Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Direct Premiums Written									$ 3,012,900	$ 2,853,200	$ 2,737,500
Net premiums earned	$ 760,094	$ 745,520	$ 731,546	$ 720,737	$ 709,368	$ 705,237	$ 697,889	$ 683,701	2,957,897	2,796,195	2,698,187
Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									2,345,800	2,223,100	2,165,600
Net premiums earned									2,308,600	2,203,000	2,163,400
Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									402,200	374,500	340,000
Net premiums earned									379,700	347,900	328,500
Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									153,500	135,900	104,700
Net premiums earned									144,400	121,800	88,300
Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									111,400	119,700	127,200
Net premiums earned									125,200	123,500	118,000
Property and Casualty Lines [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									2,983,100	2,808,900	2,674,700
Net premiums earned									2,906,600	2,737,300	2,638,400
Property and Casualty Lines [Member] | Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									2,345,800	2,223,100	2,165,600
Net premiums earned									2,308,600	2,203,000	2,163,400
Property and Casualty Lines [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									402,200	374,500	340,000
Net premiums earned									379,700	347,900	328,500
Property and Casualty Lines [Member] | Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									153,500	135,900	104,700
Net premiums earned									144,400	121,800	88,300
Property and Casualty Lines [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									81,600	75,400	64,400
Net premiums earned									73,900	64,600	58,200
Other Segments [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									29,800	44,300	62,800
Net premiums earned									51,300	58,900	59,800
Other Segments [Member] | Property and Casualty, Personal Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									0	0	0
Net premiums earned									0	0	0
Other Segments [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									0	0	0
Net premiums earned									0	0	0
Other Segments [Member] | Property and Casualty, Commercial Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									0	0	0
Net premiums earned									0	0	0
Other Segments [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Direct Premiums Written									29,800	44,300	62,800
Net premiums earned									$ 51,300	$ 58,900	$ 59,800