Allied World Assurance 10-K 17 Feb 15 2014 FY Annual report Financial data

Document_And_Entity_Informatio

Document And Entity Information (USD $)	12 Months Ended
In Billions, except Share data, unless otherwise specified	Dec. 31, 2014	Feb. 09, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Entity Registrant Name	Allied World Assurance Co Holdings, AG
Entity Central Index Key	1163348
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		96,004,829
Entity Public Float			$3.60
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
ASSETS:
Fixed maturity investments trading, at fair value (amortized cost: 2014: $6,035,240; 2013: $6,065,350)	$6,069,010	$6,100,798
Equity securities trading, at fair value (cost: 2014: $791,206; 2013: $647,301)	844,163	699,846
Other invested assets	955,509	911,392
Total investments	7,868,682	7,712,036
Cash and cash equivalents	589,339	531,936
Restricted cash	80,971	149,393
Insurance balances receivable	664,815	588,632
Funds held	724,021	632,430
Prepaid reinsurance	360,732	340,992
Reinsurance recoverable	1,340,256	1,234,504
Reinsurance recoverable on paid losses	86,075	76,099
Accrued investment income	28,456	32,236
Net deferred acquisition costs	151,546	126,661
Goodwill	278,258	268,376
Intangible assets	46,298	48,831
Balances receivable on sale of investments	47,149	76,544
Net deferred tax assets	33,615	37,469
Other assets	121,350	89,691
Total assets	12,421,563	11,945,830
LIABILITIES:
Reserve for losses and loss expenses	5,881,165	5,766,529
Unearned premiums	1,555,313	1,396,256
Reinsurance balances payable	180,060	173,023
Balances due on purchases of investments	5,428	104,740
Senior notes	798,802	798,499
Other long-term debt	19,213	0
Dividends payable	21,669	16,732
Accounts payable and accrued liabilities	181,622	170,225
Total liabilities	8,643,272	8,426,004
SHAREHOLDERS’ EQUITY:
Common shares: 2014 and 2013: par value CHF 4.10 per share (2014: 100,775,256; 2013: 103,477,452 shares issued and 2014: 96,195,482; 2013: 100,253,646 shares outstanding)	408,020	418,988
Treasury shares, at cost (2014: 4,579,774; 2013: 3,223,806)	-143,075	-79,992
Retained earnings	3,513,346	3,180,830
Total shareholders’ equity	3,778,291	3,519,826
Total liabilities and shareholders’ equity	$12,421,563	$11,945,830

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical)(USD ($))	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Fixed maturity investments trading, amortized cost	$6,035,240	$6,065,350
Equity securities trading - cost	$791,206	$647,301
Common stock, shares issued	100,775,256	103,477,452
Common stock, shares outstanding	96,195,482	100,253,646
Treasury stock (shares)	4,579,774	3,223,806

Consolidated_Statements_Of_Ope

Consolidated Statements Of Operations And Comprehensive Income (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
REVENUES:
Gross premiums written	$2,935,425	$2,738,664	$2,329,271
Premiums ceded	-613,350	-618,183	-491,448
Net premiums written	2,322,075	2,120,481	1,837,823
Change in unearned premiums	-139,317	-114,648	-88,925
Net premiums earned	2,182,758	2,005,833	1,748,898
Net investment income	176,874	157,564	167,141
Net realized investment gains	88,957	59,525	306,436
Other income	2,064	0	0
Total revenues	2,450,653	2,222,922	2,222,475
EXPENSES:
Net losses and loss expenses	1,199,190	1,123,242	1,139,264
Acquisition costs	295,122	252,673	205,722
General and administrative expenses	365,736	352,285	307,321
Other expense	8,578	0	0
Amortization of intangible assets	2,533	2,533	2,533
Interest expense	57,755	56,510	55,405
Foreign exchange loss	957	8,019	783
Total expenses	1,929,871	1,795,262	1,711,028
Income before income taxes	520,782	427,660	511,447
Income tax expense	30,523	9,780	18,440
NET INCOME	490,259	417,880	493,007
Other comprehensive loss:
Unrealized losses on investments arising during the year net of applicable deferred income tax benefit 2014: $0; 2013: $0; 2012: $(265)	0	0	-493
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax	0	0	-13,991
Other comprehensive loss	0	0	-14,484
COMPREHENSIVE INCOME	$490,259	$417,880	$478,523
PER SHARE DATA
Basic earnings per share	$5.03	$4.08	$4.56
Diluted earnings per share	$4.92	$3.98	$4.43
Weighted average common shares outstanding	97,538,319	102,464,715	108,171,435
Weighted average common shares and common share equivalents outstanding	99,591,773	104,865,834	111,209,655
Dividends paid per share	$0.78	$0.46	$0.63

Consolidated_Statements_Of_Ope1

Consolidated Statements Of Operations And Comprehensive Income (Parenthetical) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred income tax (benefit) expense for unrealized gains on investments arising during the period	$0	$0	($265)

Consolidated_Statements_Of_Sha

Consolidated Statements Of Shareholders' Equity (USD $)		Total	Share Capital	Additional Paid-In Capital	Treasury Shares	Accumulated Other Comprehensive Income	Retained Earnings
In Thousands
Balance, at beginning of period at Dec. 31, 2011		$3,149,022	$557,153	$78,225	($136,590)	$14,484	$2,635,750
Net income		493,007					493,007
Dividends — par value reduction		-53,523	-53,523
Other comprehensive loss		-14,484				-14,484
Stock compensation	[1]	16,255		-27,099	43,354
Share repurchases		-263,942			-263,942
Shares cancelled		0	-48,650	-51,126	243,360		-143,584
Balance, at end of period at Dec. 31, 2012		3,326,335	454,980	0	-113,818	0	2,985,173
Net income		417,880					417,880
Dividends — par value reduction		-12,982	-12,982
Dividends		-50,791					-50,791
Other comprehensive loss		0
Stock compensation	[1]	14,070		0	31,946		-17,876
Share repurchases		-174,686			-174,686
Shares cancelled		0	-23,010	0	176,566		-153,556
Balance, at end of period at Dec. 31, 2013		3,519,826	418,988	0	-79,992	0	3,180,830
Net income		490,259					490,259
Dividends		-81,686					-81,686
Other comprehensive loss		0
Stock compensation	[1]	25,265		0	19,638		5,627
Share repurchases		-175,373			-175,373
Shares cancelled		0	-10,968	0	92,652		-81,684
Balance, at end of period at Dec. 31, 2014		$3,778,291	$408,020	$0	($143,075)	$0	$3,513,346
[1]	Includes stock compensation expense for the period and shares issued out of treasury for awards exercised or vested. See note 2(o) to the notes to the consolidated financial statements.

Consolidated_Statements_Of_Cas

Consolidated Statements Of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$490,259	$417,880	$493,007
Adjustments to reconcile net income to cash provided by operating activities:
Net realized (gains) losses on sales of investments	-144,972	-109,084	55,865
Mark to market adjustments	21,984	48,147	-180,380
Stock compensation expense	14,245	13,969	17,642
Undistributed income of equity method investments	13,851	-11,923	0
Changes in:
Reserve for losses and loss expenses, net of reinsurance recoverables	8,884	27,586	282,215
Unearned premiums, net of prepaid reinsurance	139,317	114,649	88,924
Insurance balances receivable	-76,825	-108,412	-81,600
Reinsurance recoverable on paid losses	-8,509	-45,787	-1,594
Funds held	-91,591	-296,062	-111,548
Reinsurance balances payable	7,037	36,759	11,725
Net deferred acquisition costs	-24,885	-18,651	-7,676
Net deferred tax assets	4,186	-11,889	200
Accounts payable and accrued liabilities	8,039	22,917	27,815
Other items, net	55,829	34,332	34,843
Net cash provided by operating activities	416,849	114,431	629,438
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	-7,630,034	-7,527,670	-8,006,237
Purchases of other invested assets	-307,955	-276,926	-333,031
Sales of available for sale securities	0	0	236,978
Sales of trading securities	7,536,936	7,540,194	7,892,907
Sales of other invested assets	267,885	187,477	53,741
Purchases of fixed assets	-59,656	-5,310	-3,214
Net cash paid for acquisitions	-2,565	0	0
Change in restricted cash	68,422	34,092	-100,877
Net cash used in investing activities	-126,967	-48,143	-259,733
CASH FLOWS USED IN FINANCING ACTIVITIES:
Dividends paid — par value reduction	0	-12,981	-67,825
Dividends paid	-76,750	-34,059	0
Proceeds from the exercise of stock options	10,027	12,137	10,527
Proceeds from other long-term debt	19,213	0	0
Share repurchases	-175,910	-173,006	-263,942
Net cash used in financing activities	-223,420	-207,909	-321,240
Effect of exchange rate changes on foreign currency cash	-9,059	-8,322	-582
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	57,403	-149,943	47,883
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	531,936	681,879	633,996
CASH AND CASH EQUIVALENTS, END OF PERIOD	589,339	531,936	681,879
Supplemental disclosure of cash flow information:
Cash paid for income taxes	18,363	22,604	23,957
Cash paid for interest expense	$54,000	$54,000	$54,000

General

General	12 Months Ended
	Dec. 31, 2014
General [Abstract]
General	GENERAL
	Allied World Assurance Company Holdings, AG, a Swiss holding company (“Allied World Switzerland”), through its wholly-owned subsidiaries (collectively, the “Company”), provides property and casualty insurance and reinsurance on a worldwide basis through operations in Australia, Bermuda, the United States, Canada, Europe, Hong Kong, Labuan and Singapore. References to $ are to the lawful currency of the United States and to CHF are to the lawful currency of Switzerland.
	The Company has reached definitive agreements to acquire the Hong Kong and Singapore operations of Royal & Sun Alliance Insurance plc ("RSA") for approximately $201,761, at current exchange rates, subject to adjustments at closing. In addition to the purchase price, the Company expects to contribute an additional $90,000 to capitalize the businesses on an ongoing basis. Subject to regulatory approvals in both Hong Kong and Singapore, as well as court approval in Singapore, the acquisition is expected to be completed in the early part of the second quarter of 2015.
	On May 1, 2014, the shareholders approved a 3-for-1 stock split of the Company's common shares. All historical share and per share amounts reflect the effect of the stock split.
	During the fourth quarter of 2014, the Company reorganized how it manages its business, and as a result it realigned its executive management team and changed its reportable segments to correspond to the reorganization. The Company's Bermuda direct insurance operations, except for the trade credit line of business, which had previously been included in the international insurance segment, was combined with the U.S. insurance segment, with the new segment renamed the "North American Insurance" segment. The remaining direct insurance operations of the international insurance segment was renamed the "Global Markets Insurance" segment. The Reinsurance segment remained unchanged. The newly created segments are included in note 17 and prior periods have been recast to conform to the new presentation.

Significant_Accounting_Policie

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Significant accounting policies	SIGNIFICANT ACCOUNTING POLICIES
	These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. The significant estimates reflected in the Company’s financial statements include, but are not limited to:
	•	The premium estimates for certain reinsurance agreements,
	•	Recoverability of deferred acquisition costs,
	•	The reserve for outstanding losses and loss expenses,
	•	Valuation of ceded reinsurance recoverables,
	•	Determination of impairment of goodwill and other intangible assets, and
	•	Valuation of financial instruments.
	Intercompany accounts and transactions have been eliminated on consolidation and entities meeting consolidation requirements have been included in the consolidated financial statements. To facilitate comparison of information across periods, certain reclassifications have been made to prior year amounts to conform to the current year's presentation.
	The significant accounting policies are as follows:
	a) Premiums and Acquisition Costs
	Premiums are recorded as written on the inception date of the policy. For certain types of business written by the Company, notably assumed reinsurance, premium income may not be known at the policy inception date. In the case of quota share reinsurance treaties assumed by the Company, the underwriter makes an estimate of premium income at inception. The underwriter’s estimate is based on statistical data provided by reinsureds and the underwriter’s judgment and experience. Such estimations are refined over the reporting period of each treaty as actual written premium information is reported by ceding companies and intermediaries. Premiums resulting from changes in the estimate of the premium income are recorded in the period the estimate is changed. Certain insurance and reinsurance contracts may require that the premium be adjusted at the expiry of the contract to reflect the change in exposure or loss experience of the insured or reinsured.
	Premiums are recognized as earned over the period of policy coverage in proportion to the risks to which they relate. Reinsurance premiums under a losses-occurring reinsurance contract are earned over the coverage period. Reinsurance premiums under a risks-attaching reinsurance contract are earned over the same period as the underlying policies, or risks, covered by the contract. As a result, the earning pattern of a risks-attaching reinsurance contract may extend up to 24 months, reflecting the inception dates of the underlying policies. Premiums relating to the unexpired periods of coverage are recorded on the consolidated balance sheets as “unearned premiums”.
	Acquisition costs, comprised of commissions, brokerage fees and insurance taxes, are costs that are directly related to the successful acquisition of new and renewal business. While permitted under U.S. GAAP to defer certain internal costs that are directly related to the successful acquisition of new and renewal business, the Company does not defer such costs. Acquisition costs that are deferred are expensed as the premiums to which they relate are earned. Acquisition costs relating to the reserve for unearned premiums are deferred and carried on the balance sheets as an asset and are amortized over the period of coverage. Expected losses and loss expenses, other costs and anticipated investment income related to these unearned premiums are considered in determining the recoverability or deficiency of deferred acquisition costs. If it is determined that deferred acquisition costs are not recoverable, they are expensed. Further analysis is performed to determine if a liability is required to provide for losses which may exceed the related unearned premiums.
	b) Reserve for Losses and Loss Expenses
	The reserve for losses and loss expenses is comprised of two main elements: outstanding loss reserves (“OSLR,” also known as case reserves) and reserves for losses incurred but not reported (“IBNR”). OSLR relate to known claims and represent management’s best estimate of the likely loss payment. IBNR relates to reserves established by the Company for claims that have occurred but have not yet been reported to us as well as for changes in the values of claims that have been reported to us but are not yet settled.
	The reserve for IBNR is estimated by management for each line of business based on various factors including underwriters’ expectations about loss experience, actuarial analysis, comparisons with the results of industry benchmarks and loss experience to date. The Company’s actuaries employ generally accepted actuarial methodologies to determine estimated ultimate loss reserves. The adequacy of the reserves is re-evaluated quarterly by the Company’s actuaries. At the completion of each quarterly review of the reserves, a reserve analysis is prepared and reviewed with the Company’s loss reserve committee. This committee determines management’s best estimate for loss and loss expense reserves based upon the reserve analysis.
	While management believes that the reserves for OSLR and IBNR are sufficient to cover losses assumed by the Company, there can be no assurance that losses will not deviate from the Company’s reserves, possibly by material amounts. The methodology of estimating loss reserves is periodically reviewed to ensure that the assumptions made continue to be appropriate. The Company recognizes any changes in its loss reserve estimates and the related reinsurance recoverables in the consolidated statements of operations and comprehensive income ("consolidated income statements") in the periods in which they are determined. These are recorded in “net losses and loss expenses”.
	c) Ceded Reinsurance
	In the ordinary course of business, the Company uses both treaty and facultative reinsurance to minimize its net loss exposure to any one catastrophic loss event or to an accumulation of losses from a number of smaller events. Reinsurance premiums ceded are expensed and any commissions recorded thereon are earned over the period the reinsurance coverage is provided in proportion to the risks to which they relate. For reinsurance treaties that have contractual minimum premium provisions, premiums ceded are recorded at the inception of the treaty based on the minimum premiums. Any unearned ceding commission is included in “net deferred acquisitions costs” on the consolidated balance sheets and is recorded as a reduction to the overall net deferred acquisition cost balance. Prepaid reinsurance represents unearned premiums ceded to reinsurance companies. Reinsurance recoverable includes the balances due from those reinsurance companies under the terms of the Company’s reinsurance agreements for unpaid losses and loss reserves, including IBNR. Amounts recoverable from reinsurers are estimated in a manner consistent with the estimated claim liability associated with the reinsured policy.
	The Company determines the portion of the IBNR liability that will be recoverable under its reinsurance contracts by reference to the terms of the reinsurance protection purchased. This determination is necessarily based on the estimate of IBNR and accordingly, is subject to the same uncertainties as the estimate of IBNR.
	The Company remains liable to the extent that its reinsurers do not meet their obligations under the reinsurance contracts; therefore, the Company regularly evaluates the financial condition of its reinsurers and monitors concentration of credit risk.
	d) Investments
	All of our fixed maturity investments and equity securities are classified as trading securities as the Company has elected the fair value option permitted under U.S. GAAP for these investments. Trading securities are carried at fair value with any change in unrealized gains or losses recognized in the consolidated income statements and included in “net realized investment gains”. As a result of our investment classification, we do not record any change in unrealized gains or losses in our investments as a separate component of accumulated other comprehensive income on the consolidated balance sheets.
	Other invested assets consist primarily of investments in hedge funds and private equity funds, which have been accounted for as trading securities as the Company has elected the fair value option as permitted under U.S. GAAP. In addition, included in the Company’s other invested assets are various investments which are accounted for using the equity method of accounting. Generally, the Company uses the equity method where it does not have a controlling interest and is not the primary beneficiary. Equity method investments are recorded at cost and adjusted for the Company’s proportionate share of earnings or losses on a quarterly lag basis. An other-than-temporary impairment charge related to the equity method investments is assessed when facts and circumstances exists that indicate an impairment may exist. An other-than-temporary impairment charge is recorded when it is determined that the carrying value of the equity method investment is below its fair value and the Company does not have the intent and ability to hold to recovery. No equity method investment was deemed significant to disclose summarized financial data. Other investments are recorded based on valuation techniques depending on the nature of the individual assets.
	At each measurement date, the Company estimates the fair value of the financial instruments using various valuation techniques. The Company utilizes, to the extent available, quoted market prices in active markets or observable market inputs in estimating the fair value of financial instruments. When quoted market prices or observable market inputs are not available, the Company may utilize valuation techniques that rely on unobservable inputs to estimate the fair value of financial instruments. The Company bases its determination of whether a market is active or inactive on the spread between what a seller is asking for a security and what a buyer is bidding for that security. Spreads that are significantly above historical spreads are considered inactive markets. The Company also considers the volume of trading activity in the determination of whether a market is active or inactive. See note 5 for additional information regarding the fair value of financial instruments.
	The Company utilizes independent pricing sources to obtain market quotations for securities that have quoted prices in active markets. In general, the independent pricing sources use observable market inputs including, but not limited to, investment yields, credit risks and spreads, benchmarking of like securities, reported trades and sector groupings to determine the fair value. For a majority of the portfolio, the Company obtained two or more prices per security as of December 31, 2014. When multiple prices are obtained, a price source hierarchy is utilized to determine which price source is the best estimate of the fair value of the security. The price source hierarchy emphasizes more weighting to significant observable inputs such as index pricing and less weighting towards non-binding broker-dealer quotes. In addition, to validate all prices obtained from these pricing sources including non-binding broker-dealer quotes, the Company also obtains prices from its investment portfolio managers and other sources (e.g., another pricing vendor), and compares the prices obtained from the independent pricing sources to those obtained from the Company’s investment portfolio managers and other sources. The Company investigates any material differences between the multiple sources and determines which price best reflects the fair value of the individual security. There were no material differences between the prices obtained from the independent pricing sources and the prices obtained from the Company’s investment portfolio managers and other sources as of December 31, 2014 and 2013.
	Investment securities are recorded on a trade date basis. Investment income is recognized when earned and includes the accrual of discount or amortization of premium on fixed maturity investments using the effective yield method and is net of related expenses. Interest income for fixed maturity investments is accrued and recognized based on the contractual terms of the fixed maturity investments and is included in “net investment income” in the consolidated income statements. The Company’s share of distributed and undistributed net income from equity method investments is included in “net investment income”. The return on investments is managed on a total financial statement portfolio return basis, which includes the distributed and undistributed net income from equity method investments, and as such have classified these amounts in net investment income.
	Realized gains and losses on the disposition of investments, which are based upon the first-in first-out method of identification, are included in “net realized investment gains” in the consolidated income statements. For mortgage-backed and asset-backed securities and any other holdings for which there is a prepayment risk, prepayment assumptions are evaluated and revised on a regular basis. Revised prepayment assumptions are applied to securities on a retrospective basis to the date of acquisition. The cumulative adjustments to amortized cost required due to these changes in effective yields and maturities are recognized in net investment income in the same period as the revision of the assumptions.
	e) Variable Interest Entities
	The Company is involved in the normal course of business with variable interest entities (“VIEs”) as a passive investor in certain asset-backed securities issued by third-party VIEs and affiliated VIEs. The Company performs a qualitative assessment at the date when it becomes initially involved in the VIE, followed by ongoing reassessments related to its involvement in VIEs. The Company’s maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company’s consolidated balance sheets and any unfunded commitments.
	f) Translation of Foreign Currencies
	Monetary assets and liabilities denominated in foreign currencies are translated into U.S. dollars at the exchange rates in effect on the balance sheet date. Foreign currency revenues and expenses are translated at the average exchange rates prevailing during the period. Exchange gains and losses, including those arising from forward exchange contracts, are included in the consolidated income statements. The Company’s functional currency, and that of each its operating subsidiaries, is the U.S. dollar as it is the single largest currency in which the Company and its each of its subsidiaries transact business and hold invested assets.
	g) Cash and Cash Equivalents
	Cash and cash equivalents include amounts held in banks, time deposits, commercial paper, discount notes and U.S. Treasury Bills with maturities of less than three months from the date of purchase.
	h) Income Taxes
	Allied World Switzerland and certain of its subsidiaries operate in jurisdictions where they are subject to income taxation. Current and deferred income taxes are charged or credited to operations, or to accumulated other comprehensive income in certain cases, based upon enacted tax laws and rates applicable in the relevant jurisdiction in the period in which the tax becomes payable. Deferred income taxes are provided for all temporary differences between the bases of assets and liabilities used in the financial statements and those used in the various jurisdictional tax returns.
	It is the Company’s policy to recognize interest accrued related to unrecognized tax benefits in “interest expense” and penalties in “general and administrative expenses” in the consolidated income statements. The Company has not recorded any interest or penalties during the years ended December 31, 2014, 2013 and 2012 and the Company has not accrued any payment of interest and penalties as of December 31, 2014 and 2013.
	i) Employee Stock Option Compensation Plan
	The Company has an employee stock option plan, which is in run-off, in which the amount of Allied World Switzerland's common shares received as compensation through the issuance of stock options is determined by reference to the value of the shares. Compensation expense for stock options granted to employees is recorded on a straight-line basis over the option vesting period and is based on the fair value of the stock options on the grant date. The fair value of each stock option on the grant date is determined by using the Black-Scholes option-pricing model.
	j) Restricted Stock Units
	The Company has granted restricted stock units (“RSUs”) to certain employees. The compensation expense for the RSUs is based on the market value of Allied World Switzerland’s common shares on the grant date, and is recognized on a straight-line basis over the applicable vesting period.
	The Company has also granted cash-equivalent RSUs to certain employees that vest on a straight-line basis over the applicable vesting period. The amount payable per unit awarded will be equal to the price per share of Allied World Switzerland’s common shares and as such the Company measures the value of the award each reporting period based on the period ending share price. The effects of changes in the share price at each period end during the service period are recognized as increases or decreases in compensation expense over the service period.
	k) Performance-Based Equity Awards
	The Company has granted performance-based equity awards to key employees in order to promote the long-term growth and profitability of the Company. Each award represents the right to receive a number of common shares in the future, based upon the achievement of established performance criteria during the applicable performance period. The compensation expense for these awards is based on the market value of Allied World Switzerland’s common shares on the grant date, and is recognized on a straight-line basis over the applicable performance and vesting period. The Company will also adjust the compensation expense, as a cumulative adjustment, to the extent the Company's performance is above or below the targeted performance criteria.
	The Company has also granted cash-equivalent, performance-based awards to certain employees that vest based upon the achievement of established performance criteria during the applicable performance period. These cash-equivalent, performance-based awards vest after a three-year performance period. The amount payable per unit awarded will be equal to the price per share of Allied World Switzerland’s common shares, and as such the Company measures the value of the award each reporting period based on the period-ending share price. The effects of changes in the share price at each period end during the service period are recognized as changes in compensation expense over the service period. The Company will also adjust the compensation expense, as a cumulative adjustment, to the extent the Company's performance is above or below the targeted performance criteria.
	l) Goodwill and Intangible Assets
	The Company classifies its intangible assets into three categories: (1) intangible assets with finite lives subject to amortization, (2) intangible assets with indefinite lives not subject to amortization, and (3) goodwill. Intangible assets, other than goodwill, generally consist of trademarks, renewal rights, internally generated software, non-compete covenants and insurance licenses.
	For intangible assets with finite lives, the value of the assets is amortized over their expected useful lives and the expense is included in “amortization of intangible assets” in the consolidated income statements. The Company tests assets for impairment if conditions exist that indicate the carrying value may not be recoverable. If, as a result of the evaluation, the Company determines that the value of the intangible assets is impaired, then the value of the assets will be written-down in the period in which the determination of the impairment is made.
	For indefinite lived intangible assets the Company does not amortize the intangible asset but evaluates and compares the fair value of the assets to their carrying values on an annual basis or more frequently if circumstances warrant. If, as a result of the evaluation, the Company determines that the value of the intangible assets is impaired, then the value of the assets will be written-down in the period in which the determination of the impairment is made.
	Goodwill represents the excess of the cost of acquisitions over the fair value of net assets acquired and is not amortized. Goodwill is assigned at acquisition to the applicable reporting unit(s) based on the expected benefit to be received by the reporting units from the business combination. The Company determines the expected benefit based on several factors including the purpose of the business combination, the strategy of the Company subsequent to the business combination and structure of the acquired company subsequent to the business combination. A reporting unit is a component of the Company’s business that has discrete financial information that is reviewed by management. In determining the reporting unit, the Company analyzes the inputs, processes, outputs and overall operating performance of the reporting unit. The Company has determined that for purposes of the acquisition of Allied World Underwriters, Inc. ("AW Underwriters") (f/k/a Darwin Professional Underwriters, Inc.) the U.S. direct insurance operations of the North American Insurance segment is the reporting unit that is expected to receive the benefit of the business combination and as such all of the goodwill has been allocated to this reporting unit.
	For goodwill, the Company performs an annual impairment test, or more frequently if circumstances are warranted. The Company may first assess qualitative factors to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. The results of the qualitative assessment will determine if an entity needs to proceed with the two-step goodwill impairment test. For the year ended December 31, 2014, the Company elected to bypass the qualitative assessment and performed the first step of the goodwill impairment test.
	The first step of the goodwill impairment test is to compare the fair value of the reporting unit with its carrying value, including goodwill. If the carrying amount of the reporting unit exceeds its fair value then the second step of the goodwill impairment test is performed.
	The second step of the goodwill impairment test compares the implied fair value of the reporting unit’s goodwill with the carrying amount of that goodwill in order to determine the amount of impairment to be recognized. The implied fair value of goodwill is determined by deducting the fair value of a reporting unit’s identifiable assets and liabilities from the fair value of the reporting unit as a whole. The excess of the carrying value of goodwill above the implied goodwill, if any, would be recognized as an impairment charge in the consolidated income statements.
	We recorded no impairments during the years ended December 31, 2014, 2013 and 2012.
	m) Derivative Instruments
	The Company utilizes derivative financial instruments as part of its overall risk management strategy. The Company recognizes all derivative financial instruments at fair value as either assets or liabilities on the consolidated balance sheets. The accounting for gains and losses associated with changes in the fair value of a derivative and the effect on the consolidated financial statements depends on its hedge designation and whether the hedge is highly effective in achieving offsetting changes in the fair value of the asset or liability hedged.
	The Company uses currency forward contracts and foreign currency swaps to manage currency exposure. The Company also utilizes various derivative instruments such as interest rate futures, interest rate swaps and index options, for the purpose of managing market exposures, interest rate volatility, portfolio duration, hedging certain investments, or enhancing investment performance. These derivatives are not designated as hedges and accordingly are carried at fair value on the consolidated balance sheets with realized and unrealized gains and losses included in the consolidated income statements. Refer to Note 4 for the Company’s related disclosure.
	n) Earnings Per Share
	Basic earnings per share is defined as net income available to common shareholders divided by the weighted average number of common shares outstanding for the period, giving no effect to dilutive securities. Diluted earnings per share is defined as net income available to common shareholders divided by the weighted average number of common and common share equivalents outstanding calculated using the treasury stock method for all potentially dilutive securities, including share warrants, employee stock options, employee share repurchase plan, RSUs and performance-based awards. When the effect of dilutive securities would be anti-dilutive, these securities are excluded from the calculation of diluted earnings per share.
	o) Treasury Shares
	Common shares repurchased by the Company and not subsequently cancelled are classified as "treasury shares" on the consolidated balance sheets and are recorded at cost. When shares are reissued from treasury the historical cost, based on the first-in, first-out method, is used to determine the cost of the reissued shares. The difference between the cost of the treasury shares and the par value of the common stock shall be first reflected as additional paid-in capital, but to the extent additional paid-in capital is exhausted the remainder shall reduce retained earnings. The issuance of shares out of treasury have been related to vesting equity-based compensation of the Company's employees and directors.
	p) New Accounting Pronouncements
	In April 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update 2014-08, "Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity" ("ASU 2014-08"). ASU 2014-08 changes the requirements for reporting discontinued operations, such that a disposal of a component of the Company's operations is required to be reported as discontinued operations if the disposal represents a strategic shift that has, or will have, a major effect on the Company's operations and financial results. Examples of strategic shifts that could have a major effect on the Company's operations could include a disposal of a major geographical area, a major line of business, a major equity method investment, or other major parts of the Company. ASU 2014-08 is effective for all disposals that occur after January 1, 2015. The Company does not believe the adoption of ASU 2014-08 will have a material impact on future financial statements and related disclosures.
In May 2014, the FASB issued Accounting Standards Update 2014-09, "Revenue from Contracts with Customers" ("ASU 2014-09"). ASU 2014-09 provides a framework, through a five-step process, for recognizing revenue from customers, improves comparability and consistency of recognizing revenue across entities, industries, jurisdictions and capital markets, and requires enhanced disclosures. Certain contracts with customers are specifically excluded from the scope of ASU 2014-09, including; amongst others, insurance contracts accounted for under Accounting Standard Codification 944, Financial Services - Insurance. ASU 2014-09 is effective on January 1, 2017 with retrospective adoption required for the comparative periods. The Company is currently assessing the impact the adoption of ASU 2014-09 will have on future financial statements and related disclosures.
In August 2014, the FASB issued Accounting Standards Update 2014-15, "Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern" ("ASU 2014-15"). Currently, there is no guidance under U.S. GAAP regarding management's responsibility to assess whether there is substantial doubt about an entity's ability to continue as a going concern. Under ASU 2014-15, the Company will be required to assess its ability to continue as a going concern each interim and annual reporting period and provide certain disclosures if there is substantial doubt about the entity's ability to continue as a going concern, including management's plan to alleviate the substantial doubt. ASU 2014-15 is effective for the year ended December 31, 2016 and early adoption is permitted. The Company is currently assessing the impact the adoption of ASU 2014-15 will have on future financial statements and related disclosures.
In November 2014, the FASB issued Accounting Standards Update 2014-17, "Pushdown Accounting" ("ASU 2014-17"). ASU 2014-17 provides an acquired entity with an option to apply pushdown accounting in its separate financial statements upon an acquirer obtaining control of the acquired entity. The acquired entity may also elect to apply pushdown accounting in a subsequent reporting period, if pushdown accounting was not applied when the acquirer obtained control, but that will be treated as a change in accounting principle. Once pushdown accounting is applied, the election is irrevocable. The Company adopted ASU 2014-17 when it became effective in November 2014.

