Consolidated Statements Of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)	$ 828,954	$ (80,486)	$ 610,754
Adjustments to reconcile net income to net cash provided by operating activities:
Decrease (increase) in premiums receivable	(153,694)	(235,560)	132,986
Decrease (increase) in funds held by reinsureds, net	42,298	18,236	1,573
Decrease (increase) in reinsurance receivables	(55,274)	97,549	(62,954)
Decrease (increase) in current income taxes	(26,831)	82,702	(9,951)
Decrease (increase) in deferred tax asset	64,909	(194,029)	22,023
Decrease (increase) in prepaid reinsurance premiums	16,358	46,374	(25,281)
Increase (decrease) in reserve for losses and loss adjustment expenses	(169,595)	826,230	420,748
Increase (decrease) in future policy benefit reserve	(1,080)	4,185	(1,534)
Increase (decrease) in unearned premiums	(98,418)	(39,822)	36,883
Increase (decrease) in other net payables to reinsurers	100,566	4,911	(4,982)
Increase (decrease) in losses in course of payment	173,232	18,477	(41,539)
Change in equity adjustments in limited partnerships	(63,779)	(56,549)	(71,493)
Change in other assets and liabilities, net	76,788	108,611	(47,804)
Non-cash compensation expense	31,984	17,693	14,786
Amortization of bond premium (accrual of bond discount)	61,844	47,872	46,095
Amortization of underwriting discount on senior notes	52	49	76
Net realized capital (gains) losses	(164,400)	(6,923)	(101,911)
Net cash provided by (used in) operating activities	663,914	659,520	918,475
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from fixed maturities matured/called - available for sale, at market value	1,830,582	1,702,973	1,717,659
Proceeds from fixed maturities matured/called - available for sale, at fair value	1,300	12,775
Proceeds from fixed maturities sold - available for sale, at market value	913,139	1,732,246	1,632,719
Proceeds from fixed maturities sold - available for sale, at fair value	84,917	65,158	20,237
Proceeds from equity securities sold - available for sale, at market value	280,807	27,207	3,037
Proceeds from equity securities sold - available for sale, at fair value	558,740	247,227	234,112
Distributions from other invested assets	84,454	166,273	79,849
Cost of fixed maturities acquired - available for sale, at market value	(3,328,409)	(3,238,113)	(2,766,212)
Cost of fixed maturities acquired - available for sale, at fair value	(7,955)	(27,481)	(134,324)
Cost of equity securities acquired - available for sale, at market value	(26,571)	(127,837)	(353,265)
Cost of equity securities acquired - available for sale, at fair value	(426,507)	(755,734)	(514,092)
Cost of other invested assets acquired	(58,369)	(64,832)	(66,408)
Cost of businesses acquired		(63,100)
Net change in short-term investments	(175,043)	100,969	(110,241)
Net change in unsettled securities transactions	35,085	13,563	(13,084)
Net cash provided by (used in) investing activities	(233,830)	(208,706)	(270,013)
CASH FLOWS FROM FINANCING ACTIVITIES:
Common shares issued during the period, net	21,473	12,269	3,066
Purchase of treasury shares	(289,988)	(92,490)	(398,554)
Revolving credit borrowings		(50,000)	50,000
Net cost of senior notes maturing			(200,000)
Dividends paid to shareholders	(100,402)	(103,848)	(108,477)
Net cash provided by (used in) financing activities	(368,917)	(234,069)	(653,965)
EFFECT OF EXCHANGE RATE CHANGES ON CASH	27,232	(26,502)	16,313
Net increase (decrease) in cash	88,399	190,243	10,810
Cash, beginning of period	448,651	258,408	247,598
Cash, end of period	537,050	448,651	258,408
SUPPLEMENTAL CASH FLOW INFORMATION:
Income taxes paid (recovered)	59,754	(44,537)	(30,978)
Interest paid	53,008	51,647	60,198
Non-cash transaction:
Net assets acquired and liabilities assumed from business acquisitions		19,130
Conversion of equity securities - available for sale, at market value, to fixed maturity securities - available for sale, at market value, including accrued interest at time of conversion	$ 92,981