Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment				2
Earned Premiums	$ 1,581,500		$ 1,585,944	$ 4,720,244	$ 4,673,072
Investment Income	142,479		145,668	426,601	404,850
Other	307,363		288,115	602,963	604,618
Total revenues	2,031,342		2,019,727	5,749,808	5,682,540
Pre-Tax Income (Loss)	226,113		331,615	571,370	664,605
Net Income (Loss) to Common Stockholders	162,054		220,650	394,505	449,127
Net investment gains	183,959		175,738	276,760	189,394
Net investment gains, net of tax	119,573		114,230	179,894	123,106
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums	0		0	0	0
Investment Income	15,027		6,801	38,251	22,827
Other	123,404		112,377	326,203	415,224
Total revenues	138,431		119,178	364,454	438,051
Pre-Tax Income (Loss)	(71,681)		(81,942)	(224,716)	(190,655)
Net Income (Loss) to Common Stockholders	(45,685)		(53,035)	(146,324)	(123,610)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,433,729		1,423,635	4,262,485	4,180,985
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	1,433,729		1,423,635	4,262,485	4,180,985
Investment Income	105,924		111,300	320,552	304,904
Other	0		0	0	0
Total revenues	1,539,653		1,534,935	4,583,037	4,485,889
Pre-Tax Income (Loss)	171,478		210,498	557,605	586,651
Net Income (Loss) to Common Stockholders	123,240		140,730	381,736	394,746
Reinsurance
Segment Reporting Information [Line Items]
Earned Premiums	147,771		162,309	457,759	492,087
Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	147,771		162,309	457,759	492,087
Investment Income	21,528		27,567	67,798	77,119
Other	0		0	0	0
Total revenues	169,299		189,876	525,557	569,206
Pre-Tax Income (Loss)	(57,643)		27,321	(38,279)	79,215
Net Income (Loss) to Common Stockholders	(35,074)		18,725	(20,801)	54,885
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	49,000	$ 50,000	48,000	150,000	153,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums	0		0	0	0
Investment Income	0		0	0	0
Insurance-International Segment | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	$ 166,000		$ 187,000	$ 513,000	$ 546,000