Investments

Investments	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Investments	INVESTMENTS
	a) Trading Securities
	Securities accounted for at fair value with changes in fair value recognized in the consolidated income statements by category are as follows:
		31-Dec-14						31-Dec-13
		Fair Value			Amortized Cost			Fair Value				Amortized Cost
	U.S. government and government agencies	$	1,610,502		$	1,610,880		$	1,676,788			$	1,684,832
	Non-U.S. government and government agencies	188,199			196,332			191,776				197,082
	States, municipalities and political subdivisions	170,567			165,615			231,555				234,406
	Corporate debt:
	Financial institutions	1,024,667			1,018,777			958,794				943,518
	Industrials	1,029,729			1,037,820			1,174,047				1,165,448
	Utilities	110,997			111,599			69,426				69,658
	Mortgage-backed	1,263,517			1,219,712			1,292,502				1,267,863
	Asset-backed	670,832			674,505			505,910				502,543
	Total fixed maturity investments, trading	$	6,069,010		$	6,035,240		$	6,100,798			$	6,065,350
		December 31, 2014						December 31, 2013
		Fair Value			Cost			Fair Value				Cost
	Equity securities	$	844,163		$	791,206		$	699,846			$	647,301
	Other invested assets	812,543			725,069			764,081				658,683
		$	1,656,706		$	1,516,275		$	1,463,927			$	1,305,984
	Other invested assets, included in the table above, include investments in private equity funds, hedge funds and a high yield loan fund that are accounted for at fair value, but excludes other private securities described below in Note 3(b) that are accounted for using the equity method of accounting.
	b) Other Invested Assets
	Details regarding the carrying value, redemption characteristics and unfunded investment commitments of the other invested assets portfolio as of December 31, 2014 and 2013 were as follows:
	Fund Type	Carrying Value as of December 31, 2014			Investments			Estimated		Investments				Redemption	Redemption	Unfunded
					with			Remaining		without				Frequency(1)	Notice	Commitments
					Redemption			Restriction		Redemption					Period(1)
					Restrictions			Period		Restrictions
	Private equity (primary and secondary)	$	184,576		$	184,576		2 - 8 Years		$	—					$	223,802
	Mezzanine debt	166,905			166,905			5 - 9 Years		—						204,232
	Distressed	5,869			5,869			3 Years		—						5,180
	Real estate	—			—			9 Years		—						50,000
	Total private equity	357,350			357,350					—						483,214
	Distressed	170,169			170,169					—				Based on net asset value	60 Days	—
	Equity long/short	84,198			—					84,198				Quarterly	30 -60 Days	—
	Multi-strategy	51,507			—					51,507				Quarterly	45 -90 Days	—
	Relative value credit	119,156			—					119,156				Quarterly	60 Days	—
	Total hedge funds	425,030			170,169					254,861						—
	High yield loan fund	30,163			—					30,163				Monthly	30 days	—
	Total other invested assets at fair value	812,543			527,519					285,024						483,214
	Other private securities	142,966			—					142,966						—
	Total other invested assets	$	955,509		$	527,519				$	427,990					$	483,214
	Fund Type	Carrying Value as of December 31, 2013			Investments			Estimated		Investments				Redemption	Redemption	Unfunded
					with			Remaining		without				Frequency(1)	Notice	Commitments
					Redemption			Restriction		Redemption					Period(1)
					Restrictions			Period		Restrictions
	Private equity (primary and secondary)	$	144,422		$	144,422		2 - 9 Years		$	—					$	263,519
	Mezzanine debt	64,627			64,627			8 - 9 Years		—						198,756
	Distressed	7,776			7,776			4 Years		—						5,249
	Total private equity	216,825			216,825					—						467,524
	Distressed	151,227			151,227			1 - 2 Years		—						—
	Equity long/short	99,365			—					99,365				Quarterly	30 -60 Days	—
	Multi-strategy	136,958			—					136,958				Quarterly	45 -90 Days	—
	Event driven	14,018			—					14,018				Annual	60 Days	—
	Relative value credit	113,730			—					113,730				Quarterly	60 Days	—
	Total hedge funds	515,298			151,227					364,071						—
	High yield loan fund	31,958			—					31,958				Monthly	30 days	—
	Total other invested assets at fair value	764,081			368,052					396,029						467,524
	Other private securities	147,311			—					147,311						—
	Total other invested assets	$	911,392		$	368,052				$	543,340					$	467,524
	-1	The redemption frequency and notice periods only apply to the investments without redemption restrictions.
	In general, the Company has invested in hedge funds that require at least 30 days' notice of redemption, and may be redeemed on a monthly, quarterly, semi-annual, annual or longer basis, depending on the fund. Certain hedge funds have lock-up periods ranging from one to three years from initial investment. A lock-up period refers to the initial amount of time an investor is contractually required to invest before having the ability to redeem. Funds that provide for periodic redemptions may, depending on the funds’ governing documents, have the ability to deny or delay a redemption request, called a “gate.” The fund may implement this restriction because the aggregate amount of redemption requests as of a particular date exceeds a specified level, generally ranging from 15% to 25% of the fund’s net assets. The gate is a method for executing an orderly redemption process to reduce the possibility of adversely affecting investors in the fund. Typically, the imposition of a gate delays a portion of the requested redemption, with the remaining portion settled in cash sometime after the redemption date. Certain funds may impose a redemption fee on early redemptions. Interests in private equity funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
	The following is a summary of each investment type:
	•	Private equity funds: Primary equity funds may invest in companies and general partnership interests. Secondary equity funds buy limited partnership interests from existing limited partners of primary private equity funds. As owners of private equity funds seek liquidity, they can sell their existing investments, plus any remaining commitment, to secondary market participants. These funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
	•	Mezzanine debt funds: Mezzanine debt funds primarily focus on providing capital to upper middle market and middle market companies and private equity sponsors, in connection with leveraged buyouts, mergers and acquisitions, recapitalizations, growth financings and other corporate transactions. The most common position in the capital structure will be between the senior secured debt holder and the equity; however, the funds will utilize a flexible approach when structuring investments, which may include secured debt, subordinated debt, preferred stock and/or private equity. These funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
•	Distressed funds: In distressed debt investing, managers take positions in the debt of companies experiencing significant financial difficulties, including bankruptcy, or in certain positions of the capital structure of structured securities. The manager relies on the fundamental analysis of these securities, including the claims on the assets and the likely return to bondholders. Certain funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
•	Real estate funds: Private real estate funds invest directly in commercial real estate (multifamily units, industrial buildings, office spaces, and retail stores) and some residential property.  Real estate managers have diversified portfolios that generally follow core, core-plus, value-added, or opportunistic strategies.  These funds cannot be redeemed because the investments include restrictions that do not allow for redemption until termination of the fund.
•	Equity long/short funds: In equity long/short funds, managers take long positions in companies they deem to be undervalued and short positions in companies they deem to be overvalued. Long/short managers may invest in countries, regions or sectors and vary by their use of leverage and by their targeted net long position.
•	Multi-strategy funds: These funds may utilize many strategies employed by specialized funds including distressed investing, equity long/short, merger arbitrage, convertible arbitrage, fixed income arbitrage and macro trading.
•	Event driven funds: Event driven strategies seek to deploy capital into specific securities whose returns are affected by a specific event that affects the value of one or more securities of a company. Returns for such securities are linked primarily to the specific outcome of the events and not by the overall direction of the bond or stock markets. Examples could include mergers and acquisitions (arbitrage), corporate restructurings and spin-offs and capital structure arbitrage.
•	Relative value credit funds: These funds seek to take exposure to credit-sensitive securities, long and/or short, based upon credit analysis of issuers and securities and credit market views.
•	Other private securities: These securities include strategic non-controlling minority investments in private asset management companies, claims handling companies and other service companies related to the insurance industry.
c) Net Investment Income
	Year Ended December 31,
	2014			2013			2012
Fixed maturity investments	$	149,449		$	130,380		$	157,437
Equity securities	17,581			19,076			17,080
Other invested assets: hedge funds and private equity	12,564			8,226			6,772
Other invested assets: other private securities	13,305			14,711			—
Cash and cash equivalents	2,134			2,010			2,319
Expenses	(18,159		)	(16,839		)	(16,467		)
Net investment income	$	176,874		$	157,564		$	167,141
Net investment income from other invested assets: other private securities included the distributed and undistributed net income from investments accounted for using the equity method of accounting. The income reported for other invested assets: other private securities for the year ended December 31, 2014 included a loss of $9,348 recorded for an equity method investment due to impairment charges that it recorded.
d) Components of Realized Gains and Losses
	Year Ended December 31,
	2014			2013			2012
Gross realized gains on sale of invested assets	$	195,390		$	213,618		$	183,592
Gross realized losses on sale of invested assets	(48,239		)	(106,321		)	(51,702		)
Net realized and unrealized gains (losses) on derivatives	(39,007		)	9,485			(3,608		)
Mark-to-market gains (losses):
Debt securities, trading	(1,675		)	(117,577		)	106,328
Equity securities, trading	413			4,288			33,018
Other invested assets, trading	(17,925		)	56,032			38,808
Net realized investment gains	$	88,957		$	59,525		$	306,436
Proceeds from sale of available for sale securities	$	—		$	—		$	236,336
e) Pledged Assets
As of December 31, 2014 and 2013, $3,585,792 and $2,894,401, respectively, of cash and cash equivalents and investments were deposited, pledged or held in trust accounts in favor of ceding companies and other counterparties or government authorities to comply with reinsurance contract provisions and insurance laws.
In addition, as of December 31, 2014 and 2013, a further $571,750 and $1,053,632, respectively, of cash and cash equivalents and investments were pledged as collateral for the Company’s letter of credit facilities. See Note 10(f) for details on the Company’s credit facilities.

Derivative_Instruments

Derivative Instruments	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative instruments	DERIVATIVE INSTRUMENTS
	As of December 31, 2014 and 2013, none of the Company’s derivatives were designated as hedges for accounting purposes. The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheets:
		31-Dec-14											31-Dec-13
		Asset			Asset			Liability			Liability		Asset		Asset		Liability		Liability
		Derivative			Derivative			Derivative			Derivative		Derivative		Derivative		Derivative		Derivative
		Notional			Fair			Notional			Fair		Notional		Fair		Notional		Fair
		Amount			Value			Amount			Value		Amount		Value		Amount		Value
	Foreign exchange contracts	$	33,875		$	1,274		$	167,376		$	991	$	294,788	$	6,254	$	122,439	$	1,176
	Interest rate swaps	—			—			571,500			683		491,400		6,829		40,000		4,214
	Total derivatives	$	33,875		$	1,274		$	738,876		$	1,674	$	786,188	$	13,083	$	162,439	$	5,390
	Derivative assets and derivative liabilities are classified within “other assets” or “accounts payable and accrued liabilities” on the consolidated balance sheets.
	The following table provides the net realized and unrealized (losses) gains on derivatives not designated as hedges recorded on the consolidated income statements:
		Year Ended December 31,
		2014			2013			2012
	Foreign exchange contracts	$	897		$	1,274		$	(749	)
	Total included in foreign exchange loss	897			1,274			(749		)
	Put options	494			(3,822		)	(2,222		)
	Foreign exchange contracts	3,538			3,054			(1,024		)
	Interest rate swaps	(45,427		)	5,591			—
	Interest rate futures	2,388			4,662			(362		)
	Total included in net realized investment gains	(39,007		)	9,485			(3,608		)
	Total realized and unrealized (losses) gains on derivatives	$	(38,110	)	$	10,759		$	(4,357	)
	The loss related to interest rate swap contracts for the year ended December 31, 2014 was the result of selling interest rate swap contracts to reduce the duration of the investment portfolio. Given the decrease in interest rates during the year, the Company recorded a loss related to these interest rate swap contracts.
	Derivative Instruments Not Designated as Hedging Instruments
	The Company is exposed to foreign currency risk in its investment portfolio. Accordingly, the fair values of the Company’s investment portfolio are partially influenced by the change in foreign exchange rates. These foreign currency hedging activities have not been designated as specific hedges for financial reporting purposes.
	The Company’s insurance and reinsurance subsidiaries and branches operate in various foreign countries and consequently the Company’s underwriting portfolio is exposed to foreign currency risk. The Company manages foreign currency risk by seeking to match liabilities under the insurance policies and reinsurance contracts that it writes and that are payable in foreign currencies with cash and investments that are denominated in such currencies. When necessary, the Company may also use derivatives to economically hedge unmatched foreign currency exposures, specifically forward contracts and currency options. For example, during 2014 the Company purchased a forward contract to economically hedge a portion of its foreign currency exposure related to the consideration to be paid for the Hong Kong and Singapore operations of RSA.
	The Company also purchases and sells interest rate future and interest rate swap contracts to actively manage the duration and yield curve positioning of its fixed income portfolio. Interest rate futures and interest rate swaps can efficiently increase or decrease the overall duration of the portfolio. Additionally, interest rate future and interest rate swap contracts can be utilized to obtain the desired position along the yield curve in order to protect against certain future yield curve shapes.
	The Company also purchases options to actively manage the Company’s equity portfolio.

