Industry Segments (Financial Information of Company Operating Segments) (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Earned Premiums		$ 1,527,335	$ 1,472,013	$ 6,040,609	$ 5,744,418	$ 5,226,537
Investment Income and Funds		130,133	124,239	512,645	600,885	544,291
Other Revenues		149,743	148,427	653,203	783,625	637,706
Total revenues		1,807,211	1,744,679	7,206,457	7,128,928	6,408,534
Pre-Tax Income (Loss)		174,676	168,717	732,030	952,196	698,888
Net Income (Loss)		119,511	118,307	503,694	648,884	499,925
Net investment gains		7,343	19,044	92,324	254,852	121,544
Net investment gains, net of tax		4,773	12,379	60,011	165,654	74,118
Intersegment Eliminations and Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums	[1]	0	0	0	0	0
Investment Income and Funds	[1]	6,503	8,114	27,962	28,025	3,882
Other Revenues	[1]	115,607	103,808	464,392	421,920	408,645
Total revenues	[1]	122,110	111,922	492,354	449,945	412,527
Pre-Tax Income (Loss)	[1]	(60,379)	(58,758)	(231,739)	(244,421)	(238,743)
Net Income (Loss)	[1]	(39,649)	(38,611)	(155,230)	(158,133)	(142,479)
Insurance-Domestic Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums		1,375,358	1,311,276
Insurance-Domestic Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned Premiums		1,375,358	1,311,276	5,431,500	5,074,308	4,505,567
Investment Income and Funds		105,229	99,084	410,457	484,039	451,319
Other Revenues		26,793	25,575	96,487	106,853	107,517
Total revenues		1,507,380	1,435,935	5,938,444	5,665,200	5,064,403
Pre-Tax Income (Loss)		205,915	188,169	776,593	826,088	705,662
Net Income (Loss)		139,270	130,039	532,286	561,643	490,273
Reinsurance-Global Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums		151,977	160,737
Reinsurance-Global Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned Premiums		151,977	160,737	609,109	670,110	720,970
Investment Income and Funds		18,401	17,041	74,226	88,821	89,090
Other Revenues		0	0	0	0	0
Total revenues		170,378	177,778	683,335	758,931	810,060
Pre-Tax Income (Loss)		21,797	20,262	94,852	115,677	110,425
Net Income (Loss)		15,117	14,500	66,627	79,720	78,013
Net Investment Gains Segment [Member]
Segment Reporting Information [Line Items]
Earned Premiums		0	0	0	0	0
Investment Income and Funds		$ 0	$ 0	$ 0	$ 0	$ 0

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