Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Earned Premiums	$ 1,581,053	$ 1,568,703	$ 3,148,461	$ 3,138,744
Investment Income	153,777	135,264	328,295	284,123
Other	176,086	144,082	325,407	295,600
Total revenues	1,910,916	1,848,049	3,802,163	3,718,467
Pre-Tax Income (Loss)	229,242	160,961	440,232	345,258
Net Income (Loss) to Common Stockholders	180,075	109,004	346,472	232,451
Net investment gains	69,631	40,453	118,095	92,801
Net investment gains, net of tax	55,009	26,294	93,295	60,321
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums	0	0	0	0
Investment Income	17,321	11,054	31,327	23,226
Other	87,748	80,645	169,637	161,528
Total revenues	105,069	91,699	200,964	184,754
Pre-Tax Income (Loss)	(59,744)	(80,397)	(140,838)	(153,034)
Net Income (Loss) to Common Stockholders	(47,918)	(52,056)	(113,087)	(99,311)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,456,008	1,415,586	2,888,345	2,828,755
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	1,456,008	1,415,586	2,888,345	2,828,755
Investment Income	114,333	102,719	250,195	214,628
Other	18,707	22,984	37,675	41,271
Total revenues	1,589,048	1,541,289	3,176,215	3,084,654
Pre-Tax Income (Loss)	198,052	186,134	427,080	386,127
Net Income (Loss) to Common Stockholders	156,200	124,442	337,826	257,458
Reinsurance
Segment Reporting Information [Line Items]
Earned Premiums	125,045	153,117	260,116	309,989
Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums	125,045	153,117	260,116	309,989
Investment Income	22,123	21,491	46,773	46,269
Other	0	0	0	0
Total revenues	147,168	174,608	306,889	356,258
Pre-Tax Income (Loss)	21,303	14,771	35,895	19,364
Net Income (Loss) to Common Stockholders	16,784	10,324	28,438	13,983
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	55,000	50,000	112,000	101,000
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	$ 176,000	$ 168,000	$ 359,000	$ 347,000