Fair_Value_Of_Financial_Instru

Fair Value Of Financial Instruments	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair value of financial instruments	FAIR VALUE OF FINANCIAL INSTRUMENTS
	In accordance with U.S. GAAP, fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. There is a three-level valuation hierarchy for disclosure of fair value measurements. The valuation hierarchy is based upon whether the inputs to the valuation of an asset or liability are observable or unobservable in the market at the measurement date, with quoted market prices being the highest level (Level 1) and unobservable inputs being the lowest level (Level 3). A fair value measurement will fall within the level of the hierarchy based on the input that is significant to determining such measurement. The three levels are defined as follows:
	•	Level 1:   Observable inputs to the valuation methodology that are quoted prices (unadjusted) for identical assets or liabilities in active markets.
	•	Level 2:   Observable inputs to the valuation methodology other than quoted market prices (unadjusted) for identical assets or liabilities in active markets. Level 2 inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical assets in markets that are not active and inputs other than quoted prices that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the asset or liability.
	•	Level 3:   Inputs to the valuation methodology that are unobservable for the asset or liability.
	The following table shows the fair value of the Company’s financial instruments and where in the fair value hierarchy the fair value measurements are included as of the dates indicated below:
	31-Dec-14	Carrying			Total fair			Level 1			Level 2			Level 3
		amount			value
	Fixed maturity investments:
	U.S. government and government agencies	$	1,610,502		$	1,610,502		$	1,499,347		$	111,155		$	—
	Non-U.S. government and government agencies	188,199			188,199			—			188,199			—
	States, municipalities and political subdivisions	170,567			170,567			—			170,567			—
	Corporate debt	2,165,393			2,165,393			—			2,165,393			—
	Mortgage-backed	1,263,517			1,263,517			—			1,081,734			181,783
	Asset-backed	670,832			670,832			—			615,419			55,413
	Total fixed maturity investments	6,069,010			6,069,010			1,499,347			4,332,467			237,196
	Equity securities	844,163			844,163			800,833			—			43,330
	Other invested assets	812,543			812,543			—			—			812,543
	Total investments	$	7,725,716		$	7,725,716		$	2,300,180		$	4,332,467		$	1,093,069
	Derivative assets:
	Foreign exchange contracts	$	1,274		$	1,274		$	—		$	1,274		$	—
	Derivative liabilities:
	Foreign exchange contracts	$	991		$	991		$	—		$	991		$	—
	Interest rate swaps	683			683			—			683			—
	Senior notes	$	798,802		$	879,317		$	—		$	879,317		$	—
	Other long-term debt	$	19,213		$	22,583		$	—		$	22,583		$	—
	31-Dec-13	Carrying			Total fair			Level 1			Level 2			Level 3
		amount			value
	Fixed maturity investments:
	U.S. government and government agencies	$	1,676,788		$	1,676,788		$	1,370,088		$	306,700		$	—
	Non-U.S. government and government agencies	191,776			191,776			—			191,776			—
	States, municipalities and political subdivisions	231,555			231,555			—			231,555			—
	Corporate debt	2,202,267			2,202,267			—			2,202,267			—
	Mortgage-backed	1,292,502			1,292,502			—			1,145,164			147,338
	Asset-backed	505,910			505,910			—			412,497			93,413
	Total fixed maturity investments	6,100,798			6,100,798			1,370,088			4,489,959			240,751
	Equity securities	699,846			699,846			625,942			—			73,904
	Other invested assets	764,081			764,081			—			—			764,081
	Total investments	$	7,564,725		$	7,564,725		$	1,996,030		$	4,489,959		$	1,078,736
	Derivative assets:
	Foreign exchange contracts	$	6,254		$	6,254		$	—		$	6,254		$	—
	Interest rate swaps	6,829			6,829			—			6,829			—
	Derivative liabilities:
	Foreign exchange contracts	$	1,176		$	1,176		$	—		$	1,176		$	—
	Interest rate swaps	4,214			4,214			$	—		4,214			$	—
	Senior notes	$	798,499		$	897,601		$	—		$	897,601		$	—
	“Other invested assets” in the table above excludes other private securities that the Company did not measure at fair value, but are accounted for using the equity method of accounting. Derivative assets and derivative liabilities relating to foreign exchange contracts and interest rate swaps are classified within “other assets” or “accounts payable and accrued liabilities” on the consolidated balance sheets.
	The following describes the valuation techniques used by the Company to determine the fair value of financial instruments held as of the balance sheet date.
	Recurring Fair Value of Financial Instruments
	U.S. government and government agencies: Comprised primarily of bonds issued by the U.S. Treasury, the Federal Home Loan Bank, the Federal Home Loan Mortgage Corporation and the Federal National Mortgage Association. The fair values of the Company’s U.S. government securities are based on quoted market prices in active markets and are included in the Level 1 fair value hierarchy. The Company believes the market for U.S. Treasury securities is an actively traded market given the high level of daily trading volume. The fair values of U.S. government agency securities are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are included in the Level 2 fair value hierarchy.
	Non-U.S. government and government agencies: Comprised of fixed income obligations of non-U.S. governmental entities. The fair values of these securities are based on prices obtained from international indices and are included in the Level 2 fair value hierarchy.
	States, municipalities and political subdivisions: Comprised of fixed income obligations of U.S. domiciled state and municipality entities. The fair values of these securities are based on prices obtained from the new issue market, and are included in the Level 2 fair value hierarchy.
	Corporate debt: Comprised of bonds issued by or loan obligations of corporations that are diversified across a wide range of issuers and industries. The fair values of corporate debt that are short-term are priced using spread above the LIBOR yield curve, and the fair value of corporate debt that are long-term are priced using the spread above the risk-free yield curve. The spreads are sourced from broker-dealers, trade prices and the new issue market. As the significant inputs used to price corporate debt are observable market inputs, the fair values of corporate debt are included in the Level 2 fair value hierarchy.
	Mortgage-backed: Primarily comprised of residential and commercial mortgages originated by both U.S. government agencies (such as the Federal National Mortgage Association) and non-U.S. government agencies. The fair values of mortgage-backed securities originated by U.S. government agencies and non-U.S. government agencies are based on a pricing model that incorporates prepayment speeds and spreads to determine appropriate average life of mortgage-backed securities. The spreads are sourced from broker-dealers, trade prices and the new issue market. As the significant inputs used to price the mortgage-backed securities are observable market inputs, the fair values of these securities are included in the Level 2 fair value hierarchy, unless the significant inputs used to price the mortgage-backed securities are broker-dealer quotes and the Company is not able to determine if those quotes are based on observable market inputs, in which case the fair value is included in the Level 3 hierarchy.
	Asset-backed: Principally comprised of bonds backed by pools of automobile loan receivables, home equity loans, credit card receivables and collateralized loan obligations originated by a variety of financial institutions. The fair values of asset-backed securities are priced using prepayment speed and spread inputs that are sourced from the new issue market or broker-dealer quotes. As the significant inputs used to price the asset-backed securities are observable market inputs, the fair values of these securities are included in the Level 2 fair value hierarchy, unless the significant inputs used to price the asset-backed securities are broker-dealer quotes and the Company is not able to determine if those quotes are based on observable market inputs, in which case the fair value is included in the Level 3 hierarchy.
	Equity securities: Comprised of U.S. and foreign common and preferred stocks and mutual funds. Equities are generally included in the Level 1 fair value hierarchy as prices are obtained from market exchanges in active markets. Foreign mutual funds where the net asset value (“NAV”) is not provided on a daily basis are included in the Level 3 fair value hierarchy.
	Other invested assets: Comprised of funds invested in a range of diversified strategies. In accordance with U.S. GAAP, the fair values of the funds are based on the NAV of the funds as reported by the fund manager. The fair value of these investments are included in Level 3 fair value hierarchy as the Company believes NAV is an unobservable input and these securities are not redeemable in the near term. The Company does not measure its investments that are accounted for using the equity method of accounting at fair value.
	Derivative instruments: The fair value of foreign exchange contracts and interest rate futures and swaps are priced from quoted market prices for similar exchange-traded derivatives and pricing valuation models that utilize independent market data inputs. The fair value of derivatives are included in the Level 2 fair value hierarchy.
	Senior notes: The fair value of the senior notes is based on reported trades. The fair value of the senior notes is included in the Level 2 fair value hierarchy.
	Other long-term debt: Comprised of the mortgage and credit facility associated with the purchase of office space in Switzerland. The fair value of the other long-term debt is based on the value of the debt using current interest rates. The fair value of the long-term debt is included in the Level 2 fair value hierarchy.
	Non-recurring Fair Value of Financial Instruments
	The Company measures the fair value of certain assets on a non-recurring basis, generally quarterly, annually, or when events or changes in circumstances indicate that the carrying amount of the assets may not be recoverable. These assets include investments accounted for using the equity method, goodwill and intangible assets. The Company uses a variety of techniques to measure the fair value of these assets when appropriate, as described below:
	Investments accounted for using the equity method: When the Company determines that the carrying value of these assets may not be recoverable, the Company records the assets at fair value with the loss recognized in income. In such cases, the Company measures the fair value of these assets using the techniques discussed above.
	Goodwill and Intangible Assets: The Company tests goodwill and intangible assets for impairment whenever events or changes in circumstances indicate that the carrying amount may not be recoverable, but at least annually for goodwill and indefinite-lived intangibles. When the Company determines that goodwill and indefinite-lived intangible assets may be impaired, the Company uses techniques, including discounted expected future cash flows and market multiple models, to measure fair value.
Rollforward of Level 3 Financial Instruments
The following is a rollforward of the beginning and ending balance of financial instruments using significant unobservable inputs (Level 3):
	Fair value measurement using significant
	unobservable inputs (Level 3):
	Other invested			Mortgage-			Asset-backed			Equities
	assets			backed
Year Ended December 31, 2014
Opening balance	$	764,081		$	147,338		$	93,413		$	73,904
Realized and unrealized gains (losses) included in net income	64,566			5,505			(2,068		)	(1,277		)
Purchases	331,099			100,019			4,628			—
Sales	(347,203		)	(71,141		)	(24,463		)	(29,297		)
Transfers into Level 3 from Level 2	—			1,505			28,813			—
Transfers out of Level 3 into Level 2 (1)	—			(1,443		)	(44,910		)	—
Ending balance	$	812,543		$	181,783		$	55,413		$	43,330
Realized and unrealized losses (gains) included in net income for investments still held as of December 31, 2014	$	(17,925	)	$	4,341		$	(116	)	$	(1,277	)
Year Ended December 31, 2013
Opening balance	$	655,888		$	167,825		$	62,246		$	54,680
Realized and unrealized gains included in net income	83,621			(7,658		)	(1,176		)	9,224
Purchases	287,977			83,455			50,669			10,000
Sales	(263,405		)	(92,985		)	(28,393		)	—
Transfers into Level 3 from Level 2	—			5,393			10,067			—
Transfers out of Level 3 into Level 2 (1)	—			(8,692		)	—			—
Ending balance	$	764,081		$	147,338		$	93,413		$	73,904
Realized and unrealized gains (losses) included in net income for investments still held as of December 31, 2013	$	62,104		$	(4,914	)	$	(360	)	$	9,225
(1) Transfers out of Level 3 are primarily attributable to the availability of market observable information.
The Company attempts to verify the significant inputs used by broker-dealers in determining the fair value of the securities priced by them. If the Company could not obtain sufficient information to determine if the broker-dealers were using significant observable inputs, such securities have been transferred to the Level 3 fair value hierarchy. The Company believes the prices obtained from the broker-dealers are the best estimate of fair value of the securities being priced as the broker-dealers are typically involved in the initial pricing of the security, and the Company has compared the price per the broker-dealer to other pricing sources and noted no material differences. The Company recognizes transfers between levels at the end of the reporting period. There were no transfers between Level 1 and Level 2 during the period.
The Company’s external investment accounting service provider receives prices from internationally recognized independent pricing services to measure the fair values of its fixed maturity investments. Pricing sources are evaluated and selected in a manner to ensure that the most reliable sources are used. The independent pricing sources obtain market quotations and actual transaction prices for securities that have quoted prices in active markets. Each pricing service has its own proprietary method for determining the fair value of securities that are not actively traded. In general, these methods involve the use of “matrix pricing” in which the independent pricing service uses observable market inputs, including, but not limited to, reported trades, benchmark yields, broker-dealer quotes, interest rates, prepayment speeds, default rates and such other inputs as are available from market sources to determine a reasonable fair value.
All of the Company’s securities classified as Level 3, other than investments in other invested assets, are valued based on unadjusted broker-dealer quotes. This includes less liquid securities such as lower quality asset-backed securities, commercial mortgage-backed securities and residential mortgage-backed securities. The primary valuation inputs include monthly payment information, the probability of default, loss severity rates and estimated prepayment rates. Significant changes in these inputs in isolation would result in a significantly lower or higher fair value measurement. In general, a change in the assumption of the probability of default is accompanied by a directionally similar change in the assumption used for the loss severity in an event of default and prepayment rates.
The Company records the unadjusted price provided and validates this price through a process that includes, but is not limited to monthly and/or quarterly: (i) comparison of prices between two independent sources, with significant differences requiring additional price sources; (ii) quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to their target benchmark, with significant differences identified and investigated); (iii) evaluation of methodologies used by external parties to calculate fair value, including a review of the inputs used for pricing; (iv) comparing the price to the Company’s knowledge of the current investment market; and (v) back-testing, which includes randomly selecting purchased or sold securities and comparing the executed prices to the fair value estimates from the pricing service. In addition to internal controls, management relies on the effectiveness of the valuation controls in place at the Company’s external investment accounting service provider (supported by a Statement on Standards for Attestation Engagements No. 16 report) in conjunction with regular discussion and analysis of the investment portfolio’s structure and performance.

Reserve_For_Losses_And_Loss_Ex

Reserve For Losses And Loss Expenses	12 Months Ended
	Dec. 31, 2014
Insurance Loss Reserves [Abstract]
Reserve for losses and loss expenses	RESERVE FOR LOSSES AND LOSS EXPENSES
	The reserve for losses and loss expenses consists of the following:
		December 31,
		2014			2013
	Outstanding loss reserves	$	1,514,051		$	1,520,867
	Reserves for losses incurred but not reported	4,367,114			4,245,662
	Reserve for losses and loss expenses	$	5,881,165		$	5,766,529
	The table below is a reconciliation of the beginning and ending liability for unpaid losses and loss expenses. Losses incurred and paid are reflected net of reinsurance recoverables.
		Year Ended December 31,
		2014			2013			2012
	Gross liability at beginning of year	$	5,766,529		$	5,645,549		$	5,225,143
	Reinsurance recoverable at beginning of year	(1,234,504		)	(1,141,110		)	(1,002,919		)
	Net liability at beginning of year	4,532,025			4,504,439			4,222,224
	Net losses incurred related to:
	Current year	1,411,797			1,303,573			1,309,613
	Prior years	(212,607		)	(180,331		)	(170,349		)
	Total incurred	1,199,190			1,123,242			1,139,264
	Net paid losses related to:
	Current year	171,824			115,669			117,123
	Prior years	1,001,537			973,987			743,844
	Total paid	1,173,361			1,089,656			860,967
	Foreign exchange revaluation	(16,945		)	(6,000		)	3,918
	Net liability at end of year	4,540,909			4,532,025			4,504,439
	Reinsurance recoverable at end of year	1,340,256			1,234,504			1,141,110
	Gross liability at end of year	$	5,881,165		$	5,766,529		$	5,645,549
	For the year ended December 31, 2014, the Company recognized net favorable reserve development in each of its operating segments due to actual loss emergence being lower than initially expected. The net favorable prior year reserve development in the North American Insurance segment primarily related to the 2006, 2007, 2009 and 2010 loss years partially offset by unfavorable reserve development from the 2011 to 2013 loss years. The net favorable prior year reserve development in the Global Markets Insurance segment was primarily due to favorable reserve development for the 2008 to 2010 loss years. The net favorable reserve development in the Reinsurance segment was primarily due to benign global property catastrophe activity for the 2013 loss year.
	For the year ended December 31, 2013, the Company recognized net favorable reserve development in each of its operating segments due to actual loss emergence being lower than initially expected. The net favorable prior year reserve development in the North American Insurance segment primarily related to the 2006 through 2008 loss years partially offset by unfavorable reserve development from the 2010 and 2011 loss years. The net favorable prior year reserve development in the Global Markets Insurance segment was primarily due to favorable reserve development for the 2006 to 2008 loss years. The net favorable prior year reserve development in the Reinsurance segment was primarily due to favorable reserve development for the 2005 to 2008 loss years.
	For the year ended December 31, 2012, the Company had net favorable reserve development in each of its operating segments due to actual loss emergence being lower than initially expected. The net favorable prior year reserve development in the North American Insurance segment primarily related to the 2006 through 2010 loss years partially offset by unfavorable reserve development from the 2011 and 2012 loss years. The net favorable prior year reserve development in the Global Markets Insurance segment was primarily due to favorable reserve development for the 2006 to 2010 loss years. The net favorable reserve development in the Reinsurance segment was primarily due to lower than expected loss activity for the 2012 loss year.
	While the Company has experienced favorable development in its insurance and reinsurance lines, there is no assurance that conditions and trends that have affected the development of liabilities in the past will continue. It is not appropriate to extrapolate future redundancies based on prior years’ development. The methodology of estimating loss reserves is periodically reviewed to ensure that the key assumptions used in the actuarial models continue to be appropriate.

Ceded_Reinsurance

Ceded Reinsurance	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Ceded reinsurance	CEDED REINSURANCE
	The Company purchases reinsurance from third-party reinsurance companies to reduce its net exposure to losses. Reinsurance provides for recovery of a portion of gross losses and loss expenses from these reinsurers. The Company remains liable to the extent that its reinsurers do not meet their obligations under the related reinsurance contracts. The Company therefore regularly evaluates the financial condition of its reinsurers and monitors concentration of credit risk. The Company believes that as of December 31, 2014, its reinsurers are able to meet, and will meet, all of their obligations under the agreements. No material provision has been made for unrecoverable reinsurance as of December 31, 2014 and 2013. The amount of reinsurance recoverable is as follows:
		2014				2013
	OSLR recoverable	$	232,545			$	225,796
	IBNR recoverable	1,107,711				1,008,708
	Reinsurance recoverable	$	1,340,256			$	1,234,504
	Reinsurance recoverable on paid losses	$	86,075			$	76,099
	Direct, assumed and ceded premiums written and earned and losses and loss expenses incurred are as follows:
		Premiums				Premiums				Losses and
		Written				Earned				Loss
										Expenses
	Year Ended December 31, 2014
	Direct	$	1,996,831			$	1,835,186			$	1,031,284
	Assumed	938,594				941,182				464,728
	Ceded	(613,350		)		(593,610		)		(296,822		)
		$	2,322,075			$	2,182,758			$	1,199,190
	Year Ended December 31, 2013
	Direct	$	1,804,910			$	1,664,343			$	936,084
	Assumed	933,754				896,088				430,627
	Ceded	(618,183		)		(554,598		)		(243,469		)
		$	2,120,481			$	2,005,833			$	1,123,242
	Year Ended December 31, 2012
	Direct	$	1,569,021			$	1,446,086			$	877,961
	Assumed	760,250				743,575				513,871
	Ceded	(491,448		)		(440,763		)		(252,568		)
		$	1,837,823			$	1,748,898			$	1,139,264
	Of the premiums ceded during the years ended December 31, 2014, 2013 and 2012, approximately 42%, 45% and 48% were ceded to four reinsurers, respectively.
	The Company actively manages its reinsurance exposures by generally selecting reinsurers having a credit rating of A- or higher and monitoring the overall credit quality of its reinsurers to ensure the recoverables will be collected.
		31-Dec-14									31-Dec-13
		A.M. Best		Reinsurance				Percentage			A.M. Best		Reinsurance		Percentage
		Rating		Recoverable				of Total			Rating		Recoverable		of Total
	Munich Re	A+		$	285,918			21.3	%		A+		$	214,070	17.3	%
	Axis Capital	A+		148,371				11.1	%		A+		143,063		11.6	%
	Arch Re	A+		128,421				9.6	%		A+		143,037		11.6	%
	Markel	A		86,595				6.5	%		A		86,462		7	%
	XL Group	A		78,377				5.8	%		A		68,857		5.6	%
	Top five reinsurers			727,682				54.3	%				655,489		53.1	%
	Other reinsurers’ balances			612,574				45.7	%				579,015		46.9	%
	Total reinsurance recoverable			$	1,340,256			100	%				$	1,234,504	100	%
	Approximately 99% of ceded reserves were recoverable from reinsurers who had an A.M. Best rating of “A-” or higher as of December 31, 2014 and 2013.

Funds_Held

Funds Held	12 Months Ended
	Dec. 31, 2014
Other Insurance Industry Disclosures [Abstract]
Funds held	FUNDS HELD
	The Company has a minority interest in Aeolus Capital Management Ltd. ("ACM"), an affiliate of Aeolus Re, Ltd., a Bermuda-based property catastrophe reinsurer ("Aeolus Re"), which it accounts for under the equity method. ACM manages capital on behalf of investors seeking to invest in the property catastrophe reinsurance market.
	The Company has also entered into a collateralized property catastrophe quota share reinsurance contract with Aeolus Re, whereby the Company assumes property catastrophe business underwritten by Aeolus Re. As part of the agreement to purchase the minority interest in ACM, the Company provided a multi-year capital commitment to support the business being underwritten by Aeolus Re. To the extent that capital is not utilized to support the business being underwritten by Aeolus Re, as all obligations have been settled, the capital is returned to the Company. To the extent the losses are in excess of the premiums written, the capital is utilized to pay the claims. The capital commitment is recorded in "funds held" on the consolidated balance sheets. The funds held balance as of December 31, 2014 and 2013 was $708,047 and $621,567, respectively. For the years ended December 31, 2014, 2013 and 2012, the premiums written assumed by the Company through the collateralized property catastrophe quota share reinsurance contract with Aeolus Re were $87,275, $83,300 and $34,597, respectively.

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and intangible assets	GOODWILL AND INTANGIBLE ASSETS
	The following table shows the Company’s goodwill and intangible assets at December 31, 2014 and 2013:
		Goodwill		Indefinite-Lived		Finite-Lived Intangible Assets
				Intangible
				Assets
		Net Carrying		Net Carrying		Gross Carrying		Accumulated			Net Carrying
		Value		Value		Value		Amortization			Value
	Balance at December 31, 2012	$	268,376	$	23,920	$	41,334	$	(13,890	)	$	27,444
	Amortization of intangible assets	—		—		—		(2,533		)	(2,533		)
	Balance at December 31, 2013	268,376		23,920		41,334		(16,423		)	24,911
	Additions	9,882		—		—		—			—
	Amortization of intangible assets	—		—		—		(2,533		)	(2,533		)
	Balance at December 31, 2014	$	278,258	$	23,920	$	41,334	$	(18,956	)	$	22,378
	The Company acquired the remaining interest in a claims administration services company it did not own in 2014 and recorded goodwill of $9,882 related to the transaction. The goodwill related to this acquisition has been allocated to the U.S. direct insurance operations of the North American Insurance segment.
	Goodwill of $274,341 is allocated to the U.S. direct insurance operations of the North American Insurance segment while the remainder of the goodwill of $3,917 is allocated to the Reinsurance segment. The impairment reviews for goodwill and indefinite-lived intangibles did not result in the recognition of impairment losses for the years ended December 31, 2014, 2013 and 2012. The net carrying value of the goodwill is net of accumulated impairment charges of $156 that occurred prior to December 31, 2012.
	The net carrying value of the finite-lived intangible assets is net of accumulated impairment charges of $6,866 that occurred prior to December 31, 2012. The remaining finite-lived intangible assets as of December 31, 2013 will be amortized over an expected remaining useful life of 8.8 years. The estimated amortization expense for each of the five succeeding fiscal years and thereafter related to the Company’s finite-lived intangible assets is as follows:
		Amount
	2015	$	2,533
	2016	2,533
	2017	2,533
	2018	2,533
	2019	2,533
	2020 and thereafter	9,713
	Total	$	22,378

