Industry Segments (Financial Information of Company Operating Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Earned Premiums									$ 5,744,418		$ 5,226,537		$ 4,673,516
Investment Income									600,885		544,291		586,763
Other Revenues									783,625		637,706		563,275
Total revenues	1,784,428	1,840,605	1,796,989	1,706,906	1,696,679	1,634,124	1,570,962	1,506,769	7,128,928		6,408,534		5,823,554
Pre-Tax Income (Loss)									952,196		698,888		701,928
Net income to common stockholders	110,711	188,539	179,961	169,673	130,379	136,974	115,957	116,615	648,884		499,925		510,592
Net investment gains, before tax									254,852		121,544		210,465
Net investment gains, net of tax									165,654		74,118		136,802
Insurance-Domestic
Segment Reporting Information [Line Items]
Earned Premiums									4,271,933		3,782,416		3,417,022
Insurance-International
Segment Reporting Information [Line Items]
Earned Premiums									802,375		723,151		631,841
Reinsurance-Global
Segment Reporting Information [Line Items]
Earned Premiums									670,110		720,970		624,653
Net Investment Gains Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums									0		0		0
Operating Segments | Insurance-Domestic
Segment Reporting Information [Line Items]
Earned Premiums									4,271,933		3,782,416		3,417,022
Investment Income									428,632		404,280		424,787
Other Revenues									106,853		107,517		103,133
Total revenues									4,807,418		4,294,213		3,944,942
Pre-Tax Income (Loss)									796,309		648,740		578,500
Net income to common stockholders									539,461		449,981		397,942
Operating Segments | Insurance-International
Segment Reporting Information [Line Items]
Earned Premiums									802,375		723,151		631,841
Investment Income									55,407		47,039		45,796
Other Revenues									0		0		0
Total revenues									857,782		770,190		677,637
Pre-Tax Income (Loss)									29,779		56,922		51,639
Net income to common stockholders									22,182		40,292		35,928
Operating Segments | Reinsurance-Global
Segment Reporting Information [Line Items]
Earned Premiums									670,110		720,970		624,653
Investment Income									88,821		89,090		106,932
Other Revenues									0		0		0
Total revenues									758,931		810,060		731,585
Pre-Tax Income (Loss)									115,677		110,425		103,690
Net income to common stockholders									79,720		78,013		72,916
Corporate, Other and Eliminations Segment
Segment Reporting Information [Line Items]
Earned Premiums									0		0	[1]	0	[1]
Investment Income									28,025		3,882	[1]	9,248	[1]
Other Revenues									421,920		408,645	[1]	249,677	[1]
Total revenues									449,945	[1]	412,527	[1]	258,925	[1]
Pre-Tax Income (Loss)									(244,421)		(238,743)	[1]	(242,366)	[1]
Net income to common stockholders									$ (158,133)		$ (142,479)	[1]	$ (132,996)	[1]

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