Industry Segments (Financial Information of Company Operating Segments) (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]
Earned Premiums										$ 6,040,609	$ 5,744,418		$ 5,226,537
Investment Income										512,645	600,885		544,291
Other Revenues										653,203	783,625		637,706
Total revenues		$ 1,811,056	$ 1,860,957	$ 1,789,765	$ 1,744,679	$ 1,784,428	$ 1,840,605	$ 1,796,989	$ 1,706,906	7,206,457	7,128,928		6,408,534
Pre-Tax Income (Loss)										732,030	952,196		698,888
Net income to common stockholders		$ 109,745	$ 152,607	$ 123,035	$ 118,307	$ 110,711	$ 188,539	$ 179,961	$ 169,673	503,694	648,884		499,925
Net investment gains, before tax										92,324	254,852		121,544
Net investment gains, net of tax										60,011	165,654		74,118
Insurance-Domestic
Segment Reporting Information [Line Items]
Earned Premiums										4,659,359	4,271,933		3,782,416
Insurance-International
Segment Reporting Information [Line Items]
Earned Premiums										772,141	802,375		723,151
Reinsurance-Global
Segment Reporting Information [Line Items]
Earned Premiums										609,109	670,110		720,970
Net Investment Gains Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums										0	0		0
Investment Income										0	0		0
Operating Segments | Insurance-Domestic
Segment Reporting Information [Line Items]
Earned Premiums										4,659,359	4,271,933		3,782,416
Investment Income										358,935	428,632		404,280
Other Revenues										96,487	106,853		107,517
Total revenues										5,114,781	4,807,418		4,294,213
Pre-Tax Income (Loss)										724,667	796,309		648,740
Net income to common stockholders										495,082	539,461		449,981
Operating Segments | Insurance-International
Segment Reporting Information [Line Items]
Earned Premiums										772,141	802,375		723,151
Investment Income										51,522	55,407		47,039
Other Revenues										0	0		0
Total revenues										823,663	857,782		770,190
Pre-Tax Income (Loss)										51,926	29,779		56,922
Net income to common stockholders										37,204	22,182		40,292
Operating Segments | Reinsurance-Global
Segment Reporting Information [Line Items]
Earned Premiums										609,109	670,110		720,970
Investment Income										74,226	88,821		89,090
Other Revenues										0	0		0
Total revenues										683,335	758,931		810,060
Pre-Tax Income (Loss)										94,852	115,677		110,425
Net income to common stockholders										66,627	79,720		78,013
Corporate, Other and Eliminations Segment
Segment Reporting Information [Line Items]
Earned Premiums										0	0	[1]	0	[1]
Investment Income										27,962	28,025	[1]	3,882	[1]
Other Revenues										464,392	421,920	[1]	408,645	[1]
Total revenues	[1]									492,354	449,945		412,527
Pre-Tax Income (Loss)										(231,739)	(244,421)	[1]	(238,743)	[1]
Net income to common stockholders										$ (155,230)	$ (158,133)	[1]	$ (142,479)	[1]

[1] Corporate, other and eliminations represent corporate revenues and expenses and other items that are not allocated to business segments.