Debt_and_Financing_Arrangement

Debt and Financing Arrangements	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Debt and financing arrangements	DEBT AND FINANCING ARRANGEMENTS
	a) Financing Structure
	The following table shows the Company’s financing structure:
		Outstanding(1)		Balance(2)
	31-Dec-14
	2006 Senior notes due 2016	$	500,000	$	499,690
	2010 Senior notes due 2020	300,000		299,112
	Swiss office building mortgage	14,157		14,157
	Swiss office building credit facility	5,056		5,056
	$1,000,000 Secured letter of credit facility — uncommitted	497,332		—
	$150,000 Secured letter of credit facility — committed (3)	—		—
		$	1,316,545	$	818,015
	31-Dec-13
	2006 Senior notes due 2016	$	500,000	$	499,512
	2010 Senior notes due 2020	300,000		298,987
	$1,000,000 Secured letter of credit facility — uncommitted	707,915		—
	$450,000 Secured letter of credit facility — committed	86,102		—
		$	1,594,017	$	798,499
	-1	Indicates utilization of commitment amount, not drawn borrowings.
	-2	Represents the principal amount borrowed, net of unamortized discount.
	-3	The commitment was reduced from $450,000 to $150,000 during 2014.
	b) 2006 Senior Notes Due 2016
	In 2006, Allied World Assurance Company Holdings, Ltd, a Bermuda company ("Allied World Bermuda") issued $500,000 aggregate principal amount of 7.50% Senior Notes due August 1, 2016 (the “2006 Senior Notes”), with interest payable semi-annually commencing on February 1, 2007. The 2006 Senior Notes are Allied World Bermuda’s unsecured and unsubordinated obligations and rank equally in right of payment with all existing and future unsecured and unsubordinated indebtedness. The 2006 Senior Notes can be redeemed by Allied World Bermuda prior to maturity subject to payment of a “make-whole” premium. Allied World Bermuda has no current expectations of redeeming the notes prior to maturity. The 2006 Senior Notes include covenants and events of default that are usual and customary, but do not contain any financial covenants.
	c) 2010 Senior Notes Due 2020
	In November 2010, Allied World Bermuda issued $300,000 aggregate principal amount of 5.50% Senior Notes due November 15, 2020 (the “2010 Senior Notes”), with interest on the notes payable semi-annually commencing on May 15, 2011. The 2010 Senior Notes are Allied World Bermuda’s unsecured and unsubordinated obligations and rank equally in right of payment with all existing and future unsecured and unsubordinated indebtedness. Allied World Bermuda may redeem the 2010 Senior Notes at any time or from time to time in whole or in part at a redemption price equal to the greater of the principal amount of the 2010 Senior Notes to be redeemed or a make-whole price, plus accrued and unpaid interest. Allied World Bermuda has no current expectations of redeeming the notes prior to maturity. The 2010 Senior Notes include covenants and events of default that are usual and customary, but do not contain any financial covenants.
	d) Swiss Office Building Mortgage
	In 2014, the Company entered into a 20-year mortgage commitment with a Swiss bank for the construction of a company-used office building in Zug, Switzerland. The total proceeds received in 2014 under the mortgage are $14,157 (CHF 14,000) with a fixed annual interest rate of 3.2% payable quarterly. An additional $4,044 (CHF 4,000) of proceeds from the mortgage will be drawn during the first quarter of 2015. The mortgage payments will be $303 (CHF 300) per year, plus accrued interest, for the first 19 years with the remaining balance payable at the end of the mortgage. The outstanding balance of the mortgage is included in "other long-term debt" on the consolidated balance sheets.
	e) Swiss Office Building Credit Facility
	In conjunction with the above mortgage commitment, the Company entered into a three year credit facility with a Swiss bank that provides up to $5,056 (CHF 5,000) for general corporate purposes, however the Company will use the proceeds from the credit facility to fund the purchase of the office building in Zug, Switzerland. The Company utilized the full commitment of the credit facility to finance the purchase of the Swiss office space. The interest rate for the credit facility is 2.5%. The outstanding balance of the credit facility is included in "other long-term debt" on the consolidated balance sheets.
	f) Credit Facilities
	In the normal course of its operations, the Company enters into agreements with financial institutions to obtain secured and unsecured credit facilities.
	Allied World Bermuda has a collateralized amended letter of credit facility with Citibank Europe plc. that has been and will continue to be used to issue standby letters of credit. The maximum aggregate amount available under this credit facility as of December 31, 2014 and 2013 is $1,000,000 on an uncommitted basis.
	On November 27, 2007, Allied World Bermuda entered into an $800,000 five-year senior credit facility (the “Credit Facility”) with a syndication of lenders. The Credit Facility consisted of a $400,000 secured letter of credit facility for the issuance of standby letters of credit (the “Secured Facility”) and a $400,000 unsecured facility for the making of revolving loans and for the issuance of standby letters of credit.
	On June 7, 2012, Allied World Bermuda amended the Secured Facility. The amended $450,000 four-year secured credit facility (the “Amended Secured Credit Facility”) is primarily used for the issuance of standby letters of credit to support obligations in connection with the insurance and reinsurance business of Allied World Bermuda and its subsidiaries. A portion of the facility may also be used for revolving loans for general corporate and working capital purposes, up to a maximum of $150,000. Allied World Bermuda may request that existing lenders under the Amended Secured Credit Facility make additional commitments from time to time, up to $150,000, subject to approval by the lenders. The Amended Secured Credit Facility contains representations, warranties and covenants customary for similar bank loan facilities, including certain covenants that, among other things, require the Company to maintain a certain leverage ratio and financial strength rating.
	On November 12, 2014, Allied World Bermuda gave irrevocable notice to the administrative agent under the Amended Secured Credit Facility to reduce the aggregate commitment from $450,000 to $150,000. All other material items of the Amended Secured Credit Facility remain unchanged.
	g) Debt Maturities
	The following table reflects the Company’s debt maturities, which includes its senior notes and other long-term debt:
		Amount
	2015	$	228
	2016	500,303
	2017	5,359
	2018	303
	2019	303
	2020 and thereafter	312,717
	Total	$	819,213

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income taxes	INCOME TAXES
	Under Swiss law, a resident company is subject to income tax at the federal, cantonal and communal levels that is levied on net income. Income attributable to permanent establishments or real estate located abroad is excluded from the Swiss tax base. Allied World Switzerland is a holding company and, therefore, is exempt from cantonal and communal income tax. As a result, Allied World Switzerland is subject to Swiss income tax only at the federal level. Allied World Switzerland is resident of the Canton of Zug and, as such, is subject to an annual cantonal and communal capital tax on the taxable equity of Allied World Switzerland in Switzerland. Allied World Switzerland has a Swiss operating company resident in the Canton of Zug. The operating company is subject to federal, cantonal and communal income tax and to annual cantonal and communal capital tax.
	Under current Bermuda law, Allied World Bermuda and its Bermuda subsidiaries are not required to pay taxes in Bermuda on either income or capital gains. Allied World Bermuda and Allied World Assurance Company, Ltd have received an assurance from the Bermuda Minister of Finance under the Exempted Undertakings Tax Protection Act 1966 of Bermuda, that in the event of any such taxes being imposed, Allied World Bermuda and Allied World Assurance Company, Ltd will be exempted until March 2035.
	Certain subsidiaries of Allied World Switzerland file U.S. federal income tax returns and various U.S. state income tax returns, as well as income tax returns in Canada, Hong Kong, Ireland, Singapore, Switzerland and the United Kingdom. The Company has open tax years that are potentially subject to examinations by local tax authorities, in the following major tax jurisdictions: the U.S., 2011 to 2014; the United Kingdom, 2013 and 2014; Ireland, 2010 to 2014; Switzerland, 2013 to 2014; Hong Kong, 2009 to 2014; and Singapore, 2010 to 2014. In December 2014, the Company received notice that the U.S. Internal Revenue Service will conduct an audit of the 2012 tax return of the Company's U.S. subsidiaries. To the best of the Company’s knowledge, there are no other examinations pending by any other tax authority.
	Management has deemed all material tax positions to have a greater than 50% likelihood of being sustained based on technical merits if challenged. The Company does not expect any material unrecognized tax benefits within 12 months of December 31, 2014.
	The components of income tax expense are as follows:
		Year Ended December 31,
		2014				2013				2012
	Current income tax expense	$	26,768			$	21,669			$	18,239
	Deferred income tax expense (benefit)	3,755				(11,889		)		201
	Income tax expense	$	30,523			$	9,780			$	18,440
	Our income is primarily sourced from our Bermuda, U.S., European and Asia Pacific operations. The income before income taxes for these operations are as follows:
		Year Ended December 31,
		2014				2013				2012
	Non-U.S.	$	447,875			$	401,778			$	474,412
	United States	72,907				25,882				37,035
	Income before income taxes	$	520,782			$	427,660			$	511,447
	Deferred income taxes reflect the tax impact of temporary differences between the carrying amounts of assets and liabilities for financial reporting and income tax purposes. The significant components of the net deferred tax assets are as follows:
		December 31,
		2014				2013
	Deferred tax assets:
	Reserve for losses and loss expenses	$	21,644			$	22,567
	Equity compensation	15,316				15,828
	Unearned premium	16,921				14,299
	Deferred acquisition costs	11,184				10,604
	Other-than-temporary impairments	—				407
	Net loss carryforward	7,958				7,642
	Total deferred tax assets	73,023				71,347
	Deferred tax liabilities:
	Intangible assets	(13,856		)		(14,743		)
	Realized gains	(12,819		)		(6,980		)
	Depreciation	(2,513		)		(2,942		)
	Unrealized appreciation and timing differences on investments	(94		)		(56		)
	Market discount on bonds	(740		)		(271		)
	Other	(2,965		)		(1,244		)
	Total deferred tax liabilities	(32,987		)		(26,236		)
	Net deferred taxes before valuation allowance	40,036				45,111
	Valuation allowance	(6,421		)		(7,642		)
	Net deferred tax assets	$	33,615			$	37,469
	The valuation allowance reported in the current period relates to net loss carryforwards for the European and Asia Pacific operations as it is unlikely those operations will have sufficient income to utilize the net loss carryforwards in the near term. The change in valuation allowance of $1,221 was included in income tax expense for the year ended December 31, 2014. The capital loss carryforwards from the United Kingdom and Asia Pacific operations do not expire as along as minimum capital requirements are maintained. The net loss carryforwards from the Switzerland operation expire between 2020 and 2021.
	Current tax receivable and payable has been included in “other assets” and “accounts payable and accrued liabilities” on the consolidated balance sheets, respectively. Current taxes receivable or payable was as follows:
		December 31,
		2014				2013
	Current tax receivable	$	8,640			$	11,684
	Current tax payable	$	829			$	1,452
	The expected tax provision has been calculated using the pre-tax accounting income in each jurisdiction multiplied by that jurisdiction’s applicable statutory tax rate. The reconciliation between the Company’s effective tax rate on pre-tax accounting income and the expected tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Expected tax rate	7.8	%		7.8	%		7.8	%
	Income not subject to income tax	(7.1	)%		(8.2	)%		(7.3	)%
	Foreign taxes at local expected tax rates	4.4	%		1.4	%		2.1	%
	Disallowed expenses and capital allowances	0.2	%		1	%		0.7	%
	Prior year refunds and adjustments	1.2	%		0.8	%		0.4	%
	Other	(0.6	)%		(0.5	)%		(0.1	)%
	Effective tax rate	5.9	%		2.3	%		3.6	%

Shareholders_Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Shareholders' equity	SHAREHOLDERS’ EQUITY
	a) Authorized shares
	The issued share capital consists of the following:
		December 31,
		2014			2013
	Common shares issued and fully paid, 2014 and 2013: CHF 4.10 per share	100,775,256			103,477,452
	Share capital at end of year	$	408,020		$	418,988
		2014		2013
	Shares issued at beginning of year	103,477,452		109,109,604
	Shares cancelled	(2,702,196	)	(5,632,152	)
	Total shares issued at end of year	100,775,256		103,477,452
	Treasury shares issued at beginning of year	3,223,806		4,716,261
	Shares repurchased	4,906,785		5,544,432
	Shares issued out of treasury	(848,621	)	(1,404,735	)
	Shares cancelled	(2,702,196	)	(5,632,152	)
	Total treasury shares at end of year	4,579,774		3,223,806
	Total shares outstanding at end of year	96,195,482		100,253,646
	During the years ended December 31, 2014, and 2013, 2,702,196 and 5,544,432 voting shares repurchased and designated for cancellation, respectively, were constructively retired and cancelled.
	Effective July 9, 2013, the Company cancelled 87,720 non-voting shares held in treasury and 4,616,058 shares previously repurchased and constructively retired, following a required filing with the Swiss Commercial Register in Zug.
	As required under Swiss law, the Company cannot hold more than 10% of its registered capital in treasury shares, unless it receives shareholder approval to do so.
	b) Dividends
	As a holding company, Allied World Switzerland’s principal source of income is dividends or other sources of permitted payments from its subsidiaries. These funds provide the cash flow required for dividend payments to the Company’s shareholders. The ability of our insurance and reinsurance subsidiaries to make dividend payments is limited by the applicable laws and regulations of the various states and countries in which they operate.
	The Company’s primary restrictions on net assets of subsidiaries consist of regulatory requirements placed upon the regulated insurance and reinsurance subsidiaries to hold minimum amounts of total statutory capital and surplus and there were no other material restrictions on net assets in place as of December 31, 2014.
	Under Swiss law, distributions to shareholders may be paid only if the Company has sufficient distributable profits from previous fiscal years, or if the Company has freely distributable reserves, each as presented on the audited stand-alone statutory balance sheet. Distributions to shareholders out of the share and participation capital may be made by way of a capital reduction in the form of a reduction to par value to achieve a similar result as the payment of a dividend. Under Swiss law, if the Company’s general capital reserves amount to less than 20% of the share and participation capital recorded in the Swiss Commercial Register, then at least 5% of the Company’s annual profit must be retained as general reserves.
	On May 2, 2013, the shareholders approved the Company’s proposal to pay cash dividends in the form of a distribution out of general legal reserve from capital contributions. The distribution amounts were paid to shareholders in quarterly dividends of $0.167 per share in July 2013, October 2013, January 2014 and April 2014.
	On May 1, 2014, the shareholders approved the Company’s proposal to pay cash dividends in the form of a distribution out of general legal reserve from capital contributions. The distribution amount will be paid to shareholders in quarterly dividends of $0.225 per share. The first dividend was on July 2, 2014, the second dividend was on October 2, 2014 and the third dividend was on January 2, 2015. The Company expects to distribute the remaining dividend in April 2015.
	The Company paid the following dividends during the years ended December 31, 2014, 2013 and 2012:
	Dividend Paid	Partial			Dividend		Total
		Par Value			Per		Amount
		Reduction			Share		Paid
		Per Share
	2-Oct-14		N/A		$	0.225	$	21,686
	2-Jul-14		N/A		$	0.225	$	21,870
	3-Apr-14		N/A		$	0.167	$	16,495
	2-Jan-14		N/A		$	0.167	$	16,732
	3-Oct-13		N/A		$	0.167	$	16,952
	3-Jul-13		N/A		$	0.167	$	17,117
	12-Mar-13	CHF	0.11		$	0.125	$	12,981
	December 18, 2012	CHF	0.11		$	0.125	$	13,010
	September 25, 2012	CHF	0.12		$	0.125	$	13,324
	August 6, 2012	CHF	0.12		$	0.125	$	13,394
	April 6, 2012	CHF	0.11		$	0.125	$	13,795
	January 6, 2012	CHF	0.12		$	0.125	$	14,302
	c) Share Repurchases
	On May 1, 2014, the shareholders approved a new share repurchase program in order for the Company to repurchase up to $500,000 of Allied World Switzerland's common shares. This new share repurchase program supersedes the 2012 share repurchase program and no further repurchases will be made under the 2012 share repurchase program. Repurchases may be effected from time to time through open market purchases, privately negotiated transactions, tender offers or otherwise. The timing, form and amount of the share repurchases under the program will depend on a variety of factors, including market conditions, the Company’s capital position, legal requirements and other factors. Under the terms of this share repurchase program, the first three million of common shares repurchased will remain in treasury and will be used by Allied World Switzerland to satisfy share delivery obligations under its equity-based compensation plans. Any additional common shares repurchased will be designated for cancellation at acquisition and will be canceled upon shareholder approval. Shares repurchased and designated for cancellation are constructively retired and recorded as a share cancellation.
	Shares repurchased by the Company and not designated for cancellation are classified as “Treasury shares, at cost” on the consolidated balance sheets. The Company will issue shares out of treasury principally related to the Company’s employee benefit plans. Shares repurchased and designated for cancellation are constructively retired and recorded as a share cancellation.
	The Company’s share repurchases were as follows:
		Year Ended December 31,
		2014			2013		2012
	Common shares repurchased	4,906,785			5,544,432		10,967,877
	Total cost of shares repurchased	$	175,373		$	174,686	$	263,942
	Average price per share	$	35.74		$	31.51	$	24.07

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee benefit plans	EMPLOYEE BENEFIT PLANS
	The Company implemented the Allied World Assurance Company Holdings, AG 2012 Omnibus Incentive Compensation Plan (the “2012 Omnibus Plan”). Under the 2012 Omnibus Plan, up to 4,500,000 common shares may be issued, subject to adjustment. The 2012 Omnibus Plan provides for the grant of options intended to qualify as incentive stock options under the Internal Revenue Code, non-qualified stock options, stock appreciation rights, restricted shares, RSUs, deferred share units, cash incentive awards, performance-based compensation awards and other equity-based and equity-related awards. These awards generally vest pro rata over four years from the date of grant, except for the performance-based awards that will generally vest over a three-year period based on the achievement of certain performance conditions.
	The Allied World Assurance Company Holdings, AG Third Amended and Restated 2001 Employee Stock Option Plan (the “Plan”), the Allied World Assurance Company Holdings, AG Third Amended and Restated 2004 Stock Incentive Plan and the and the Allied World Assurance Company Holdings, AG Third Amended and Restated Long-Term Incentive Plan (the “LTIP”) were automatically terminated, replaced and superseded by the 2012 Omnibus Plan on May 3, 2012, except that any outstanding awards granted under such plans remain in effect pursuant to their terms.
	a) Employee option plan
	Options under the Plan are exercisable in certain limited conditions, expire after 10 years, and generally vest pro-rata over four years from the date of grant. The exercise price of options issued were approved by the Compensation Committee but were not less than the fair market value of the common shares of Allied World Switzerland on the date the option award is granted. The Company has not granted stock options since 2011.
	The following table summarizes the activity related to options granted and exercised:
		Year Ended December 31,
		2014				2013				2012
	Options granted	—				—				—
	Weighted average grant date fair value	$	—			$	—			$	—
	Options exercised	(479,831		)		(674,187		)		(801,810		)
	Total intrinsic value of options exercised	$	10,313			$	12,212			$	9,650
	Proceeds from option exercises	$	10,027			$	12,137			$	10,527
	The activity related to the Company’s stock options is as follows:
		Year Ended December 31, 2014
		Options			Weighted				Weighted		Aggregate
					Average				Average		Intrinsic
					Exercise Price				Contractual		Value
									Term
	Outstanding at beginning of year	2,928,312			$	16.07
	Granted	—			$	—
	Exercised	(479,831	)		$	(14.25	)
	Forfeited	(15,096	)		$	(20.50	)
	Expired	(6,711	)		$	(16.35	)
	Outstanding at end of year	2,426,674			$	16.4			4.7 years		$	52,212
	Exercisable at end of year	2,151,790			$	15.88			4.5 years		$	47,424
	As of December 31, 2014, there was remaining $299 of total unrecognized compensation expense related to unvested options granted under the Plan. This expense is expected to be recognized over a weighted-average period of 0.1 years.
	b) Restricted stock units and performance-based equity awards
	The RSUs vest pro-rata over four years from the date of grant. The compensation expense for the RSUs is based on the fair market value of Allied World Switzerland’s common shares at the date of grant. The Company estimates the expected forfeitures of RSUs at the date of grant and recognizes compensation expense only for those awards that the Company expects to vest. The forfeiture assumption is ultimately adjusted to the actual forfeiture rate.
	Each award for the performance-based RSUs represents the right to receive a number of shares in the future, based upon the achievement of established performance criteria during the applicalbe performance period. The compensation expense for the performance-based RSUs is based on the fair market value of Allied World Switzerland's common shares at the time of grant. For the performance-based RSUs granted in 2014, 2013 and 2012, the Company anticipates that the performance goals are likely to be achieved. Based on the performance goals, the performance-based RSUs granted in 2014, 2013 and 2012 are expensed at 100%, 110% and 135%, respectively, of the fair market value of Allied World Switzerland’s common shares on the date of grant. The expense is recognized over the performance period.
	The activity related to the Company’s RSUs awards is as follows:
		Year Ended December 31, 2014
		Number of Awards			Weighted				Aggregate
					Average				Intrinsic
					Grant Date				Value
					Fair Value
	Outstanding at beginning of year	143,697			$	21.69
	RSUs granted	454,176			33.56
	RSUs forfeited	(16,386	)		(31.22		)
	RSUs fully vested	(78,981	)		(21.76		)
	Outstanding at end of year	502,506			$	32.1			$	19,055
	As of December 31, 2014, there was remaining $12,057 of total unrecognized compensation expense related to 502,506 unvested RSUs awarded. This expense is expected to be recognized over a weighted-average period of 1.9 years.
	The activity related to the Company’s performance-based equity awards is as follows:
		Year Ended December 31, 2014
		Number of Awards			Weighted				Aggregate
					Average				Intrinsic
					Grant Date				Value
					Fair Value
	Outstanding at beginning of year	804,519			$	23.21
	Performance-based equity awards granted	166,302			33.56
	Additional awards granted due to achievement of performance criteria	104,895			20.5
	Performance-based equity awards forfeited	(4,635	)		(25.20		)
	Performance-based equity awards fully vested	(454,440	)		(20.50		)
	Outstanding at end of year	616,641			$	27.52			$	23,383
	As of December 31, 2014, there was remaining $5,814 of total unrecognized compensation expense related to 616,641 unvested performance-based equity awarded. This expense is expected to be recognized over a weighted-average period of 1.6 years.
	The RSUs and performance-based equity awards vested in 2014, 2013 and 2012 had aggregate intrinsic values of $14,366, $28,836 and $13,947 at the time of vesting, respectively.
	c) Cash-equivalent stock awards
	As part of the Company’s annual year-end compensation awards, the Company granted both stock-based awards and cash-equivalent stock awards. The cash-equivalent awards were granted to employees who received RSU and performance-based awards and were granted in lieu of granting the full award as a stock-based award. The cash-equivalent RSU awards vest pro-rata over four years from the date of grant. The cash-equivalent performance-based awards vest after a three-year performance period. As the cash-equivalent awards are settled in cash, the Company establishes a liability equal to the product of the fair market value of Allied World Switzerland’s common shares as of the end of the reporting period and the total awards outstanding. At December 31, 2014, the liability was $51,343, and payments for awards vesting in the period are typically settled within the first three months of the year. The liability is included in “accounts payable and accrued expenses” in the consolidated balance sheets and changes in the liability are recorded in “general and administrative expenses” in the consolidated income statements.
	The activity related to the Company's cash-equivalent RSUs and performance-based equity awards is as follows:
		RSU's							Performance-based Awards
	Year Ended December 31, 2014	Number of Awards			Weighted Average Grant Date Fair Value				Number of Awards			Weighted Average Grant Date Fair Value
	Outstanding at beginning of year	2,049,084			$	24.69			1,031,961			23.67
	Granted	438,162			33.56				249,438			33.56
	Additional awards granted due to achievement of performance criteria	—			—				104,895			20.5
Forfeited	(77,322	)		(27.12		)		(6,948	)		(25.20		)
Fully vested	(755,860	)		(22.51		)		(454,440	)		(20.50		)
Outstanding at end of year	1,654,064			$	27.99			924,906			$	27.52
d) Total Stock-Related Compensation Expense
The following table shows the total stock-related compensation expense relating to the stock options, RSUs, LTIP and cash equivalent awards.
	Year Ended December 31,
	2014				2013				2012
Stock options	$	1,949			$	3,269			$	4,256
RSUs and performance-based equity awards	12,295				10,700				13,010
LTIP	—				—				376
Cash-equivalent stock awards	37,790				50,878				32,760
Total	$	52,034			$	64,847			$	50,402
e) Pension Plans
The Company provides defined contribution retirement plans for its employees and officers.  Contributions are made by the Company, and in some locations, these contributions are supplemented by the local plan participants. Contributions are based on a percentage of the participant’s base salary depending upon competitive local market practice and vesting provisions meeting legal compliance standards.  The amount that an individual employee or officer can contribute may also be subject to regulatory requirements relating to the country of which the individual is a citizen. The Company incurred expenses for these defined contribution arrangements of $10,648, $9,287 and $6,602 for the years ended December 31, 2014, 2013 and 2012, respectively.
f) Employee Share Purchase Plan
Under the Allied World Assurance Company Holdings, AG Amended and Restated 2008 Employee Share Purchase Plan (“ESPP”), eligible employees of the Company may purchase common shares of the Company at a 15% discount from the fair market value of one common share on the last trading day of each offering period. Employees purchase a variable number of common shares through payroll deductions elected as of the beginning of the offering period. The Company may sell up to 3,000,000 common shares to eligible employees under the ESPP.

