adjustment Expense, Claims Paid [Abstract]
Paid claims and allocated claim adjustment expenses related to current year	$ 1,663