Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 1,934,956	$ 2,031,342	$ 1,848,049	$ 1,870,418	$ 1,971,333	$ 2,019,727	$ 1,855,914	$ 1,807,211	$ 7,684,764	$ 7,654,184	$ 7,206,457
Net premiums earned									6,311,419	6,293,348	6,040,609
Insurance
Segment Reporting Information [Line Items]
Net premiums earned									5,706,443	5,618,842	5,393,166
Insurance | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,843,826	1,761,748	1,614,453
Insurance | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,481,507	1,402,611	1,355,631
Insurance | Short-tail lines
Segment Reporting Information [Line Items]
Net premiums earned									1,184,799	1,280,091	1,277,538
Insurance | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned									650,441	642,452	674,078
Insurance | Professional Liability
Segment Reporting Information [Line Items]
Net premiums earned									545,870	531,940	471,466
Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									604,976	674,506	647,443
Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned									377,650	405,470	438,800
Reinsurance | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 227,326	$ 269,036	$ 208,643