Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Earned Premiums	$ 1,567,408	$ 1,570,042
Investment Income	174,518	148,858
Other	149,321	151,518
Total revenues	1,891,247	1,870,418
Pre-Tax Income (Loss)	210,990	184,297
Net Income (Loss) to Common Stockholders	166,396	123,447
Net investment gains	48,464	52,348
Net investment gains, net of tax	38,286	34,026
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums	0	0
Investment Income	14,006	12,171
Other	81,889	80,883
Total revenues	95,895	93,054
Pre-Tax Income (Loss)	(81,094)	(72,639)
Net Income (Loss) to Common Stockholders	(65,170)	(47,773)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,432,337	1,413,170
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	1,432,337	1,413,170
Investment Income	135,862	111,909
Other	18,968	18,287
Total revenues	1,587,167	1,543,366
Pre-Tax Income (Loss)	229,028	199,994
Net Income (Loss) to Common Stockholders	181,626	134,095
Reinsurance
Segment Reporting Information [Line Items]
Earned Premiums	135,071	156,872
Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	135,071	156,872
Investment Income	24,650	24,778
Other	0	0
Total revenues	159,721	181,650
Pre-Tax Income (Loss)	14,592	4,594
Net Income (Loss) to Common Stockholders	11,654	3,099
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	56,000	49,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums	0	0
Investment Income	0	0
Insurance-International Segment | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	$ 183,000	$ 184,000