Employee Benefit Plans (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years	Dec. 31, 2010	Dec. 31, 2009
Schedule Of Employee Benefit Plans [Line Items]
Options are exercisable in certain limited conditions, expiration date in years	10
Additional paid-in capital	$ 78,225	$ 170,239
Accounts payable and accrued liabilities	116,063	106,010
Total intrinsic value of options exercised during the year	4,070	5,888	4,283
Cash received upon the exercise of stock options	6,837	9,598
Compensation expense related to unvested options granted, expected time period for recognition in years	1.7
Average market value per share of vested shares	$ 57.52	$ 47.49	$ 39.79
Vesting period in years	four
Expense recognition period	1.7
Percentage of participant's contribution to pension plans	5.00%
Percentage of officer's maximum contribution to pension plans	25.00%
Percentage of employer's contribution to participants under pension plans	5.00%
Percentage of employer's contribution to officers under pension plans	10.00%
Base salary capped for pension purposes under supplemental executive retirement plan	600
Percentage of employer's contribution to pension plans, under supplemental executive retirement plan	10.00%
Amounts funded and expensed under pension plans	7,165	8,011	4,886
Employee share purchase plan, discount from fair market price on purchase date	15.00%
Amount of compensation expense recorded	42,589	49,375	40,399
RSUs [Member]
Schedule Of Employee Benefit Plans [Line Items]
Percentage of weighted average annual forfeiture rate used in determining in compensation expense over the service period	2.80%
Unrecognized compensation expense related to unvested options granted	15,139
Compensation expense related to unvested options granted, expected time period for recognition in years	1.4
Intrinsic value of vested shares	19,732	17,302	6,212
Actual market value per share of unvested shares	$ 62.93
Intrinsic value of unvested shares awarded	41,102
Expense related to unvested awards in percentage of fair value	100.00%
Vesting period in years	four
Expense recognition period	1.4
RSUs [Member] | Allied World Switzerland (Parent Guarantor) [Member]
Schedule Of Employee Benefit Plans [Line Items]
Expense related to unvested awards in percentage of fair value	100.00%
LTIP Award [Member]
Schedule Of Employee Benefit Plans [Line Items]
Common shares, issued	2,000,000
Unrecognized compensation expense related to unvested options granted	375
Compensation expense related to unvested options granted, expected time period for recognition in years	1
Actual market value per share of unvested shares	$ 62.93
Intrinsic value of unvested shares awarded	30,587
Expense related to unvested awards in percentage of fair value	150.00%
Expense recognition period	1
2001 Employee Stock Option Plan [Member]
Schedule Of Employee Benefit Plans [Line Items]
Common shares, issued	4,000,000
2004 Stock Incentive Plan [Member]
Schedule Of Employee Benefit Plans [Line Items]
Common shares, issued	2,000,000
Employee Stock Option [Member]
Schedule Of Employee Benefit Plans [Line Items]
Percentage of weighted average annual forfeiture rate used in determining in compensation expense over the service period	6.72%
Unrecognized compensation expense related to unvested options granted	10,887
Vesting period in years	four
Employee Stock Option [Member] | Allied World Switzerland (Parent Guarantor) [Member]
Schedule Of Employee Benefit Plans [Line Items]
Exercise price of options issued shall not be less than percentage of fair market value of the common shares granted	100.00%
Employee Share Purchase Plan [Member]
Schedule Of Employee Benefit Plans [Line Items]
Amount of compensation expense recorded	207	160	162
DR Long Term Incentive Plan [Member]
Schedule Of Employee Benefit Plans [Line Items]
Underwriting profit, percentage	20.00%
Investment income based on average net assets outstanding	20.00%
Net premiums earned, percentage	5.00%
Vesting period in years	four
General And Administrative Expense [Member] | RSUs [Member]
Schedule Of Employee Benefit Plans [Line Items]
Amount of compensation expense recorded	13,140	13,473	9,003
General And Administrative Expense [Member] | LTIP Award [Member]
Schedule Of Employee Benefit Plans [Line Items]
Amount of compensation expense recorded	5,056	17,817	25,580
General And Administrative Expense [Member] | Cash-Equivalent Stock Awards [Member]
Schedule Of Employee Benefit Plans [Line Items]
Amount of compensation expense recorded	20,295	15,132	3,260
General And Administrative Expense [Member] | Employee Stock Option [Member]
Schedule Of Employee Benefit Plans [Line Items]
Amount of compensation expense recorded	4,098	2,953	2,556
General And Administrative Expense [Member] | DR Long Term Incentive Plan [Member]
Schedule Of Employee Benefit Plans [Line Items]
Amount of compensation expense recorded	3,219	932	5,851
Additional Paid-In Capital [Member] | RSUs [Member]
Schedule Of Employee Benefit Plans [Line Items]
Additional paid-in capital	28,380	37,991
Additional Paid-In Capital [Member] | LTIP Award [Member]
Schedule Of Employee Benefit Plans [Line Items]
Additional paid-in capital	49,171	77,728
Additional Paid-In Capital [Member] | Employee Stock Option [Member]
Schedule Of Employee Benefit Plans [Line Items]
Additional paid-in capital	42,203	41,505
Accounts Payable And Accrued Liabilities [Member] | DR Long Term Incentive Plan [Member]
Schedule Of Employee Benefit Plans [Line Items]
Accounts payable and accrued liabilities	$ 3,425	$ 4,919
Maximum [Member]
Schedule Of Employee Benefit Plans [Line Items]
Employee share purchase plan, number of shares available for sale	1,000,000