Earnings_Per_Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings per share	EARNINGS PER SHARE
	The following table sets forth the comparison of basic and diluted earnings per share:
		Year Ended December 31,
		2014		2013		2012
	Basic earnings per share:
	Net income	$	490,259	$	417,880	$	493,007
	Weighted average common shares outstanding	97,538,319		102,464,715		108,171,435
	Basic earnings per share	$	5.03	$	4.08	$	4.56
		Year Ended December 31,
		2014		2013		2012
	Diluted earnings per share:
	Net income	$	490,259	$	417,880	$	493,007
	Weighted average common shares outstanding	97,538,319		102,464,715		108,171,435
	Share equivalents:
	Warrants and options	1,432,960		1,536,939		1,306,002
	RSU and LTIP awards	605,808		864,180		1,727,265
	Employee share purchase plan	14,686		—		4,953
	Weighted average common shares and common share equivalents outstanding — diluted	99,591,773		104,865,834		111,209,655
	Diluted earnings per share	$	4.92	$	3.98	$	4.43
	For the years ended December 31, 2014, 2013 and 2012, a weighted average of nil, nil and 678,312 employee stock options and RSUs were considered anti-dilutive and were therefore excluded from the calculation of the diluted earnings per share, respectively.

Commitments_And_Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	COMMITMENTS AND CONTINGENCIES
	a)    Concentrations of Credit Risk
	Credit risk arises out of the failure of a counterparty to perform according to the terms of the contract.
	The Company’s investment portfolio is managed pursuant to guidelines that follow prudent standards of diversification. The guidelines limit the allowable holdings of a single issue and issuers. The Company believes that there are no significant concentrations of credit risk associated with its investment portfolio. As of December 31, 2014 and 2013, substantially all of the Company’s cash and investments were held with one custodian.
	Insurance balances receivable primarily consist of net premiums due from insureds and reinsureds. The Company believes that the counterparties to these receivables are able to meet, and will meet, all of their obligations. The Company’s credit risk is further reduced by the contractual right to offset loss obligations or unearned premiums against premiums receivable. Consequently, the Company has not included any material allowance for doubtful accounts against the receivable balance.
	b)    Operating Leases
	The Company leases office space under operating leases expiring in various years through 2030. The following are future minimum rental payments as of December 31, 2014:
		Amount
	2015	$	12,568
	2016	16,903
	2017	20,596
	2018	20,096
	2019	19,013
	2020 and thereafter	138,602
		$	227,778
	Total rent expense for the years ended December 31, 2014, 2013 and 2012 was $18,950, $11,392 and $11,929, respectively. The rent expense for the year ended December 31, 2014 included $963 of accelerated rent expense due to the Company vacating one of its leased properties before the lease expired.
	c)    Producers
	The three largest individual producers as a percentage of gross premiums written are as follows:
		Year Ended December 31,
		2014		2013		2012
	Marsh & McLennan Companies, Inc.	25	%	26	%	25	%
	Aon Corporation	17	%	18	%	19	%
	Willis Group Holdings	10	%	12	%	11	%
	d)    Legal Proceedings
	The Company, in common with the insurance industry in general, is subject to litigation and arbitration in the normal course of its business. These legal proceedings generally relate to claims asserted by or against the Company in the ordinary course of insurance or reinsurance operations. Estimated amounts payable under these proceedings are included in the reserve for losses and loss expenses in the Company’s consolidated balance sheets. As of December 31, 2014, the Company was not a party to any material legal proceedings arising outside the ordinary course of business that management believes will have a material adverse effect on the Company’s results of operations, financial position or cash flow.

Statutory_Capital_and_Surplus

Statutory Capital and Surplus	12 Months Ended
	Dec. 31, 2014
Disclosure Dividend Restrictions And Statutory Requirements [Abstract]
Statutory capital and surplus	STATUTORY CAPITAL AND SURPLUS
	Allied World Switzerland’s ability to pay dividends is subject to certain regulatory restrictions on the payment of dividends by its subsidiaries. The payment of such dividends is limited by applicable laws and statutory requirements of the jurisdictions in which Allied World Switzerland and its subsidiaries operate. The total amount of restricted net assets for the Company’s consolidated subsidiaries as of December 31, 2014 was $2,953,535.
	The minimum required statutory capital and surplus is the amount of statutory capital and surplus necessary to satisfy regulatory requirements based on the Company’s current operations. The statutory capital and surplus and minimum required statutory capital and surplus for the Company’s most significant regulatory jurisdictions at December 31, 2014 and 2013 were as follows:
		December 31, 2014						December 31, 2013
		Statutory			Minimum			Statutory		Minimum
		Capital and			Required			Capital and		Required
		Surplus			Statutory			Surplus		Statutory
					Capital and					Capital and
					Surplus					Surplus
	Bermuda	$	3,273,730		$	876,235		$	3,208,776	$	806,012
	United States	1,220,299			226,876			1,187,023		205,699
	Ireland	410,841			29,204			407,600		27,098
	Switzerland	172,306			19,943			167,840		22,905
	United Kingdom	231,659			203,742			229,745		229,689
	There were no state-prescribed or permitted regulatory accounting practices for any of our insurance entities that resulted in reported statutory surplus that differed from that which would have been reported under the prescribed practices of the respective regulatory authorities, including the National Association of Insurance Commissioners. Statutory accounting under the prescribed practices of the respective regulatory authorities differs from U.S. GAAP accounting in the treatment of various items, including reporting of investments, acquisition costs and deferred income taxes.
	The statutory net income (loss) for the Company’s most significant regulatory jurisdictions for the years ended December 31, 2014, 2013 and 2012 was as follows:
		Year Ended December 31,
		2014			2013			2012
	Bermuda	$	487,051		$	467,283		$	528,712
	United States	45,445			36,744			25,390
	Ireland	3,241			(4,880		)	9,325
	Switzerland	7,408			(6,418		)	10,657
	United Kingdom	(7,819		)	9,362			1,800
	At December 31, 2014, the maximum amount of ordinary dividends or distributions that can be paid, without prior regulatory approval, for the Company’s most significant regulatory jurisdictions, were as follows:
		December 31, 2014
	Bermuda	$	818,433
	United States	102,283
	Ireland	21,637
	Switzerland	6,323
	United Kingdom	1,601
	a)    Bermuda
	The Company’s Bermuda subsidiary, Allied World Assurance Company, Ltd, is registered under the Bermuda Insurance Act 1978 and Related Regulations as amended. As a Class 4 insurer, Allied World Assurance Company, Ltd is required to maintain minimum solvency standards and to hold available statutory capital and surplus equal to or exceeding the enhanced capital requirements as determined by the Bermuda Monetary Authority under the Bermuda Solvency Capital Requirement model (“BSCR model”). The BSCR model is a risk-based capital model that provides a method for determining an insurer’s minimum required capital taking into account the risk characteristics of different aspects of the company’s business. In addition, this subsidiary is required to maintain a minimum liquidity ratio. As of December 31, 2014 and 2013, this subsidiary met the requirements.
	b)    United States
	The Company’s U.S. insurance subsidiaries are subject to the insurance laws and regulations of the states in which they are domiciled, and also states in which they are licensed or authorized to transact business. These laws also restrict the amount of ordinary shareholder dividends the subsidiaries can pay. The restrictions are generally based on statutory surplus and/or statutory net income as determined in accordance with the relevant statutory accounting requirements of the individual domiciliary states. The U.S. subsidiaries are required to file annual statements with insurance regulatory authorities prepared on an accounting basis prescribed or permitted by such authorities. The U.S. subsidiaries are also required to maintain minimum levels of solvency and liquidity as determined by law, and comply with capital requirements and licensing rules. As of December 31, 2014 and 2013, the actual levels of solvency, liquidity and capital of each U.S. subsidiary were in excess of the minimum levels required.
	c)    Ireland
	The Company’s Irish insurance subsidiary is regulated by the Central Bank of Ireland pursuant to the Insurance Acts 1909 to 2000, the Central Bank Acts 1942 to 2014 and all statutory instruments relating to insurance made or adopted under the European Communities Acts 1972 to 2014. This subsidiary is required to maintain a minimum level of capital. As of December 31, 2014 and 2013, these requirements were met. The amount of dividends that this subsidiary is permitted to distribute is restricted to accumulated realized profits that have not been capitalized or distributed, less accumulated realized losses that have not been written off. The solvency and capital requirements must still be met following any distribution.
	d)    Switzerland
	The Company’s Swiss insurance subsidiary, Allied World Assurance Company, AG, is regulated by the Swiss Financial Market Supervisory Authority (“FINMA”) pursuant to the Insurance Supervisory Law 2004 to 2014. This subsidiary’s accounts are prepared in accordance with the Swiss Code of Obligation 1911 to 2013 and the Insurance Supervisory Law 2004 to 2014. This subsidiary is obligated to maintain a minimum level of capital based on the Swiss Code of Obligation, a minimum of tied assets based on the Insurance Supervisory Law and a minimum solvency margin based on the Solvency I and Swiss Solvency Test regulations as stipulated by the Insurance Supervisory Law. As of December 31, 2014 and 2013, this subsidiary met the requirements. The amount of dividends that this subsidiary is permitted to distribute is restricted to freely distributable reserves which consist of retained earnings, the current year profit and legal reserves to a certain extent. Any dividend requires approval of the shareholders and in case of the dividend exceeding the current year profit, approval is also required from FINMA. The solvency and capital requirements must still be met following any distribution.
	e)    United Kingdom
	Allied World Capital (Europe) Limited is the sole corporate member of Syndicate 2232. Syndicate 2232 is managed by Allied World Managing Agency Limited, which is authorized and regulated by the Prudential Regulatory Authority ("PRA") and the Financial Conduct Authority. As a member of Lloyd’s, Allied World Capital (Europe) Limited is obliged to comply with Lloyd’s byelaws and regulations (made pursuant to the Lloyd’s Acts 1871 to 1982) and applicable provisions of the Financial and Services and Markets Act 2000. The Council of Lloyd’s has wide discretionary powers to regulate members’ underwriting at Lloyd’s and its exercise of these powers might affect the return on an investment of the corporate member in a given underwriting year. The capital required to support a Syndicate’s underwriting capacity, referred to as “funds at Lloyd’s”, is assessed annually and is determined by Lloyd’s in accordance with the capital adequacy rules established by the PRA. If a member of Lloyd’s is unable to pay its debts to policyholders, such debts may be payable from the Lloyd’s Central Fund, which in many respects acts as an equivalent to a state guaranty fund in the United States. The Company has provided capital to support the underwriting of Syndicate 2232 in the form of pledged assets provided by Allied World Assurance Company, Ltd. The amount which the Company provides as funds at Lloyd’s is not available for distribution to the Company for the payment of dividends. Lloyd’s is supervised by the PRA and required to implement certain rules prescribed by the PRA under the Lloyd’s Act of 1982 regarding the operation of the Lloyd’s market. With respect to managing agents and corporate members, Lloyd’s prescribes certain minimum standards relating to management and control, solvency and other requirements and monitors managing agents’ compliance with such standards.
	f)    Branch Offices
	The Company’s insurance subsidiaries maintain branch offices in Australia, Bermuda, England, Switzerland, Hong Kong, Canada, Singapore and Labuan. As branch offices are not considered separate legal entities, the required and actual statutory capital and surplus amounts for each jurisdiction in the table above include amounts related to the branch offices. These branch offices are subject to additional minimum capital or asset requirements in their countries of domicile. At December 31, 2014 and 2013, the actual capital and surplus for each of these branches exceeded the relevant local regulatory requirements.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment information	SEGMENT INFORMATION
	The determination of reportable segments is based on how the Company's chief operating decision maker, the Chief Executive Officer, monitors the Company’s underwriting operations. Management monitors the performance of its direct underwriting operations based on the geographic location of the Company’s offices, the markets and customers served and the type of accounts written. The Company is currently organized into three operating segments: North American Insurance, Global Markets Insurance and Reinsurance. All product lines fall within these classifications.
	The North American Insurance segment includes the Company’s direct specialty insurance operations in the United States, Bermuda and Canada, as well as the Company's claim administration services operation. This segment provides both direct property and specialty casualty insurance to North American domiciled accounts. The Global Markets Insurance segment includes the Company’s direct insurance operations in Europe and Asia Pacific, which includes offices in Australia, Hong Kong and Singapore. This segment provides both direct property and casualty insurance primarily to non-North American domiciled accounts. The Reinsurance segment includes the Company’s reinsurance operations in the United States, Bermuda, Europe and Singapore. This segment provides reinsurance of property, general casualty, professional liability, specialty lines and property catastrophe coverages written by insurance companies. The Company presently writes reinsurance on both a treaty and a facultative basis, targeting several niche reinsurance markets.
	Responsibility and accountability for the results of underwriting operations are assigned by major line of business within each segment. Because the Company does not manage its assets by segment, investment income, interest expense and total assets are not allocated to individual reportable segments. General and administrative expenses are allocated to segments based on various factors, including staff count and each segment’s proportional share of gross premiums written.
	The Company measures its segment profit or loss as underwriting income or loss plus other insurance-related income and expenses, which may include the net earnings from our claims administration services operations and other income or expense that is not directly related to our underwriting operations. Management measures results of each segment's underwriting income or loss on the basis of the “loss and loss expense ratio,” “acquisition cost ratio,” “general and administrative expense ratio”, “expense ratio” and the “combined ratio.” The “loss and loss expense ratio” is derived by dividing net losses and loss expenses by net premiums earned. The “acquisition cost ratio” is derived by dividing acquisition costs by net premiums earned. The “general and administrative expense ratio” is derived by dividing general and administrative expenses by net premiums earned. The expense ratio is the sum of the “acquisition cost ratio” and the “general and administrative expense ratio”. The “combined ratio” is the sum of the “loss and loss expense ratio,” the “acquisition cost ratio” and the “general and administrative expense ratio.”
	The following tables provide a summary of the segment results:
	Year Ended December 31, 2014	North American  Insurance			Global Markets			Reinsurance			Total
					Insurance
	Gross premiums written	$	1,716,288		$	280,543		$	938,594		$	2,935,425
	Net premiums written	1,230,839			187,997			903,239			2,322,075
	Net premiums earned	1,111,164			162,566			909,028			2,182,758
	Net losses and loss expenses	(683,836		)	(61,057		)	(454,297		)	(1,199,190		)
	Acquisition costs	(105,914		)	(18,222		)	(170,986		)	(295,122		)
	General and administrative expenses	(219,665		)	(68,106		)	(77,965		)	(365,736		)
	Underwriting income	101,749			15,181			205,780			322,710
	Other insurance-related revenue	2,064			—			—			2,064
	Other insurance-related expenses	(1,854		)	(6,724		)	—			(8,578		)
	Segment income	101,959			8,457			205,780			316,196
	Net investment income										176,874
	Net realized investment gains										88,957
	Amortization of intangible assets										(2,533		)
	Interest expense										(57,755		)
	Foreign exchange loss										(957		)
	Income before income taxes										$	520,782
	Loss and loss expense ratio	61.5		%	37.6		%	50		%	54.9		%
	Acquisition cost ratio	9.5		%	11.2		%	18.8		%	13.5		%
	General and administrative expense ratio	19.8		%	41.9		%	8.6		%	16.8		%
	Expense ratio	29.3		%	53.1		%	27.4		%	30.3		%
	Combined ratio	90.8		%	90.7		%	77.4		%	85.2		%
	Year Ended December 31, 2013	North American  Insurance			Global Markets			Reinsurance			Total
					Insurance
	Gross premiums written	$	1,572,355		$	232,555		$	933,754		$	2,738,664
	Net premiums written	1,082,441			145,002			893,038			2,120,481
	Net premiums earned	1,022,979			126,008			856,846			2,005,833
	Net losses and loss expenses	(651,267		)	(50,395		)	(421,580		)	(1,123,242		)
	Acquisition costs	(94,869		)	(10,051		)	(147,753		)	(252,673		)
	General and administrative expenses	(209,042		)	(63,164		)	(80,079		)	(352,285		)
	Underwriting income	67,801			2,398			207,434			277,633
	Other insurance-related revenue	—			—			—			—
	Other insurance-related expenses	—			—			—			—
	Segment income	67,801			2,398			207,434			277,633
	Net investment income										157,564
	Net realized investment gains										59,525
	Amortization of intangible assets										(2,533		)
	Interest expense										(56,510		)
	Foreign exchange loss										(8,019		)
	Income before income taxes										$	427,660
	Loss and loss expense ratio	63.7		%	40		%	49.2		%	56		%
	Acquisition cost ratio	9.3		%	8		%	17.2		%	12.6		%
	General and administrative expense ratio	20.4		%	50.1		%	9.3		%	17.6		%
	Expense ratio	29.7		%	58.1		%	26.5		%	30.2		%
	Combined ratio	93.4		%	98.1		%	75.7		%	86.2		%
	Year Ended December 31, 2012	North American  Insurance			Global Markets			Reinsurance			Total
					Insurance
	Gross premiums written	$	1,376,040		$	192,981		$	760,250		$	2,329,271
	Net premiums written	964,199			125,577			748,047			1,837,823
	Net premiums earned	892,721			115,660			740,517			1,748,898
	Net losses and loss expenses	(608,762		)	(19,536		)	(510,966		)	(1,139,264		)
	Acquisition costs	(77,701		)	(8,483		)	(119,538		)	(205,722		)
	General and administrative expenses	(185,231		)	(48,365		)	(73,725		)	(307,321		)
	Underwriting income	21,027			39,276			36,288			96,591
	Other insurance-related revenue	—			—			—			—
	Other insurance-related expenses	—			—			—			—
	Segment income	21,027			39,276			36,288			96,591
Net investment income										167,141
Net realized investment gains										306,436
Amortization of intangible assets										(2,533		)
Interest expense										(55,405		)
Foreign exchange loss										(783		)
Income before income taxes										$	511,447
Loss and loss expense ratio	68.2		%	16.9		%	69		%	65.1		%
Acquisition cost ratio	8.7		%	7.3		%	16.1		%	11.8		%
General and administrative expense ratio	20.7		%	41.8		%	10		%	17.6		%
Expense ratio	29.4		%	49.1		%	26.1		%	29.4		%
Combined ratio	97.6		%	66		%	95.1		%	94.5		%
The following table shows an analysis of the Company’s gross premiums written by geographic location of the Company’s subsidiaries. All intercompany premiums have been eliminated.
	Year Ended December 31,
	2014			2013			2012
United States	$	1,795,564		$	1,636,010		$	1,360,190
Bermuda	640,948			676,174			611,382
Europe	318,626			264,867			228,791
Asia Pacific	167,334			161,613			128,908
Canada	12,953			—			—
Total gross premiums written	$	2,935,425		$	2,738,664		$	2,329,271
Gross premiums written attributable to Switzerland were $67,564, $61,042 and $43,664 for the years ended December 31, 2014, 2013 and 2012, respectively.
The following table shows the Company’s net premiums earned by line of business for each segment for the years ended December 31, 2014, 2013 and 2012.
	Year Ended December 31,
	2014			2013			2012
North American Insurance:
General casualty	$	334,603		$	269,114		$	211,238
Professional liability	276,105			259,215			245,583
Healthcare	168,476			194,344			187,053
General property	126,646			141,896			137,002
Programs	119,756			107,870			88,180
Inland marine	43,666			25,939			10,612
Environmental	19,507			17,017			12,627
Other	22,405			7,584			426
Total	1,111,164			1,022,979			892,721
Global Markets Insurance:
Professional liability	57,176			45,967			43,621
General property	33,957			31,364			36,751
Aviation	22,342			9,521			—
Trade credit	17,623			14,099			9,471
General casualty	15,266			13,903			16,762
Healthcare	11,963			11,154			9,055
Other	4,239			—			—
Total	162,566			126,008			115,660
Reinsurance:
Property	411,968			336,009			273,167
Specialty	321,538			349,044			283,415
Casualty	175,522			171,793			183,935
Total	909,028			856,846			740,517
Total net premiums earned	$	2,182,758		$	2,005,833		$	1,748,898

