Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	$ 507,271	$ 429,747	$ 970,499	$ 831,637
Net investment income	37,635	42,451	71,023	89,660
Net realized investment losses	(115,198)	8,663	(35,561)	142,244
Net losses and loss expenses	(275,128)	(240,380)	(530,306)	(465,582)
Acquisition costs	(64,617)	(51,588)	(121,302)	(98,726)
General and administrative expenses	(80,585)	(73,979)	(163,265)	(144,345)
Amortization of intangible assets	(634)	(634)	(1,267)	(1,267)
Interest expense	(14,188)	(14,001)	(28,322)	(27,757)
Foreign exchange (loss) gain	(490)	1,019	(3,008)	1,100
Income tax (expense) benefit	4,072	(4,947)	(1,361)	(12,457)
Equity in earnings of consolidated subsidiaries	0	0	0	0
NET (LOSS) INCOME	(1,862)	96,351	157,130	314,507
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and six months ended June 30, 2012: $68 and $96, respectively	0	231	0	179
Reclassification adjustment for net realized investment gains included in net (loss) income, net of applicable income tax	0	(1,142)	0	(13,249)
Other comprehensive loss	0	(911)	0	(13,070)
COMPREHENSIVE INCOME (LOSS)	(1,862)	95,440	157,130	301,437
Parenthetical disclosures:
Deferred income tax (benefit) for unrealized gains on investments arising during the period	0	(68)	0	(96)
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	0	0	0	0
Net investment income	1	5	8	14
Net realized investment losses	0	0	0	0
Other income - termination fee				0
Net losses and loss expenses	0	0	0	0
Acquisition costs	0	0	0	0
General and administrative expenses	(8,566)	(4,278)	(19,552)	(8,234)
Amortization of intangible assets	0	0	0	0
Interest expense	0	0	0	0
Foreign exchange (loss) gain	2	460	274	549
Income tax (expense) benefit	0	(373)	0	71
Equity in earnings of consolidated subsidiaries	6,701	100,537	176,400	322,107
NET (LOSS) INCOME	(1,862)	96,351	157,130	314,507
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and six months ended June 30, 2012: $68 and $96, respectively		231		179
Reclassification adjustment for net realized investment gains included in net (loss) income, net of applicable income tax		(1,142)		(13,249)
Other comprehensive loss	0	(911)	0	(13,070)
COMPREHENSIVE INCOME (LOSS)	(1,862)	95,440	157,130	301,437
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	0	0	0	0
Net investment income	2	8	4	11
Net realized investment losses	0	0	0	0
Other income - termination fee				0
Net losses and loss expenses	0	0	0	0
Acquisition costs	0	0	0	0
General and administrative expenses	(455)	(1,302)	(912)	(2,454)
Amortization of intangible assets	0	0	0	0
Interest expense	(13,835)	(14,001)	(27,665)	(27,757)
Foreign exchange (loss) gain	(628)	(42)	(723)	(67)
Income tax (expense) benefit	0	0	0	0
Equity in earnings of consolidated subsidiaries	21,147	115,102	202,627	349,409
NET (LOSS) INCOME	6,231	99,765	173,331	319,142
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and six months ended June 30, 2012: $68 and $96, respectively		0		0
Reclassification adjustment for net realized investment gains included in net (loss) income, net of applicable income tax		0		0
Other comprehensive loss	0	0	0	0
COMPREHENSIVE INCOME (LOSS)	6,231	99,765	173,331	319,142
Other Allied World Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	507,271	429,747	970,499	831,637
Net investment income	37,632	42,438	71,011	89,635
Net realized investment losses	(115,198)	8,663	(35,561)	142,244
Other income - termination fee				0
Net losses and loss expenses	(275,128)	(240,380)	(530,306)	(465,582)
Acquisition costs	(64,617)	(51,588)	(121,302)	(98,726)
General and administrative expenses	(71,564)	(68,399)	(142,801)	(133,657)
Amortization of intangible assets	(634)	(634)	(1,267)	(1,267)
Interest expense	(353)	0	(657)	0
Foreign exchange (loss) gain	136	601	(2,559)	618
Income tax (expense) benefit	4,072	(4,574)	(1,361)	(12,528)
Equity in earnings of consolidated subsidiaries	0	0	0	0
NET (LOSS) INCOME	21,617	115,874	205,696	352,374
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and six months ended June 30, 2012: $68 and $96, respectively		231		179
Reclassification adjustment for net realized investment gains included in net (loss) income, net of applicable income tax		(1,142)		(13,249)
Other comprehensive loss	0	(911)	0	(13,070)
COMPREHENSIVE INCOME (LOSS)	21,617	114,963	205,696	339,304
Parenthetical disclosures:
Deferred income tax (benefit) for unrealized gains on investments arising during the period		(68)	(81)	(3,051)
Consolidating Adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Net premiums earned	0	0	0	0
Net investment income	0	0	0	0
Net realized investment losses	0	0	0	0
Other income - termination fee				0
Net losses and loss expenses	0	0	0	0
Acquisition costs	0	0	0	0
General and administrative expenses	0	0	0	0
Amortization of intangible assets	0	0	0	0
Interest expense	0	0	0	0
Foreign exchange (loss) gain	0	0	0	0
Income tax (expense) benefit	0	0	0	0
Equity in earnings of consolidated subsidiaries	(27,848)	(215,639)	(379,027)	(671,516)
NET (LOSS) INCOME	(27,848)	(215,639)	(379,027)	(671,516)
Unrealized gains on investments arising during the period net of applicable deferred income tax benefit for three and six months ended June 30, 2012: $68 and $96, respectively		(231)		(179)
Reclassification adjustment for net realized investment gains included in net (loss) income, net of applicable income tax		1,142		13,249
Other comprehensive loss	0	911	0	13,070
COMPREHENSIVE INCOME (LOSS)	$ (27,848)	$ (214,728)	$ (379,027)	$ (658,446)