Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 1,971,333	$ 1,807,211	$ 1,811,056	$ 1,744,679	$ 1,855,914	$ 1,789,765	$ 2,019,727	$ 1,860,957	$ 7,654,184	$ 7,206,457	$ 7,128,928
Net premiums earned									6,293,348	6,040,609	5,744,418
Consolidation Items [Domain]
Segment Reporting Information [Line Items]
Revenues									7,654,184	7,206,457	7,128,928
Operating Segments
Segment Reporting Information [Line Items]
Income Before Income Taxes									896,438	732,030	952,196
Insurance
Segment Reporting Information [Line Items]
Net premiums earned									5,652,903	5,431,500	5,074,308
Insurance | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,798,771	1,650,131	1,541,836
Insurance | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,408,911	1,363,513	1,198,701
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned									1,299,545	1,298,883	1,291,021
Insurance | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned									642,452	674,078	642,713
Insurance | Professional Liability
Segment Reporting Information [Line Items]
Net premiums earned									503,224	444,895	400,037
Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									640,445	609,109	670,110
Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned									390,863	421,811	487,264
Reinsurance | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 249,582	$ 187,298	$ 182,846