Condensed_Consolidated_Guarant

Condensed Consolidated Guarantor Financial Statements	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed consolidated guarantor financial statements	CONDENSED CONSOLIDATED PARENT COMPANY AND GUARANTOR FINANCIAL STATEMENTS
	The following tables present condensed consolidating financial information as of December 31, 2014 and 2013 and for the years ended December 31, 2014, 2013 and 2012 for Allied World Switzerland (the “Parent Guarantor”) and Allied World Bermuda (the “Subsidiary Issuer”). The Subsidiary Issuer is a direct 100%-owned subsidiary of the Parent Guarantor. Investments in subsidiaries are accounted for by the Parent Guarantor under the equity method for purposes of the supplemental consolidating presentation. Earnings of subsidiaries are reflected in the Parent Guarantor’s investment accounts and earnings. The Parent Guarantor fully and unconditionally guarantees the senior notes issued by the Subsidiary Issuer.
	Condensed Consolidating Balance Sheet:
	As of December 31, 2014	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	ASSETS:
	Investments	$	—		$	—		$	7,868,682		$	—		$	7,868,682
	Cash and cash equivalents	32,579			1,734			555,026			—			589,339
	Insurance balances receivable	—			—			664,815			—			664,815
	Funds held	—			—			724,021			—			724,021
	Reinsurance recoverable	—			—			1,340,256			—			1,340,256
	Reinsurance recoverable on paid losses	—			—			86,075						86,075
	Net deferred acquisition costs	—			—			151,546			—			151,546
	Goodwill and intangible assets	—			—			324,556			—			324,556
	Balances receivable on sale of investments	—			—			47,149			—			47,149
	Investments in subsidiaries	3,629,301			4,218,028			—			(7,847,329		)	—
	Due from subsidiaries	147,072			19,190			14,396			(180,658		)	—
	Other assets	1,470			3,192			620,462			—			625,124
	Total assets	$	3,810,422		$	4,242,144		$	12,396,984		$	(8,027,987	)	$	12,421,563
	LIABILITIES:
	Reserve for losses and loss expenses	$	—		$	—		$	5,881,165		$	—		$	5,881,165
	Unearned premiums	—			—			1,555,313			—			1,555,313
	Reinsurance balances payable	—			—			180,060			—			180,060
	Balances due on purchases of investments	—						5,428			—			5,428
	Senior notes	—			798,802			—			—			798,802
	Other long-term debt	—			—			19,213			—			19,213
	Due to subsidiaries	7,599			6,797			166,262			(180,658		)	—
	Other liabilities	24,532			19,618			159,141			—			203,291
	Total liabilities	32,131			825,217			7,966,582			(180,658		)	8,643,272
	Total shareholders’ equity	3,778,291			3,416,927			4,430,402			(7,847,329		)	3,778,291
	Total liabilities and shareholders’ equity	$	3,810,422		$	4,242,144		$	12,396,984		$	(8,027,987	)	$	12,421,563
	As of December 31, 2013	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	ASSETS:
	Investments	$	—		$	—		$	7,712,036		$	—		$	7,712,036
	Cash and cash equivalents	10,790			2,775			518,371			—			531,936
	Insurance balances receivable	—			—			588,632			—			588,632
	Funds held	—			—			632,430			—			632,430
	Reinsurance recoverable	—			—			1,234,504			—			1,234,504
	Reinsurance recoverable on paid losses	—			—			76,099			—			76,099
	Net deferred acquisition costs	—			—			126,661			—			126,661
	Goodwill and intangible assets	—			—			317,207			—			317,207
	Balances receivable on sale of investments	—			—			76,544			—			76,544
	Investments in subsidiaries	3,413,001			4,018,619			—			(7,431,620		)	—
	Due from subsidiaries	111,172			122,846			123,479			(357,497		)	—
	Other assets	1,757			4,671			643,353			—			649,781
	Total assets	$	3,536,720		$	4,148,911		$	12,049,316		$	(7,789,117	)	$	11,945,830
	LIABILITIES:
	Reserve for losses and loss expenses	$	—		$	—		$	5,766,529		$	—		$	5,766,529
	Unearned premiums	—			—			1,396,256			—			1,396,256
	Reinsurance balances payable	—			—			173,023			—			173,023
	Balances due on purchases of investments	—			—			104,740			—			104,740
	Senior notes	—			798,499			—			—			798,499
	Due to subsidiaries	12,945			110,534			234,018			(357,497		)	—
	Other liabilities	3,949			17,797			165,211			—			186,957
	Total liabilities	16,894			926,830			7,839,777			(357,497		)	8,426,004
	Total shareholders’ equity	3,519,826			3,222,081			4,209,539			(7,431,620		)	3,519,826
	Total liabilities and shareholders’ equity	$	3,536,720		$	4,148,911		$	12,049,316		$	(7,789,117	)	$	11,945,830
	Condensed Consolidating Income Statement:
	Year Ended December 31, 2014	Allied World			Allied World			Other Allied			Consolidating			Allied World
Switzerland			Bermuda			World			Adjustments			Switzerland
(Parent			(Subsidiary			Subsidiaries						Consolidated
Guarantor)			Issuer)
Net premiums earned	$	—		$	—		$	2,182,758		$	—		$	2,182,758
Net investment income	50			—			176,824			—			176,874
Net realized investment gains	—			—			88,957			—			88,957
Other income	—			—			2,064			—			2,064
Net losses and loss expenses	—			—			(1,199,190		)	—			(1,199,190		)
Acquisition costs	—			—			(295,122		)	—			(295,122		)
General and administrative expenses	(35,596		)	(3,088		)	(327,052		)	—			(365,736		)
Other expense	—			—			(8,578		)	—			(8,578		)
Amortization of intangible assets	—			—			(2,533		)	—			(2,533		)
Interest expense	—			(55,418		)	(2,337		)	—			(57,755		)
Foreign exchange gain (loss)	20			104			(1,081		)	—			(957		)
Income tax (expense) benefit	(89		)	—			(30,434		)	—			(30,523		)
Equity in earnings of consolidated subsidiaries	525,874			564,648			—			(1,090,522		)	—
NET INCOME (LOSS)	$	490,259		$	506,246		$	584,276		$	(1,090,522	)	$	490,259
Other comprehensive income	—			—			—			—			—
COMPREHENSIVE INCOME (LOSS)	$	490,259		$	506,246		$	584,276		$	(1,090,522	)	$	490,259
Year Ended December 31, 2013	Allied World			Allied World			Other Allied			Consolidating			Allied World
	Switzerland			Bermuda			World			Adjustments			Switzerland
	(Parent			(Subsidiary			Subsidiaries						Consolidated
	Guarantor)			Issuer)
Net premiums earned	$	—		$	—		$	2,005,833		$	—		$	2,005,833
Net investment income	13			4			157,547			—			157,564
Net realized investment gains	—			—			59,525			—			59,525
Other income	—			—			—			—			—
Net losses and loss expenses	—			—			(1,123,242		)	—			(1,123,242		)
Acquisition costs	—			—			(252,673		)	—			(252,673		)
General and administrative expenses	(42,122		)	(1,320		)	(308,843		)	—			(352,285		)
Other expense	—			—			—			—			—
Amortization of intangible assets	—			—			(2,533		)	—			(2,533		)
Interest expense	—			(55,347		)	(1,163		)	—			(56,510		)
Foreign exchange gain (loss)	263			(1,045		)	(7,237		)	—			(8,019		)
Income tax (expense) benefit	98			—			(9,878		)	—			(9,780		)
Equity in earnings of consolidated subsidiaries	459,628			515,156			—			(974,784		)	—
NET INCOME (LOSS)	$	417,880		$	457,448		$	517,336		$	(974,784	)	$	417,880
Other comprehensive income	—			—			—			—			—
COMPREHENSIVE INCOME (LOSS)	$	417,880		$	457,448		$	517,336		$	(974,784	)	$	417,880
Year Ended December 31, 2012	Allied World			Allied World			Other Allied			Consolidating			Allied World
	Switzerland			Bermuda			World			Adjustments			Switzerland
	(Parent			(Subsidiary			Subsidiaries						Consolidated
	Guarantor)			Issuer)
Net premiums earned	$	—		$	—		$	1,748,898		$	—		$	1,748,898
Net investment income	26			20			167,095			—			167,141
Net realized investment gains (loss)	—			—			306,436			—			306,436
Other income	—			—			—			—			—
Net losses and loss expenses	—			—			(1,139,264		)	—			(1,139,264		)
Acquisition costs	—			—			(205,722		)	—			(205,722		)
General and administrative expenses	(35,258		)	(2,973		)	(269,090		)	—			(307,321		)
Other expense	—			—			—			—			—
Amortization of intangible assets	—			—			(2,533		)	—			(2,533		)
Interest expense	—			(55,405		)	—			—			(55,405		)
Foreign exchange gain (loss)	159			(319		)	(623		)	—			(783		)
Income tax (expense) benefit	358			—			(18,798		)	—			(18,440		)
Equity in earnings of consolidated subsidiaries	527,722			573,256			—			(1,100,978		)	—
NET INCOME (LOSS)	$	493,007		$	514,579		$	586,399		$	(1,100,978	)	$	493,007
Unrealized gains on investments arising during the year net of applicable deferred income tax benefit of $265	(493		)	—			(493		)	493			(493		)
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax	(13,991		)	—			(13,991		)	13,991			(13,991		)
Other comprehensive loss	(14,484		)	—			(14,484		)	14,484			(14,484		)
COMPREHENSIVE INCOME (LOSS)	$	478,523		$	514,579		$	571,915		$	(1,086,494	)	$	478,523
Condensed Consolidating Statement of Cash Flows:
Year Ended December 31, 2014	Allied World			Allied World			Other Allied			Consolidating			Allied World
	Switzerland			Bermuda			World			Adjustments			Switzerland
	(Parent			(Subsidiary			Subsidiaries						Consolidated
	Guarantor)			Issuer)
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES	$	264,422		$	292,959		$	493,659		$	(643,250	)	$	407,790
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases trading securities	—			—			(7,630,034		)	—			(7,630,034		)
Purchases of other invested assets	—			—			(307,955		)	—			(307,955		)
Sales of trading securities	—			—			7,536,936			—			7,536,936
Sales of other invested assets	—			—			267,885			—			267,885
Other	—			—			6,201			—			6,201
Net cash provided by (used in) investing activities	—			—			(126,967		)	—			(126,967		)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Dividends paid	(76,750		)	—			—			—			(76,750		)
Intercompany dividends paid	—			(294,000		)	(349,250		)	643,250			—
Proceeds from the exercise of stock options	10,027			—			—			—			10,027
Proceeds from other long-term debt	—			—			19,213			—			19,213
Share repurchases	(175,910		)	—			—			—			(175,910		)
Net cash provided by (used in) financing activities	(242,633		)	(294,000		)	(330,037		)	643,250			(223,420		)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	21,789			(1,041		)	36,655			—			57,403
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	10,790			2,775			518,371			—			531,936
CASH AND CASH EQUIVALENTS, END OF YEAR	$	32,579		$	1,734		$	555,026		$	—		$	589,339
Year Ended December 31, 2013	Allied World			Allied World			Other Allied			Consolidating			Allied World
	Switzerland			Bermuda			World			Adjustments			Switzerland
	(Parent			(Subsidiary			Subsidiaries						Consolidated
	Guarantor)			Issuer)
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES	$	198,702		$	265,951		$	237,956		$	(596,500	)	$	106,109
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	—			—			(7,527,670		)	—			(7,527,670		)
Purchases of other invested assets	—			—			(276,926		)	—			(276,926		)
Sales of trading securities	—			—			7,540,194			—			7,540,194
Sales of other invested assets	—			—			187,477			—			187,477
Other	—			—			28,782			—			28,782
Net cash provided by (used in) investing activities	—			—			(48,143		)	—			(48,143		)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Partial par value reduction	(12,981		)	—			—			—			(12,981		)
Dividends paid	(34,059		)	—			—			—			(34,059		)
Intercompany dividends paid	—			(274,500		)	(322,000		)	596,500			—
Proceeds from the exercise of stock options	12,137			—			—			—			12,137
Share repurchases	(173,006		)	—			—			—			(173,006		)
Net cash provided by (used in) financing activities	(207,909		)	(274,500		)	(322,000		)	596,500			(207,909		)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(9,207		)	(8,549		)	(132,187		)	—			(149,943		)
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	19,997			11,324			650,558			—			681,879
CASH AND CASH EQUIVALENTS, END OF YEAR	$	10,790		$	2,775		$	518,371		$	—		$	531,936
Year Ended December 31, 2012	Allied World			Allied World			Other Allied			Consolidating			Allied World
	Switzerland			Bermuda			World			Adjustments			Switzerland
	(Parent			(Subsidiary			Subsidiaries						Consolidated
	Guarantor)			Issuer)
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$	338,565		$	357,438		$	702,853		$	(770,000	)	$	628,856
CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
Purchases of trading securities	—			—			(8,006,237		)	—			(8,006,237		)
Purchases of other invested assets	—			—			(333,031		)	—			(333,031		)
Sales of available for sale securities	—			—			236,978			—			236,978
Sales of trading securities	—			—			7,892,907			—			7,892,907
Sales of other invested assets	—			—			53,741			—			53,741
Capital contribution to subsidiary	(110,000		)	—			—			110,000			—
Other	—			—			(104,091		)	—			(104,091		)
Net cash provided by (used in) investing activities	(110,000		)	—			(259,733		)	110,000			(259,733		)
CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
Partial par value reduction	(67,825		)	—			—			—			(67,825		)
Intercompany dividends paid	—			(355,000		)	(415,000		)	770,000			—
Proceeds from the exercise of stock options	10,527			—			—			—			10,527
Share repurchases	(263,942		)	—			—			—			(263,942		)
Capital contribution from parent	—			—			110,000			(110,000		)	—
Net cash provided by (used in) financing activities	(321,240		)	(355,000		)	(305,000		)	660,000			(321,240		)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(92,675		)	2,438			138,120			—			47,883
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	112,672			8,886			512,438			—			633,996
CASH AND CASH EQUIVALENTS, END OF YEAR	$	19,997		$	11,324		$	650,558		$	—		$	681,879
Notes to Parent Company Condensed Financial Information
a) Dividends
Allied World Switzerland received cash dividends from its subsidiaries of $294,000, $274,500 and $355,000 for the years ended December 31, 2014, 2013 and 2012, respectively. Such dividends are included in “cash flows provided by (used in) operating activities” in the condensed consolidating statement of cash flows.

Unaudited_Quarterly_Financial_

Unaudited Quarterly Financial Data	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Unaudited quarterly financial data	UNAUDITED QUARTERLY FINANCIAL DATA
	The following are the unaudited consolidated statements of income by quarter for the years ended December 31, 2014 and 2013:
		Quarter Ended
		December 31,			September 30,			June 30,			March 31,
		2014			2014			2014			2014
	REVENUES:
	Gross premiums written	$	565,743		$	707,884		$	760,405		$	901,393
	Premiums ceded	(137,948		)	(139,142		)	(206,481		)	(129,779		)
	Net premiums written	427,795			568,742			553,924			771,614
	Change in unearned premiums	145,694			(27,005		)	(16,677		)	(241,329		)
	Net premiums earned	573,489			541,737			537,247			530,285
	Net investment income	49,050			43,412			36,793			47,619
	Net realized investment (losses) gains	(15,329		)	(35,136		)	85,217			54,205
	Other income	1,032			1,032			—			—
		608,242			551,045			659,257			632,109
	EXPENSES:
	Net losses and loss expenses	272,959			336,090			314,855			275,286
	Acquisition costs	80,718			72,403			74,279			67,722
	General and administrative expenses	100,914			88,294			96,188			80,340
	Other expense	2,003			6,575			—			—
	Amortization of intangible assets	633			633			634			633
	Interest expense	14,304			14,325			14,592			14,534
	Foreign exchange (gain) loss	(21		)	278			651			49
		471,510			518,598			501,199			438,564
	Income before income taxes	136,732			32,447			158,058			193,545
	Income tax expense	6,223			1,532			6,195			16,573
	NET INCOME	$	130,509		$	30,915		$	151,863		$	176,972
	PER SHARE DATA
	Basic earnings per share	$	1.35		$	0.32		$	1.55		$	1.78
	Diluted earnings per share	$	1.33		$	0.31		$	1.52		$	1.74
	Weighted average common shares outstanding	96,386,796			96,458,231			97,809,639			99,545,187
	Weighted average common shares and common share equivalents outstanding	98,394,432			98,444,238			99,724,802			101,584,662
		Quarter Ended
		December 31,			September 30,			June 30,			March 31,
		2013			2013			2013			2013
	REVENUES:
	Gross premiums written	$	555,490		$	580,893		$	765,200		$	837,081
	Premiums ceded	(164,360		)	(127,816		)	(183,978		)	(142,029		)
	Net premiums written	391,130			453,077			581,222			695,052
	Change in unearned premiums	133,431			57,696			(73,951		)	(231,824		)
	Net premiums earned	524,561			510,773			507,271			463,228
	Net investment income	47,270			39,271			37,635			33,388
	Net realized investment gains (losses)	67,599			27,487			(115,198		)	79,637
		639,430			577,531			429,708			576,253
	EXPENSES:
	Net losses and loss expenses	315,966			276,970			275,128			255,178
	Acquisition costs	66,257			65,114			64,617			56,685
	General and administrative expenses	100,467			88,553			80,585			82,680
	Amortization of intangible assets	633			633			634			633
	Interest expense	14,094			14,094			14,188			14,134
	Foreign exchange loss	658			4,353			490			2,518
		498,075			449,717			435,642			411,828
	Income (loss) before income taxes	141,355			127,814			(5,934		)	164,425
	Income tax expense (benefit)	3,448			4,971			(4,072		)	5,433
	NET INCOME (LOSS)	$	137,907		$	122,843		$	(1,862	)	$	158,992
	PER SHARE DATA
	Basic earnings (loss) per share	1.37			1.2			(0.02		)	1.53
	Diluted earnings (loss) per share	1.34			1.18			(0.02		)	1.5
	Weighted average common shares outstanding	100,814,955			101,974,077			103,267,659			103,840,818
	Weighted average common shares and common share equivalents outstanding	103,129,812			104,184,579			103,267,659			106,295,529

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent event	SUBSEQUENT EVENTS
	a) Dividend
	On January 2, 2015, the Company paid a quarterly dividend of $0.225 per share to shareholders of record on December 24, 2014.

Schedule_III_Supplementary_Ins

Schedule III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2014
Supplementary Insurance Information [Abstract]
Schedule III - Supplementary insurance information	chedule III
	ALLIED WORLD ASSURANCE COMPANY HOLDINGS, AG
	SUPPLEMENTARY INSURANCE INFORMATION
	(Expressed in thousands of United States dollars
	Year Ended December 31, 2014
		Net		Reserve for		Unearned		Net		Net		Losses and		Amortization  of		Other		Net
		Deferred		Losses		Premiums		Premiums		Investment		Loss		Deferred		Operating		Premiums
		Acquisition		and Loss				Earned		Income		Expenses		Acquisition		Costs		Written
		Costs		Expenses										Costs
	North American Insurance	$	55,756	$	3,806,621	$	1,003,151	$	1,111,164	$	—	$	683,836	$	105,914	$	219,665	$	1,230,839
	Global Markets Insurance	23,336		568,126		223,402		162,566		—		61,057		18,222		68,106		187,997
	Reinsurance	72,454		1,506,418		328,760		909,028		—		454,297		170,986		77,965		903,239
	Corporate	—		—		—		—		176,874		—		—		—		—
	Total	$	151,546	$	5,881,165	$	1,555,313	$	2,182,758	$	176,874	$	1,199,190	$	295,122	$	365,736	$	2,322,075
	Year Ended December 31, 2013
		Net Deferred		Reserve for		Unearned		Net		Net		Losses and		Amortization of		Other		Net
		Acquisition		Losses		Premiums		Premiums		Investment		Loss		Deferred		Operating		Premiums
		Costs		and Loss				Earned		Income		Expenses		Acquisition Costs		Costs		Written
				Expenses
	North American Insurance	$	45,446	$	3,701,744	$	872,628	$	1,022,979	$	—	$	651,267	$	94,869	$	209,042	$	1,082,441
	Global Markets Insurance	9,828		568,675		164,792		126,008		—		50,395		10,051		63,164		145,002
	Reinsurance	71,387		1,496,110		358,836		856,846		—		421,580		147,753		80,079		893,038
	Corporate	—		—		—		—		157,564		—		—		—		—
	Total	$	126,661	$	5,766,529	$	1,396,256	$	2,005,833	$	157,564	$	1,123,242	$	252,673	$	352,285	$	2,120,481
	Year Ended December 31, 2012
		Net Deferred		Reserve for		Unearned		Net		Net		Losses and		Amortization of		Other		Net
		Acquisition		Losses		Premiums		Premiums		Investment		Loss		Deferred		Operating		Premiums
		Costs		and Loss				Earned		Income		Expenses		Acquisition Costs		Costs		Written
				Expenses
	North American Insurance	$	38,413	$	3,597,226	$	763,743	$	892,721	$	—	$	608,762	$	77,701	$	185,231	$	964,199
	Global Markets Insurance	8,326		567,675		131,130		115,660		—		19,536		8,483		48,365		125,577
	Reinsurance	61,271		1,480,648		323,148		740,517		—		510,966		119,538		73,725		748,047
	Corporate	—		—		—		—		167,141		—		—		—		—
	Total	$	108,010	$	5,645,549	$	1,218,021	$	1,748,898	$	167,141	$	1,139,264	$	205,722	$	307,321	$	1,837,823

Schedule_IV_Supplementary_Rein

Schedule IV - Supplementary Reinsurance Information	12 Months Ended
	Dec. 31, 2014
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Schedule IV - Supplementary reinsurance information	Schedule IV
	ALLIED WORLD ASSURANCE COMPANY HOLDINGS, AG
	SUPPLEMENTARY REINSURANCE INFORMATION
	(Expressed in thousands of United States dollars)
	Property and Casualty:	(a)		(b)		(c)		(d)		Percentage of
		Gross		Ceded to		Assumed from		Net		Amount Assumed
				Other		Other		Amount		to Net
				Companies		Companies		(a) - (b) +		(c) / (d)
								(c)
	Year Ended December 31, 2014	$	1,996,831	$	613,350	$	938,594	$	2,322,075	40%
	Year Ended December 31, 2013	$	1,804,910	$	618,183	$	933,754	$	2,120,481	44%
	Year Ended December 31, 2012	$	1,569,021	$	491,448	$	760,250	$	1,837,823	41%

