Industry Segments Industry Segments (Net Premiums Earned By Major Line of Business) (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
Revenue from External Customer [Line Items]
Premiums Earned, Net		$ 1,527,335	$ 1,472,013	$ 6,040,609	$ 5,744,418	$ 5,226,537
Insurance-Domestic Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		1,375,358	1,311,276
Insurance-Domestic Segment [Member] | Other Insurance Liabilities [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		422,956	396,904
Insurance-Domestic Segment [Member] | Workers' Compensation [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		345,503	314,738
Insurance-Domestic Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net	[1]	331,857	323,090
Insurance-Domestic Segment [Member] | Commercial Automobile [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		156,817	165,560
Insurance-Domestic Segment [Member] | Products Liability [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		118,225	110,984
Reinsurance-Global Segment [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		151,977	160,737
Reinsurance-Global Segment [Member] | Property [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		51,643	47,921
Reinsurance-Global Segment [Member] | Casualty [Member]
Revenue from External Customer [Line Items]
Premiums Earned, Net		$ 100,334	$ 112,816

[1] For the Three Months Ended March 31,(In thousands)2016 2015 Insurance: Other liability$422,956 $396,904Workers’ compensation345,503 314,738Short-tail lines 331,857 323,090Commercial automobile156,817 165,560Professional liability118,225 110,984Total Insurance1,375,358 1,311,276 Reinsurance: Casualty100,334 112,816Property51,643 47,921Total Reinsurance151,977 160,737 Total$1,527,335 $1,472,013