Business Segments (Financial Information of Company Operating Segments) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Earned Premiums (1)	$ 1,646,841	$ 1,581,053	$ 3,239,697	$ 3,148,461
Investment Income	188,333	153,777	346,587	328,295
Other	188,210	176,086	374,122	325,407
Total revenues	2,023,384	1,910,916	3,960,406	3,802,163
Pre-Tax Income (Loss)	273,736	229,242	503,175	440,232
Net Income (Loss) to Common Stockholders	216,709	180,075	397,431	346,472
Net investment gains	73,574	69,631	142,226	118,095
Net investment gains, net of tax	58,123	55,008	112,359	93,295
Corporate, other and eliminations
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	11,481	17,321	30,120	31,327
Other	100,949	87,748	204,965	169,637
Total revenues	112,430	105,069	235,085	200,964
Pre-Tax Income (Loss)	(78,344)	(59,744)	(146,928)	(140,838)
Net Income (Loss) to Common Stockholders	(62,519)	(47,917)	(117,551)	(113,087)
Insurance
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,475,184	1,415,579	2,902,218	2,806,868
Insurance | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	1,475,184	1,415,579	2,902,218	2,806,868
Investment Income	132,403	94,542	232,444	208,040
Other	13,687	18,707	26,931	37,675
Total revenues	1,621,274	1,528,828	3,161,593	3,052,583
Pre-Tax Income (Loss)	225,871	163,181	410,387	362,109
Net Income (Loss) to Common Stockholders	179,241	128,693	325,234	286,426
Reinsurance & Monoline Excess
Segment Reporting Information [Line Items]
Earned Premiums (1)	171,657	165,474	337,479	341,593
Reinsurance & Monoline Excess | Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums (1)	171,657	165,474	337,479	341,593
Investment Income	44,449	41,914	84,023	88,928
Other	0	0	0	0
Total revenues	216,106	207,388	421,502	430,521
Pre-Tax Income (Loss)	52,635	56,174	97,490	100,866
Net Income (Loss) to Common Stockholders	41,864	44,291	77,389	79,838
Re-insurance - International | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	62,000	55,000	121,000	112,000
Net investment gains
Segment Reporting Information [Line Items]
Earned Premiums (1)	0	0	0	0
Investment Income	0	0	0	0
Insurance-International Segment | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	$ 179,000	$ 176,000	$ 349,000	$ 359,000