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of accounting	These consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”).
	Intercompany accounts and transactions have been eliminated on consolidation and entities meeting consolidation requirements have been included in the consolidated financial statements. To facilitate comparison of information across periods, certain reclassifications have been made to prior year amounts to conform to the current year's presentation.
Use of estimates	The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. The significant estimates reflected in the Company’s financial statements include, but are not limited to:
	•	The premium estimates for certain reinsurance agreements,
	•	Recoverability of deferred acquisition costs,
	•	The reserve for outstanding losses and loss expenses,
	•	Valuation of ceded reinsurance recoverables,
	•	Determination of impairment of goodwill and other intangible assets, and
	•	Valuation of financial instruments.
Premiums	Premiums are recorded as written on the inception date of the policy. For certain types of business written by the Company, notably assumed reinsurance, premium income may not be known at the policy inception date. In the case of quota share reinsurance treaties assumed by the Company, the underwriter makes an estimate of premium income at inception. The underwriter’s estimate is based on statistical data provided by reinsureds and the underwriter’s judgment and experience. Such estimations are refined over the reporting period of each treaty as actual written premium information is reported by ceding companies and intermediaries. Premiums resulting from changes in the estimate of the premium income are recorded in the period the estimate is changed. Certain insurance and reinsurance contracts may require that the premium be adjusted at the expiry of the contract to reflect the change in exposure or loss experience of the insured or reinsured.
	Premiums are recognized as earned over the period of policy coverage in proportion to the risks to which they relate. Reinsurance premiums under a losses-occurring reinsurance contract are earned over the coverage period. Reinsurance premiums under a risks-attaching reinsurance contract are earned over the same period as the underlying policies, or risks, covered by the contract. As a result, the earning pattern of a risks-attaching reinsurance contract may extend up to 24 months, reflecting the inception dates of the underlying policies. Premiums relating to the unexpired periods of coverage are recorded on the consolidated balance sheets as “unearned premiums”.
Acquisition costs	Acquisition costs, comprised of commissions, brokerage fees and insurance taxes, are costs that are directly related to the successful acquisition of new and renewal business. While permitted under U.S. GAAP to defer certain internal costs that are directly related to the successful acquisition of new and renewal business, the Company does not defer such costs. Acquisition costs that are deferred are expensed as the premiums to which they relate are earned. Acquisition costs relating to the reserve for unearned premiums are deferred and carried on the balance sheets as an asset and are amortized over the period of coverage. Expected losses and loss expenses, other costs and anticipated investment income related to these unearned premiums are considered in determining the recoverability or deficiency of deferred acquisition costs. If it is determined that deferred acquisition costs are not recoverable, they are expensed. Further analysis is performed to determine if a liability is required to provide for losses which may exceed the related unearned premiums.
Reserve for losses and loss expenses	The reserve for losses and loss expenses is comprised of two main elements: outstanding loss reserves (“OSLR,” also known as case reserves) and reserves for losses incurred but not reported (“IBNR”). OSLR relate to known claims and represent management’s best estimate of the likely loss payment. IBNR relates to reserves established by the Company for claims that have occurred but have not yet been reported to us as well as for changes in the values of claims that have been reported to us but are not yet settled.
	The reserve for IBNR is estimated by management for each line of business based on various factors including underwriters’ expectations about loss experience, actuarial analysis, comparisons with the results of industry benchmarks and loss experience to date. The Company’s actuaries employ generally accepted actuarial methodologies to determine estimated ultimate loss reserves. The adequacy of the reserves is re-evaluated quarterly by the Company’s actuaries. At the completion of each quarterly review of the reserves, a reserve analysis is prepared and reviewed with the Company’s loss reserve committee. This committee determines management’s best estimate for loss and loss expense reserves based upon the reserve analysis.
	While management believes that the reserves for OSLR and IBNR are sufficient to cover losses assumed by the Company, there can be no assurance that losses will not deviate from the Company’s reserves, possibly by material amounts. The methodology of estimating loss reserves is periodically reviewed to ensure that the assumptions made continue to be appropriate. The Company recognizes any changes in its loss reserve estimates and the related reinsurance recoverables in the consolidated statements of operations and comprehensive income ("consolidated income statements") in the periods in which they are determined. These are recorded in “net losses and loss expenses”.
Ceded reinsurance	In the ordinary course of business, the Company uses both treaty and facultative reinsurance to minimize its net loss exposure to any one catastrophic loss event or to an accumulation of losses from a number of smaller events. Reinsurance premiums ceded are expensed and any commissions recorded thereon are earned over the period the reinsurance coverage is provided in proportion to the risks to which they relate. For reinsurance treaties that have contractual minimum premium provisions, premiums ceded are recorded at the inception of the treaty based on the minimum premiums. Any unearned ceding commission is included in “net deferred acquisitions costs” on the consolidated balance sheets and is recorded as a reduction to the overall net deferred acquisition cost balance. Prepaid reinsurance represents unearned premiums ceded to reinsurance companies. Reinsurance recoverable includes the balances due from those reinsurance companies under the terms of the Company’s reinsurance agreements for unpaid losses and loss reserves, including IBNR. Amounts recoverable from reinsurers are estimated in a manner consistent with the estimated claim liability associated with the reinsured policy.
	The Company determines the portion of the IBNR liability that will be recoverable under its reinsurance contracts by reference to the terms of the reinsurance protection purchased. This determination is necessarily based on the estimate of IBNR and accordingly, is subject to the same uncertainties as the estimate of IBNR.
	The Company remains liable to the extent that its reinsurers do not meet their obligations under the reinsurance contracts; therefore, the Company regularly evaluates the financial condition of its reinsurers and monitors concentration of credit risk.
Investments	All of our fixed maturity investments and equity securities are classified as trading securities as the Company has elected the fair value option permitted under U.S. GAAP for these investments. Trading securities are carried at fair value with any change in unrealized gains or losses recognized in the consolidated income statements and included in “net realized investment gains”. As a result of our investment classification, we do not record any change in unrealized gains or losses in our investments as a separate component of accumulated other comprehensive income on the consolidated balance sheets.
	Other invested assets consist primarily of investments in hedge funds and private equity funds, which have been accounted for as trading securities as the Company has elected the fair value option as permitted under U.S. GAAP. In addition, included in the Company’s other invested assets are various investments which are accounted for using the equity method of accounting. Generally, the Company uses the equity method where it does not have a controlling interest and is not the primary beneficiary. Equity method investments are recorded at cost and adjusted for the Company’s proportionate share of earnings or losses on a quarterly lag basis. An other-than-temporary impairment charge related to the equity method investments is assessed when facts and circumstances exists that indicate an impairment may exist. An other-than-temporary impairment charge is recorded when it is determined that the carrying value of the equity method investment is below its fair value and the Company does not have the intent and ability to hold to recovery. No equity method investment was deemed significant to disclose summarized financial data. Other investments are recorded based on valuation techniques depending on the nature of the individual assets.
	Investment securities are recorded on a trade date basis. Investment income is recognized when earned and includes the accrual of discount or amortization of premium on fixed maturity investments using the effective yield method and is net of related expenses. Interest income for fixed maturity investments is accrued and recognized based on the contractual terms of the fixed maturity investments and is included in “net investment income” in the consolidated income statements. The Company’s share of distributed and undistributed net income from equity method investments is included in “net investment income”. The return on investments is managed on a total financial statement portfolio return basis, which includes the distributed and undistributed net income from equity method investments, and as such have classified these amounts in net investment income.
	Realized gains and losses on the disposition of investments, which are based upon the first-in first-out method of identification, are included in “net realized investment gains” in the consolidated income statements. For mortgage-backed and asset-backed securities and any other holdings for which there is a prepayment risk, prepayment assumptions are evaluated and revised on a regular basis. Revised prepayment assumptions are applied to securities on a retrospective basis to the date of acquisition. The cumulative adjustments to amortized cost required due to these changes in effective yields and maturities are recognized in net investment income in the same period as the revision of the assumptions.
Fair value measurements	At each measurement date, the Company estimates the fair value of the financial instruments using various valuation techniques. The Company utilizes, to the extent available, quoted market prices in active markets or observable market inputs in estimating the fair value of financial instruments. When quoted market prices or observable market inputs are not available, the Company may utilize valuation techniques that rely on unobservable inputs to estimate the fair value of financial instruments. The Company bases its determination of whether a market is active or inactive on the spread between what a seller is asking for a security and what a buyer is bidding for that security. Spreads that are significantly above historical spreads are considered inactive markets. The Company also considers the volume of trading activity in the determination of whether a market is active or inactive. See note 5 for additional information regarding the fair value of financial instruments.
	The Company utilizes independent pricing sources to obtain market quotations for securities that have quoted prices in active markets. In general, the independent pricing sources use observable market inputs including, but not limited to, investment yields, credit risks and spreads, benchmarking of like securities, reported trades and sector groupings to determine the fair value. For a majority of the portfolio, the Company obtained two or more prices per security as of December 31, 2014. When multiple prices are obtained, a price source hierarchy is utilized to determine which price source is the best estimate of the fair value of the security. The price source hierarchy emphasizes more weighting to significant observable inputs such as index pricing and less weighting towards non-binding broker-dealer quotes. In addition, to validate all prices obtained from these pricing sources including non-binding broker-dealer quotes, the Company also obtains prices from its investment portfolio managers and other sources (e.g., another pricing vendor), and compares the prices obtained from the independent pricing sources to those obtained from the Company’s investment portfolio managers and other sources. The Company investigates any material differences between the multiple sources and determines which price best reflects the fair value of the individual security. There were no material differences between the prices obtained from the independent pricing sources and the prices obtained from the Company’s investment portfolio managers and other sources as of December 31, 2014 and 2013.
Variable interest entities	The Company is involved in the normal course of business with variable interest entities (“VIEs”) as a passive investor in certain asset-backed securities issued by third-party VIEs and affiliated VIEs. The Company performs a qualitative assessment at the date when it becomes initially involved in the VIE, followed by ongoing reassessments related to its involvement in VIEs. The Company’s maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company’s consolidated balance sheets and any unfunded commitments.
Translation of foreign currencies	Monetary assets and liabilities denominated in foreign currencies are translated into U.S. dollars at the exchange rates in effect on the balance sheet date. Foreign currency revenues and expenses are translated at the average exchange rates prevailing during the period. Exchange gains and losses, including those arising from forward exchange contracts, are included in the consolidated income statements. The Company’s functional currency, and that of each its operating subsidiaries, is the U.S. dollar as it is the single largest currency in which the Company and its each of its subsidiaries transact business and hold invested assets.
Cash and cash equivalents	Cash and cash equivalents include amounts held in banks, time deposits, commercial paper, discount notes and U.S. Treasury Bills with maturities of less than three months from the date of purchase.
Income taxes	Allied World Switzerland and certain of its subsidiaries operate in jurisdictions where they are subject to income taxation. Current and deferred income taxes are charged or credited to operations, or to accumulated other comprehensive income in certain cases, based upon enacted tax laws and rates applicable in the relevant jurisdiction in the period in which the tax becomes payable. Deferred income taxes are provided for all temporary differences between the bases of assets and liabilities used in the financial statements and those used in the various jurisdictional tax returns.
	It is the Company’s policy to recognize interest accrued related to unrecognized tax benefits in “interest expense” and penalties in “general and administrative expenses” in the consolidated income statements. The Company has not recorded any interest or penalties during the years ended December 31, 2014, 2013 and 2012 and the Company has not accrued any payment of interest and penalties as of December 31, 2014 and 2013.
Employee stock options, restricted stock units and performance-based equity awards	The Company has an employee stock option plan, which is in run-off, in which the amount of Allied World Switzerland's common shares received as compensation through the issuance of stock options is determined by reference to the value of the shares. Compensation expense for stock options granted to employees is recorded on a straight-line basis over the option vesting period and is based on the fair value of the stock options on the grant date. The fair value of each stock option on the grant date is determined by using the Black-Scholes option-pricing model.
	The Company has granted performance-based equity awards to key employees in order to promote the long-term growth and profitability of the Company. Each award represents the right to receive a number of common shares in the future, based upon the achievement of established performance criteria during the applicable performance period. The compensation expense for these awards is based on the market value of Allied World Switzerland’s common shares on the grant date, and is recognized on a straight-line basis over the applicable performance and vesting period. The Company will also adjust the compensation expense, as a cumulative adjustment, to the extent the Company's performance is above or below the targeted performance criteria.
	The Company has also granted cash-equivalent, performance-based awards to certain employees that vest based upon the achievement of established performance criteria during the applicable performance period. These cash-equivalent, performance-based awards vest after a three-year performance period. The amount payable per unit awarded will be equal to the price per share of Allied World Switzerland’s common shares, and as such the Company measures the value of the award each reporting period based on the period-ending share price. The effects of changes in the share price at each period end during the service period are recognized as changes in compensation expense over the service period. The Company will also adjust the compensation expense, as a cumulative adjustment, to the extent the Company's performance is above or below the targeted performance criteria.
	The Company has granted restricted stock units (“RSUs”) to certain employees. The compensation expense for the RSUs is based on the market value of Allied World Switzerland’s common shares on the grant date, and is recognized on a straight-line basis over the applicable vesting period.
	The Company has also granted cash-equivalent RSUs to certain employees that vest on a straight-line basis over the applicable vesting period. The amount payable per unit awarded will be equal to the price per share of Allied World Switzerland’s common shares and as such the Company measures the value of the award each reporting period based on the period ending share price. The effects of changes in the share price at each period end during the service period are recognized as increases or decreases in compensation expense over the service period.
Goodwill and intangible assets	The Company classifies its intangible assets into three categories: (1) intangible assets with finite lives subject to amortization, (2) intangible assets with indefinite lives not subject to amortization, and (3) goodwill. Intangible assets, other than goodwill, generally consist of trademarks, renewal rights, internally generated software, non-compete covenants and insurance licenses.
	For intangible assets with finite lives, the value of the assets is amortized over their expected useful lives and the expense is included in “amortization of intangible assets” in the consolidated income statements. The Company tests assets for impairment if conditions exist that indicate the carrying value may not be recoverable. If, as a result of the evaluation, the Company determines that the value of the intangible assets is impaired, then the value of the assets will be written-down in the period in which the determination of the impairment is made.
	For indefinite lived intangible assets the Company does not amortize the intangible asset but evaluates and compares the fair value of the assets to their carrying values on an annual basis or more frequently if circumstances warrant. If, as a result of the evaluation, the Company determines that the value of the intangible assets is impaired, then the value of the assets will be written-down in the period in which the determination of the impairment is made.
	Goodwill represents the excess of the cost of acquisitions over the fair value of net assets acquired and is not amortized. Goodwill is assigned at acquisition to the applicable reporting unit(s) based on the expected benefit to be received by the reporting units from the business combination. The Company determines the expected benefit based on several factors including the purpose of the business combination, the strategy of the Company subsequent to the business combination and structure of the acquired company subsequent to the business combination. A reporting unit is a component of the Company’s business that has discrete financial information that is reviewed by management. In determining the reporting unit, the Company analyzes the inputs, processes, outputs and overall operating performance of the reporting unit. The Company has determined that for purposes of the acquisition of Allied World Underwriters, Inc. ("AW Underwriters") (f/k/a Darwin Professional Underwriters, Inc.) the U.S. direct insurance operations of the North American Insurance segment is the reporting unit that is expected to receive the benefit of the business combination and as such all of the goodwill has been allocated to this reporting unit.
	For goodwill, the Company performs an annual impairment test, or more frequently if circumstances are warranted. The Company may first assess qualitative factors to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. The results of the qualitative assessment will determine if an entity needs to proceed with the two-step goodwill impairment test. For the year ended December 31, 2014, the Company elected to bypass the qualitative assessment and performed the first step of the goodwill impairment test.
	The first step of the goodwill impairment test is to compare the fair value of the reporting unit with its carrying value, including goodwill. If the carrying amount of the reporting unit exceeds its fair value then the second step of the goodwill impairment test is performed.
	The second step of the goodwill impairment test compares the implied fair value of the reporting unit’s goodwill with the carrying amount of that goodwill in order to determine the amount of impairment to be recognized. The implied fair value of goodwill is determined by deducting the fair value of a reporting unit’s identifiable assets and liabilities from the fair value of the reporting unit as a whole. The excess of the carrying value of goodwill above the implied goodwill, if any, would be recognized as an impairment charge in the consolidated income statements.
Derivative instruments	The Company utilizes derivative financial instruments as part of its overall risk management strategy. The Company recognizes all derivative financial instruments at fair value as either assets or liabilities on the consolidated balance sheets. The accounting for gains and losses associated with changes in the fair value of a derivative and the effect on the consolidated financial statements depends on its hedge designation and whether the hedge is highly effective in achieving offsetting changes in the fair value of the asset or liability hedged.
	The Company uses currency forward contracts and foreign currency swaps to manage currency exposure. The Company also utilizes various derivative instruments such as interest rate futures, interest rate swaps and index options, for the purpose of managing market exposures, interest rate volatility, portfolio duration, hedging certain investments, or enhancing investment performance. These derivatives are not designated as hedges and accordingly are carried at fair value on the consolidated balance sheets with realized and unrealized gains and losses included in the consolidated income statements. Refer to Note 4 for the Company’s related disclosure.
Earnings per share	Basic earnings per share is defined as net income available to common shareholders divided by the weighted average number of common shares outstanding for the period, giving no effect to dilutive securities. Diluted earnings per share is defined as net income available to common shareholders divided by the weighted average number of common and common share equivalents outstanding calculated using the treasury stock method for all potentially dilutive securities, including share warrants, employee stock options, employee share repurchase plan, RSUs and performance-based awards. When the effect of dilutive securities would be anti-dilutive, these securities are excluded from the calculation of diluted earnings per share.
Treasury shares	Common shares repurchased by the Company and not subsequently cancelled are classified as "treasury shares" on the consolidated balance sheets and are recorded at cost. When shares are reissued from treasury the historical cost, based on the first-in, first-out method, is used to determine the cost of the reissued shares. The difference between the cost of the treasury shares and the par value of the common stock shall be first reflected as additional paid-in capital, but to the extent additional paid-in capital is exhausted the remainder shall reduce retained earnings. The issuance of shares out of treasury have been related to vesting equity-based compensation of the Company's employees and directors.
New accounting pronouncements	In April 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update 2014-08, "Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity" ("ASU 2014-08"). ASU 2014-08 changes the requirements for reporting discontinued operations, such that a disposal of a component of the Company's operations is required to be reported as discontinued operations if the disposal represents a strategic shift that has, or will have, a major effect on the Company's operations and financial results. Examples of strategic shifts that could have a major effect on the Company's operations could include a disposal of a major geographical area, a major line of business, a major equity method investment, or other major parts of the Company. ASU 2014-08 is effective for all disposals that occur after January 1, 2015. The Company does not believe the adoption of ASU 2014-08 will have a material impact on future financial statements and related disclosures.
	In May 2014, the FASB issued Accounting Standards Update 2014-09, "Revenue from Contracts with Customers" ("ASU 2014-09"). ASU 2014-09 provides a framework, through a five-step process, for recognizing revenue from customers, improves comparability and consistency of recognizing revenue across entities, industries, jurisdictions and capital markets, and requires enhanced disclosures. Certain contracts with customers are specifically excluded from the scope of ASU 2014-09, including; amongst others, insurance contracts accounted for under Accounting Standard Codification 944, Financial Services - Insurance. ASU 2014-09 is effective on January 1, 2017 with retrospective adoption required for the comparative periods. The Company is currently assessing the impact the adoption of ASU 2014-09 will have on future financial statements and related disclosures.
	In August 2014, the FASB issued Accounting Standards Update 2014-15, "Disclosure of Uncertainties about an Entity's Ability to Continue as a Going Concern" ("ASU 2014-15"). Currently, there is no guidance under U.S. GAAP regarding management's responsibility to assess whether there is substantial doubt about an entity's ability to continue as a going concern. Under ASU 2014-15, the Company will be required to assess its ability to continue as a going concern each interim and annual reporting period and provide certain disclosures if there is substantial doubt about the entity's ability to continue as a going concern, including management's plan to alleviate the substantial doubt. ASU 2014-15 is effective for the year ended December 31, 2016 and early adoption is permitted. The Company is currently assessing the impact the adoption of ASU 2014-15 will have on future financial statements and related disclosures.
	In November 2014, the FASB issued Accounting Standards Update 2014-17, "Pushdown Accounting" ("ASU 2014-17"). ASU 2014-17 provides an acquired entity with an option to apply pushdown accounting in its separate financial statements upon an acquirer obtaining control of the acquired entity. The acquired entity may also elect to apply pushdown accounting in a subsequent reporting period, if pushdown accounting was not applied when the acquirer obtained control, but that will be treated as a change in accounting principle. Once pushdown accounting is applied, the election is irrevocable. The Company adopted ASU 2014-17 when it became effective in November 2014.

Investments_Tables

Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Schedule of trading securities	Securities accounted for at fair value with changes in fair value recognized in the consolidated income statements by category are as follows:
		31-Dec-14						31-Dec-13
		Fair Value			Amortized Cost			Fair Value				Amortized Cost
	U.S. government and government agencies	$	1,610,502		$	1,610,880		$	1,676,788			$	1,684,832
	Non-U.S. government and government agencies	188,199			196,332			191,776				197,082
	States, municipalities and political subdivisions	170,567			165,615			231,555				234,406
	Corporate debt:
	Financial institutions	1,024,667			1,018,777			958,794				943,518
	Industrials	1,029,729			1,037,820			1,174,047				1,165,448
	Utilities	110,997			111,599			69,426				69,658
	Mortgage-backed	1,263,517			1,219,712			1,292,502				1,267,863
	Asset-backed	670,832			674,505			505,910				502,543
	Total fixed maturity investments, trading	$	6,069,010		$	6,035,240		$	6,100,798			$	6,065,350
		December 31, 2014						December 31, 2013
		Fair Value			Cost			Fair Value				Cost
	Equity securities	$	844,163		$	791,206		$	699,846			$	647,301
	Other invested assets	812,543			725,069			764,081				658,683
		$	1,656,706		$	1,516,275		$	1,463,927			$	1,305,984
Schedule of other invested assets	Details regarding the carrying value, redemption characteristics and unfunded investment commitments of the other invested assets portfolio as of December 31, 2014 and 2013 were as follows:
	Fund Type	Carrying Value as of December 31, 2014			Investments			Estimated		Investments				Redemption	Redemption	Unfunded
					with			Remaining		without				Frequency(1)	Notice	Commitments
					Redemption			Restriction		Redemption					Period(1)
					Restrictions			Period		Restrictions
	Private equity (primary and secondary)	$	184,576		$	184,576		2 - 8 Years		$	—					$	223,802
	Mezzanine debt	166,905			166,905			5 - 9 Years		—						204,232
	Distressed	5,869			5,869			3 Years		—						5,180
	Real estate	—			—			9 Years		—						50,000
	Total private equity	357,350			357,350					—						483,214
	Distressed	170,169			170,169					—				Based on net asset value	60 Days	—
	Equity long/short	84,198			—					84,198				Quarterly	30 -60 Days	—
	Multi-strategy	51,507			—					51,507				Quarterly	45 -90 Days	—
	Relative value credit	119,156			—					119,156				Quarterly	60 Days	—
	Total hedge funds	425,030			170,169					254,861						—
	High yield loan fund	30,163			—					30,163				Monthly	30 days	—
	Total other invested assets at fair value	812,543			527,519					285,024						483,214
	Other private securities	142,966			—					142,966						—
	Total other invested assets	$	955,509		$	527,519				$	427,990					$	483,214
	Fund Type	Carrying Value as of December 31, 2013			Investments			Estimated		Investments				Redemption	Redemption	Unfunded
					with			Remaining		without				Frequency(1)	Notice	Commitments
					Redemption			Restriction		Redemption					Period(1)
					Restrictions			Period		Restrictions
	Private equity (primary and secondary)	$	144,422		$	144,422		2 - 9 Years		$	—					$	263,519
	Mezzanine debt	64,627			64,627			8 - 9 Years		—						198,756
	Distressed	7,776			7,776			4 Years		—						5,249
	Total private equity	216,825			216,825					—						467,524
	Distressed	151,227			151,227			1 - 2 Years		—						—
	Equity long/short	99,365			—					99,365				Quarterly	30 -60 Days	—
	Multi-strategy	136,958			—					136,958				Quarterly	45 -90 Days	—
	Event driven	14,018			—					14,018				Annual	60 Days	—
	Relative value credit	113,730			—					113,730				Quarterly	60 Days	—
	Total hedge funds	515,298			151,227					364,071						—
	High yield loan fund	31,958			—					31,958				Monthly	30 days	—
	Total other invested assets at fair value	764,081			368,052					396,029						467,524
	Other private securities	147,311			—					147,311						—
	Total other invested assets	$	911,392		$	368,052				$	543,340					$	467,524
	-1	The redemption frequency and notice periods only apply to the investments without redemption restrictions.
Schedule of net investment income
		Year Ended December 31,
		2014			2013			2012
	Fixed maturity investments	$	149,449		$	130,380		$	157,437
	Equity securities	17,581			19,076			17,080
	Other invested assets: hedge funds and private equity	12,564			8,226			6,772
	Other invested assets: other private securities	13,305			14,711			—
	Cash and cash equivalents	2,134			2,010			2,319
	Expenses	(18,159		)	(16,839		)	(16,467		)
	Net investment income	$	176,874		$	157,564		$	167,141
Components of realized gains and losses
		Year Ended December 31,
		2014			2013			2012
	Gross realized gains on sale of invested assets	$	195,390		$	213,618		$	183,592
	Gross realized losses on sale of invested assets	(48,239		)	(106,321		)	(51,702		)
	Net realized and unrealized gains (losses) on derivatives	(39,007		)	9,485			(3,608		)
	Mark-to-market gains (losses):
	Debt securities, trading	(1,675		)	(117,577		)	106,328
	Equity securities, trading	413			4,288			33,018
	Other invested assets, trading	(17,925		)	56,032			38,808
	Net realized investment gains	$	88,957		$	59,525		$	306,436
	Proceeds from sale of available for sale securities	$	—		$	—		$	236,336

Derivative_Instruments_Tables

Derivative Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Location and amounts of derivative fair values on the unaudited condensed consolidated balance sheets	The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheets:
		31-Dec-14											31-Dec-13
		Asset			Asset			Liability			Liability		Asset		Asset		Liability		Liability
		Derivative			Derivative			Derivative			Derivative		Derivative		Derivative		Derivative		Derivative
		Notional			Fair			Notional			Fair		Notional		Fair		Notional		Fair
		Amount			Value			Amount			Value		Amount		Value		Amount		Value
	Foreign exchange contracts	$	33,875		$	1,274		$	167,376		$	991	$	294,788	$	6,254	$	122,439	$	1,176
	Interest rate swaps	—			—			571,500			683		491,400		6,829		40,000		4,214
	Total derivatives	$	33,875		$	1,274		$	738,876		$	1,674	$	786,188	$	13,083	$	162,439	$	5,390
Unrealized and realized gains (losses) on derivatives recorded in the unaudited condensed consolidated income statements	The following table provides the net realized and unrealized (losses) gains on derivatives not designated as hedges recorded on the consolidated income statements:
		Year Ended December 31,
		2014			2013			2012
	Foreign exchange contracts	$	897		$	1,274		$	(749	)
	Total included in foreign exchange loss	897			1,274			(749		)
	Put options	494			(3,822		)	(2,222		)
	Foreign exchange contracts	3,538			3,054			(1,024		)
	Interest rate swaps	(45,427		)	5,591			—
	Interest rate futures	2,388			4,662			(362		)
	Total included in net realized investment gains	(39,007		)	9,485			(3,608		)
	Total realized and unrealized (losses) gains on derivatives	$	(38,110	)	$	10,759		$	(4,357	)

