Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net premiums earned	$ 915,012	$ 858,135	$ 2,674,034	$ 2,500,178
Direct Premiums Written	997,000	917,500	2,865,200	2,673,100
Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	698,200	659,600	2,055,100	1,932,100
Direct Premiums Written	743,900	695,100	2,146,800	2,035,100
Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	133,500	120,000	376,700	340,500
Direct Premiums Written	162,900	140,700	452,800	394,000
Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	53,500	48,700	154,500	140,300
Direct Premiums Written	55,200	50,300	165,400	149,900
Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29,800	29,800	87,700	87,300
Direct Premiums Written	35,000	31,400	100,200	94,100
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	907,900	850,500	2,652,800	2,476,900
Direct Premiums Written	987,800	910,600	2,841,200	2,652,600
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	698,200	659,600	2,055,100	1,932,100
Direct Premiums Written	743,900	695,100	2,146,800	2,035,100
Property and Casualty Lines [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	133,500	120,000	376,700	340,500
Direct Premiums Written	162,900	140,700	452,800	394,000
Property and Casualty Lines [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	53,500	48,700	154,500	140,300
Direct Premiums Written	55,200	50,300	165,400	149,900
Property and Casualty Lines [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	22,700	22,200	66,500	64,000
Direct Premiums Written	25,800	24,500	76,200	73,600
Other Segments [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,600	21,200	23,300
Direct Premiums Written	9,200	6,900	24,000	20,500
Other Segments [Member] | Private Passenger Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct Premiums Written	0	0	0	0
Other Segments [Member] | Homeowners Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct Premiums Written	0	0	0	0
Other Segments [Member] | Commercial Automobile Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Direct Premiums Written	0	0	0	0
Other Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	7,100	7,600	21,200	23,300
Direct Premiums Written	$ 9,200	$ 6,900	$ 24,000	$ 20,500