UNAUDITED CONDENSED QUARTERLY FINANCIAL DATA (DETAILS) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 941,911	$ 945,242	$ 945,873	$ 874,039	$ 856,049	$ 862,447	$ 850,603	$ 846,362	$ 3,707,065	$ 3,415,463	$ 3,314,961
Net investment income	113,863	103,429	83,112	108,908	86,847	103,638	74,449	116,023	409,312	380,957	362,430
Net realized investment gains (losses)	19,558	(4,708)	16,235	44,478	31,771	50,803	30,405	14,491	75,564	127,469	121,439
Underwriting income (loss)	94,159	153,088	9,921	170,799	(74,291)	154,999	119,739	62,689
Net income (loss) available to common shareholders	$ 171,524	$ 137,121	$ 72,447	$ 302,816	$ (18,551)	$ 223,407	$ 168,152	$ 121,997	$ 683,910	$ 495,004	$ 9,430
Basic earnings per common share	$ 1.55	$ 1.23	$ 0.63	$ 2.59	$ (0.16)	$ 1.84	$ 1.36	$ 0.97	$ 6.02	$ 4.05	$ 0.08
Diluted earnings per common share	$ 1.52	$ 1.21	$ 0.62	$ 2.55	$ (0.16)	$ 1.82	$ 1.35	$ 0.96	$ 5.93	$ 4.00	$ 0.07