Industry Segments (Financial Information of Company Operating Segments) (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]
Earned Premiums										$ 6,293,348	$ 6,040,609		$ 5,744,418
Investment Income										564,163	512,645		600,885
Other Income										376	337		1,308
Total 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 income to common stockholders		$ 152,790	$ 119,511	$ 109,745	$ 118,307	$ 108,967	$ 123,035	$ 220,650	$ 152,607	601,916	503,694		648,884
Insurance
Segment Reporting Information [Line Items]
Earned Premiums										5,652,903	5,431,500		5,074,308
Reinsurance
Segment Reporting Information [Line Items]
Earned Premiums										640,445	609,109		670,110
Net Investment Gains Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums										0	0		0
Investment Income										0	0		0
Other Income										267,005	92,324		254,852
Investment Income, Net										267,005	92,324		254,852
Income (Loss) from Subsidiaries, Net of Tax										173,553	60,011		165,654
Pre-Tax Income (Loss)										267,005	92,324		254,852
Premiums Receivable [Member]
Segment Reporting Information [Line Items]
Earned Premiums										6,293,348	6,040,609		5,744,418
Investment Income [Member]
Segment Reporting Information [Line Items]
Investment Income										564,163	512,645		600,885
Other funds
Segment Reporting Information [Line Items]
Other Revenues										796,673	653,203		783,625
Operating Segments
Segment Reporting Information [Line Items]
Net income to common stockholders										601,916	503,694		648,884
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Earned Premiums										5,652,903	5,431,500		5,074,308
Investment Income										455,139	410,457		484,039
Other Revenues										97,879	96,487		106,853
Total revenues										6,205,921	5,938,444		5,665,200
Pre-Tax Income (Loss)										822,617	776,593		826,088
Net income to common stockholders										551,482	532,286		561,643
Operating Segments | Reinsurance
Segment Reporting Information [Line Items]
Earned Premiums										640,445	609,109		670,110
Investment Income										78,967	74,226		88,821
Other Revenues										0	0		0
Total revenues										719,412	683,335		758,931
Pre-Tax Income (Loss)										74,799	94,852		115,677
Net income to common stockholders										51,531	66,627		79,720
Corporate, Other and Eliminations Segment
Segment Reporting Information [Line Items]
Earned Premiums										0	0	[1]	0	[1]
Investment Income										30,057	27,962	[1]	28,025	[1]
Other Revenues										431,789	464,392	[1]	421,920	[1]
Total revenues	[1]									461,846	492,354		449,945
Pre-Tax Income (Loss)										(267,983)	(231,739)	[1]	(244,421)	[1]
Net income to common stockholders										$ (174,650)	$ (155,230)	[1]	$ (158,133)	[1]

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