UNAUDITED CONDENSED QUARTERLY FINANCIAL DATA (DETAILS) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 921,879	$ 934,415	$ 946,990	$ 902,340	$ 921,812	$ 919,341	$ 941,211	$ 904,053	$ 3,705,625	$ 3,686,417	$ 3,870,999
Net investment income	95,517	116,923	91,730	49,164	79,000	45,685	88,544	92,107	353,335	305,336	342,766
Net realized investment gains (losses)	(20,229)	5,205	21,010	(66,508)	(14,872)	(69,957)	(11,110)	(42,553)	(60,525)	(138,491)	132,108
Underwriting income	66,265	103,998	9,860	98,951	88,358	56,245	56,848	100,790
Net income available to common shareholders	$ 130,912	$ 176,644	$ 119,491	$ 38,417	$ 134,787	$ 247,620	$ 63,349	$ 155,803	$ 465,462	$ 601,562	$ 770,657
Basic earnings per common share (in dollars per share)	$ 1.50	$ 1.97	$ 1.30	$ 0.41	$ 1.40	$ 2.52	$ 0.63	$ 1.56	$ 5.13	$ 6.10	$ 7.38
Diluted earnings per common share (in dollars per share)	$ 1.48	$ 1.96	$ 1.29	$ 0.41	$ 1.39	$ 2.50	$ 0.63	$ 1.54	$ 5.08	$ 6.04	$ 7.29