Fair_Value_Of_Financial_Instru1

Fair Value Of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair value hierarchy and related disclosures	The following table shows the fair value of the Company’s financial instruments and where in the fair value hierarchy the fair value measurements are included as of the dates indicated below:
	31-Dec-14	Carrying			Total fair			Level 1			Level 2			Level 3
		amount			value
	Fixed maturity investments:
	U.S. government and government agencies	$	1,610,502		$	1,610,502		$	1,499,347		$	111,155		$	—
	Non-U.S. government and government agencies	188,199			188,199			—			188,199			—
	States, municipalities and political subdivisions	170,567			170,567			—			170,567			—
	Corporate debt	2,165,393			2,165,393			—			2,165,393			—
	Mortgage-backed	1,263,517			1,263,517			—			1,081,734			181,783
	Asset-backed	670,832			670,832			—			615,419			55,413
	Total fixed maturity investments	6,069,010			6,069,010			1,499,347			4,332,467			237,196
	Equity securities	844,163			844,163			800,833			—			43,330
	Other invested assets	812,543			812,543			—			—			812,543
	Total investments	$	7,725,716		$	7,725,716		$	2,300,180		$	4,332,467		$	1,093,069
	Derivative assets:
	Foreign exchange contracts	$	1,274		$	1,274		$	—		$	1,274		$	—
	Derivative liabilities:
	Foreign exchange contracts	$	991		$	991		$	—		$	991		$	—
	Interest rate swaps	683			683			—			683			—
	Senior notes	$	798,802		$	879,317		$	—		$	879,317		$	—
	Other long-term debt	$	19,213		$	22,583		$	—		$	22,583		$	—
	31-Dec-13	Carrying			Total fair			Level 1			Level 2			Level 3
		amount			value
	Fixed maturity investments:
	U.S. government and government agencies	$	1,676,788		$	1,676,788		$	1,370,088		$	306,700		$	—
	Non-U.S. government and government agencies	191,776			191,776			—			191,776			—
	States, municipalities and political subdivisions	231,555			231,555			—			231,555			—
	Corporate debt	2,202,267			2,202,267			—			2,202,267			—
	Mortgage-backed	1,292,502			1,292,502			—			1,145,164			147,338
	Asset-backed	505,910			505,910			—			412,497			93,413
	Total fixed maturity investments	6,100,798			6,100,798			1,370,088			4,489,959			240,751
	Equity securities	699,846			699,846			625,942			—			73,904
	Other invested assets	764,081			764,081			—			—			764,081
	Total investments	$	7,564,725		$	7,564,725		$	1,996,030		$	4,489,959		$	1,078,736
	Derivative assets:
	Foreign exchange contracts	$	6,254		$	6,254		$	—		$	6,254		$	—
	Interest rate swaps	6,829			6,829			—			6,829			—
	Derivative liabilities:
	Foreign exchange contracts	$	1,176		$	1,176		$	—		$	1,176		$	—
	Interest rate swaps	4,214			4,214			$	—		4,214			$	—
	Senior notes	$	798,499		$	897,601		$	—		$	897,601		$	—
Fair value measurements using significant unobservable inputs (Level 3)	The following is a rollforward of the beginning and ending balance of financial instruments using significant unobservable inputs (Level 3):
		Fair value measurement using significant
		unobservable inputs (Level 3):
		Other invested			Mortgage-			Asset-backed			Equities
		assets			backed
	Year Ended December 31, 2014
	Opening balance	$	764,081		$	147,338		$	93,413		$	73,904
	Realized and unrealized gains (losses) included in net income	64,566			5,505			(2,068		)	(1,277		)
	Purchases	331,099			100,019			4,628			—
	Sales	(347,203		)	(71,141		)	(24,463		)	(29,297		)
	Transfers into Level 3 from Level 2	—			1,505			28,813			—
	Transfers out of Level 3 into Level 2 (1)	—			(1,443		)	(44,910		)	—
	Ending balance	$	812,543		$	181,783		$	55,413		$	43,330
	Realized and unrealized losses (gains) included in net income for investments still held as of December 31, 2014	$	(17,925	)	$	4,341		$	(116	)	$	(1,277	)
	Year Ended December 31, 2013
	Opening balance	$	655,888		$	167,825		$	62,246		$	54,680
	Realized and unrealized gains included in net income	83,621			(7,658		)	(1,176		)	9,224
	Purchases	287,977			83,455			50,669			10,000
	Sales	(263,405		)	(92,985		)	(28,393		)	—
	Transfers into Level 3 from Level 2	—			5,393			10,067			—
	Transfers out of Level 3 into Level 2 (1)	—			(8,692		)	—			—
	Ending balance	$	764,081		$	147,338		$	93,413		$	73,904
	Realized and unrealized gains (losses) included in net income for investments still held as of December 31, 2013	$	62,104		$	(4,914	)	$	(360	)	$	9,225
	(1) Transfers out of Level 3 are primarily attributable to the availability of market observable information.

Reserve_For_Losses_And_Loss_Ex1

Reserve For Losses And Loss Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Insurance Loss Reserves [Abstract]
Components of liabilty for unpaid losses and loss expenses	The reserve for losses and loss expenses consists of the following:
		December 31,
		2014			2013
	Outstanding loss reserves	$	1,514,051		$	1,520,867
	Reserves for losses incurred but not reported	4,367,114			4,245,662
	Reserve for losses and loss expenses	$	5,881,165		$	5,766,529
Reconciliation of beginning and ending liability for unpaid losses and loss expenses	The table below is a reconciliation of the beginning and ending liability for unpaid losses and loss expenses. Losses incurred and paid are reflected net of reinsurance recoverables.
		Year Ended December 31,
		2014			2013			2012
	Gross liability at beginning of year	$	5,766,529		$	5,645,549		$	5,225,143
	Reinsurance recoverable at beginning of year	(1,234,504		)	(1,141,110		)	(1,002,919		)
	Net liability at beginning of year	4,532,025			4,504,439			4,222,224
	Net losses incurred related to:
	Current year	1,411,797			1,303,573			1,309,613
	Prior years	(212,607		)	(180,331		)	(170,349		)
	Total incurred	1,199,190			1,123,242			1,139,264
	Net paid losses related to:
	Current year	171,824			115,669			117,123
	Prior years	1,001,537			973,987			743,844
	Total paid	1,173,361			1,089,656			860,967
	Foreign exchange revaluation	(16,945		)	(6,000		)	3,918
	Net liability at end of year	4,540,909			4,532,025			4,504,439
	Reinsurance recoverable at end of year	1,340,256			1,234,504			1,141,110
	Gross liability at end of year	$	5,881,165		$	5,766,529		$	5,645,549

Ceded_Reinsurance_Tables

Ceded Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Components of reinsurance recoverable	The amount of reinsurance recoverable is as follows:
		2014				2013
	OSLR recoverable	$	232,545			$	225,796
	IBNR recoverable	1,107,711				1,008,708
	Reinsurance recoverable	$	1,340,256			$	1,234,504
	Reinsurance recoverable on paid losses	$	86,075			$	76,099
Effects of reinsurance	Direct, assumed and ceded premiums written and earned and losses and loss expenses incurred are as follows:
		Premiums				Premiums				Losses and
		Written				Earned				Loss
										Expenses
	Year Ended December 31, 2014
	Direct	$	1,996,831			$	1,835,186			$	1,031,284
	Assumed	938,594				941,182				464,728
	Ceded	(613,350		)		(593,610		)		(296,822		)
		$	2,322,075			$	2,182,758			$	1,199,190
	Year Ended December 31, 2013
	Direct	$	1,804,910			$	1,664,343			$	936,084
	Assumed	933,754				896,088				430,627
	Ceded	(618,183		)		(554,598		)		(243,469		)
		$	2,120,481			$	2,005,833			$	1,123,242
	Year Ended December 31, 2012
	Direct	$	1,569,021			$	1,446,086			$	877,961
	Assumed	760,250				743,575				513,871
	Ceded	(491,448		)		(440,763		)		(252,568		)
		$	1,837,823			$	1,748,898			$	1,139,264
Schedule of ceded credit risk by reinsurer
		31-Dec-14									31-Dec-13
		A.M. Best		Reinsurance				Percentage			A.M. Best		Reinsurance		Percentage
		Rating		Recoverable				of Total			Rating		Recoverable		of Total
	Munich Re	A+		$	285,918			21.3	%		A+		$	214,070	17.3	%
	Axis Capital	A+		148,371				11.1	%		A+		143,063		11.6	%
	Arch Re	A+		128,421				9.6	%		A+		143,037		11.6	%
	Markel	A		86,595				6.5	%		A		86,462		7	%
	XL Group	A		78,377				5.8	%		A		68,857		5.6	%
	Top five reinsurers			727,682				54.3	%				655,489		53.1	%
	Other reinsurers’ balances			612,574				45.7	%				579,015		46.9	%
	Total reinsurance recoverable			$	1,340,256			100	%				$	1,234,504	100	%

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of goodwill and intangible assets	The following table shows the Company’s goodwill and intangible assets at December 31, 2014 and 2013:
		Goodwill		Indefinite-Lived		Finite-Lived Intangible Assets
				Intangible
				Assets
		Net Carrying		Net Carrying		Gross Carrying		Accumulated			Net Carrying
		Value		Value		Value		Amortization			Value
	Balance at December 31, 2012	$	268,376	$	23,920	$	41,334	$	(13,890	)	$	27,444
	Amortization of intangible assets	—		—		—		(2,533		)	(2,533		)
	Balance at December 31, 2013	268,376		23,920		41,334		(16,423		)	24,911
	Additions	9,882		—		—		—			—
	Amortization of intangible assets	—		—		—		(2,533		)	(2,533		)
	Balance at December 31, 2014	$	278,258	$	23,920	$	41,334	$	(18,956	)	$	22,378
Schedule of estimated amortization expense	The estimated amortization expense for each of the five succeeding fiscal years and thereafter related to the Company’s finite-lived intangible assets is as follows:
		Amount
	2015	$	2,533
	2016	2,533
	2017	2,533
	2018	2,533
	2019	2,533
	2020 and thereafter	9,713
	Total	$	22,378

Debt_and_Financing_Arrangement1

Debt and Financing Arrangements (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of financing structure	The following table shows the Company’s financing structure:
		Outstanding(1)		Balance(2)
	31-Dec-14
	2006 Senior notes due 2016	$	500,000	$	499,690
	2010 Senior notes due 2020	300,000		299,112
	Swiss office building mortgage	14,157		14,157
	Swiss office building credit facility	5,056		5,056
	$1,000,000 Secured letter of credit facility — uncommitted	497,332		—
	$150,000 Secured letter of credit facility — committed (3)	—		—
		$	1,316,545	$	818,015
	31-Dec-13
	2006 Senior notes due 2016	$	500,000	$	499,512
	2010 Senior notes due 2020	300,000		298,987
	$1,000,000 Secured letter of credit facility — uncommitted	707,915		—
	$450,000 Secured letter of credit facility — committed	86,102		—
		$	1,594,017	$	798,499
	-1	Indicates utilization of commitment amount, not drawn borrowings.
	-2	Represents the principal amount borrowed, net of unamortized discount.
	-3	The commitment was reduced from $450,000 to $150,000 during 2014.
Schedule of debt maturities	The following table reflects the Company’s debt maturities, which includes its senior notes and other long-term debt:
		Amount
	2015	$	228
	2016	500,303
	2017	5,359
	2018	303
	2019	303
	2020 and thereafter	312,717
	Total	$	819,213

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule of components of income tax expense	The components of income tax expense are as follows:
		Year Ended December 31,
		2014				2013				2012
	Current income tax expense	$	26,768			$	21,669			$	18,239
	Deferred income tax expense (benefit)	3,755				(11,889		)		201
	Income tax expense	$	30,523			$	9,780			$	18,440
Schedule of sources of income before income tax	Our income is primarily sourced from our Bermuda, U.S., European and Asia Pacific operations. The income before income taxes for these operations are as follows:
		Year Ended December 31,
		2014				2013				2012
	Non-U.S.	$	447,875			$	401,778			$	474,412
	United States	72,907				25,882				37,035
	Income before income taxes	$	520,782			$	427,660			$	511,447
Schedule of significant components of net deferred tax assets	The significant components of the net deferred tax assets are as follows:
		December 31,
		2014				2013
	Deferred tax assets:
	Reserve for losses and loss expenses	$	21,644			$	22,567
	Equity compensation	15,316				15,828
	Unearned premium	16,921				14,299
	Deferred acquisition costs	11,184				10,604
	Other-than-temporary impairments	—				407
	Net loss carryforward	7,958				7,642
	Total deferred tax assets	73,023				71,347
	Deferred tax liabilities:
	Intangible assets	(13,856		)		(14,743		)
	Realized gains	(12,819		)		(6,980		)
	Depreciation	(2,513		)		(2,942		)
	Unrealized appreciation and timing differences on investments	(94		)		(56		)
	Market discount on bonds	(740		)		(271		)
	Other	(2,965		)		(1,244		)
	Total deferred tax liabilities	(32,987		)		(26,236		)
	Net deferred taxes before valuation allowance	40,036				45,111
	Valuation allowance	(6,421		)		(7,642		)
	Net deferred tax assets	$	33,615			$	37,469
Schedule of current taxes receivable or payable	Current taxes receivable or payable was as follows:
		December 31,
		2014				2013
	Current tax receivable	$	8,640			$	11,684
	Current tax payable	$	829			$	1,452
Schedule of reconciliation between effective and expected tax rate	The reconciliation between the Company’s effective tax rate on pre-tax accounting income and the expected tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Expected tax rate	7.8	%		7.8	%		7.8	%
	Income not subject to income tax	(7.1	)%		(8.2	)%		(7.3	)%
	Foreign taxes at local expected tax rates	4.4	%		1.4	%		2.1	%
	Disallowed expenses and capital allowances	0.2	%		1	%		0.7	%
	Prior year refunds and adjustments	1.2	%		0.8	%		0.4	%
	Other	(0.6	)%		(0.5	)%		(0.1	)%
	Effective tax rate	5.9	%		2.3	%		3.6	%

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Issued share capital	The issued share capital consists of the following:
		December 31,
		2014			2013
	Common shares issued and fully paid, 2014 and 2013: CHF 4.10 per share	100,775,256			103,477,452
	Share capital at end of year	$	408,020		$	418,988
Schedule of common stock outstanding roll forward
		2014		2013
	Shares issued at beginning of year	103,477,452		109,109,604
	Shares cancelled	(2,702,196	)	(5,632,152	)
	Total shares issued at end of year	100,775,256		103,477,452
	Treasury shares issued at beginning of year	3,223,806		4,716,261
	Shares repurchased	4,906,785		5,544,432
	Shares issued out of treasury	(848,621	)	(1,404,735	)
	Shares cancelled	(2,702,196	)	(5,632,152	)
	Total treasury shares at end of year	4,579,774		3,223,806
	Total shares outstanding at end of year	96,195,482		100,253,646
Dividends paid	The Company paid the following dividends during the years ended December 31, 2014, 2013 and 2012:
	Dividend Paid	Partial			Dividend		Total
		Par Value			Per		Amount
		Reduction			Share		Paid
		Per Share
	2-Oct-14		N/A		$	0.225	$	21,686
	2-Jul-14		N/A		$	0.225	$	21,870
	3-Apr-14		N/A		$	0.167	$	16,495
	2-Jan-14		N/A		$	0.167	$	16,732
	3-Oct-13		N/A		$	0.167	$	16,952
	3-Jul-13		N/A		$	0.167	$	17,117
	12-Mar-13	CHF	0.11		$	0.125	$	12,981
	December 18, 2012	CHF	0.11		$	0.125	$	13,010
	September 25, 2012	CHF	0.12		$	0.125	$	13,324
	August 6, 2012	CHF	0.12		$	0.125	$	13,394
	April 6, 2012	CHF	0.11		$	0.125	$	13,795
	January 6, 2012	CHF	0.12		$	0.125	$	14,302
Share repurchases	The Company’s share repurchases were as follows:
		Year Ended December 31,
		2014			2013		2012
	Common shares repurchased	4,906,785			5,544,432		10,967,877
	Total cost of shares repurchased	$	175,373		$	174,686	$	263,942
	Average price per share	$	35.74		$	31.51	$	24.07

Employee_Benefit_Plans_Tables

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of activity related to options	The following table summarizes the activity related to options granted and exercised:
		Year Ended December 31,
		2014				2013				2012
	Options granted	—				—				—
	Weighted average grant date fair value	$	—			$	—			$	—
	Options exercised	(479,831		)		(674,187		)		(801,810		)
	Total intrinsic value of options exercised	$	10,313			$	12,212			$	9,650
	Proceeds from option exercises	$	10,027			$	12,137			$	10,527
	The activity related to the Company’s stock options is as follows:
		Year Ended December 31, 2014
		Options			Weighted				Weighted		Aggregate
					Average				Average		Intrinsic
					Exercise Price				Contractual		Value
									Term
	Outstanding at beginning of year	2,928,312			$	16.07
	Granted	—			$	—
	Exercised	(479,831	)		$	(14.25	)
	Forfeited	(15,096	)		$	(20.50	)
	Expired	(6,711	)		$	(16.35	)
	Outstanding at end of year	2,426,674			$	16.4			4.7 years		$	52,212
	Exercisable at end of year	2,151,790			$	15.88			4.5 years		$	47,424
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of compensation expense by type of award	The following table shows the total stock-related compensation expense relating to the stock options, RSUs, LTIP and cash equivalent awards.
		Year Ended December 31,
		2014				2013				2012
	Stock options	$	1,949			$	3,269			$	4,256
	RSUs and performance-based equity awards	12,295				10,700				13,010
	LTIP	—				—				376
	Cash-equivalent stock awards	37,790				50,878				32,760
	Total	$	52,034			$	64,847			$	50,402
Rsus Other Than Performance Based Rsus
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of activity related to equity based awards granted and exercised	The activity related to the Company’s RSUs awards is as follows:
		Year Ended December 31, 2014
		Number of Awards			Weighted				Aggregate
					Average				Intrinsic
					Grant Date				Value
					Fair Value
	Outstanding at beginning of year	143,697			$	21.69
	RSUs granted	454,176			33.56
	RSUs forfeited	(16,386	)		(31.22		)
	RSUs fully vested	(78,981	)		(21.76		)
	Outstanding at end of year	502,506			$	32.1			$	19,055
Performance Based Rsus
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of activity related to equity based awards granted and exercised	The activity related to the Company’s performance-based equity awards is as follows:
		Year Ended December 31, 2014
		Number of Awards			Weighted				Aggregate
					Average				Intrinsic
					Grant Date				Value
					Fair Value
	Outstanding at beginning of year	804,519			$	23.21
	Performance-based equity awards granted	166,302			33.56
	Additional awards granted due to achievement of performance criteria	104,895			20.5
	Performance-based equity awards forfeited	(4,635	)		(25.20		)
	Performance-based equity awards fully vested	(454,440	)		(20.50		)
	Outstanding at end of year	616,641			$	27.52			$	23,383
Cash-equivalent stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of activity related to cash-equivalent equity awards	The activity related to the Company's cash-equivalent RSUs and performance-based equity awards is as follows:
		RSU's							Performance-based Awards
	Year Ended December 31, 2014	Number of Awards			Weighted Average Grant Date Fair Value				Number of Awards			Weighted Average Grant Date Fair Value
	Outstanding at beginning of year	2,049,084			$	24.69			1,031,961			23.67
	Granted	438,162			33.56				249,438			33.56
	Additional awards granted due to achievement of performance criteria	—			—				104,895			20.5
	Forfeited	(77,322	)		(27.12		)		(6,948	)		(25.20		)
	Fully vested	(755,860	)		(22.51		)		(454,440	)		(20.50		)
	Outstanding at end of year	1,654,064			$	27.99			924,906			$	27.52

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Comparison of basic and diluted earnings per share	The following table sets forth the comparison of basic and diluted earnings per share:
		Year Ended December 31,
		2014		2013		2012
	Basic earnings per share:
	Net income	$	490,259	$	417,880	$	493,007
	Weighted average common shares outstanding	97,538,319		102,464,715		108,171,435
	Basic earnings per share	$	5.03	$	4.08	$	4.56
		Year Ended December 31,
		2014		2013		2012
	Diluted earnings per share:
	Net income	$	490,259	$	417,880	$	493,007
	Weighted average common shares outstanding	97,538,319		102,464,715		108,171,435
	Share equivalents:
	Warrants and options	1,432,960		1,536,939		1,306,002
	RSU and LTIP awards	605,808		864,180		1,727,265
	Employee share purchase plan	14,686		—		4,953
	Weighted average common shares and common share equivalents outstanding — diluted	99,591,773		104,865,834		111,209,655
	Diluted earnings per share	$	4.92	$	3.98	$	4.43

Commitments_And_Contingencies_

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Summary of future minimum rental payments on operating leases	The Company leases office space under operating leases expiring in various years through 2030. The following are future minimum rental payments as of December 31, 2014:
		Amount
	2015	$	12,568
	2016	16,903
	2017	20,596
	2018	20,096
	2019	19,013
	2020 and thereafter	138,602
		$	227,778
Summary of three largest individual producers	The three largest individual producers as a percentage of gross premiums written are as follows:
		Year Ended December 31,
		2014		2013		2012
	Marsh & McLennan Companies, Inc.	25	%	26	%	25	%
	Aon Corporation	17	%	18	%	19	%
	Willis Group Holdings	10	%	12	%	11	%

Statutory_Capital_and_Surplus_

Statutory Capital and Surplus (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure Dividend Restrictions And Statutory Requirements [Abstract]
Schedule of statutory capital	The statutory capital and surplus and minimum required statutory capital and surplus for the Company’s most significant regulatory jurisdictions at December 31, 2014 and 2013 were as follows:
		December 31, 2014						December 31, 2013
		Statutory			Minimum			Statutory		Minimum
		Capital and			Required			Capital and		Required
		Surplus			Statutory			Surplus		Statutory
					Capital and					Capital and
					Surplus					Surplus
	Bermuda	$	3,273,730		$	876,235		$	3,208,776	$	806,012
	United States	1,220,299			226,876			1,187,023		205,699
	Ireland	410,841			29,204			407,600		27,098
	Switzerland	172,306			19,943			167,840		22,905
	United Kingdom	231,659			203,742			229,745		229,689
Schedule of statutory net income loss	The statutory net income (loss) for the Company’s most significant regulatory jurisdictions for the years ended December 31, 2014, 2013 and 2012 was as follows:
		Year Ended December 31,
		2014			2013			2012
	Bermuda	$	487,051		$	467,283		$	528,712
	United States	45,445			36,744			25,390
	Ireland	3,241			(4,880		)	9,325
	Switzerland	7,408			(6,418		)	10,657
	United Kingdom	(7,819		)	9,362			1,800
Schedule of allowable dividends by jurisdiction	At December 31, 2014, the maximum amount of ordinary dividends or distributions that can be paid, without prior regulatory approval, for the Company’s most significant regulatory jurisdictions, were as follows:
		December 31, 2014
	Bermuda	$	818,433
	United States	102,283
	Ireland	21,637
	Switzerland	6,323
	United Kingdom	1,601

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Summary of segment results	The following tables provide a summary of the segment results:
	Year Ended December 31, 2014	North American  Insurance			Global Markets			Reinsurance			Total
					Insurance
	Gross premiums written	$	1,716,288		$	280,543		$	938,594		$	2,935,425
	Net premiums written	1,230,839			187,997			903,239			2,322,075
	Net premiums earned	1,111,164			162,566			909,028			2,182,758
	Net losses and loss expenses	(683,836		)	(61,057		)	(454,297		)	(1,199,190		)
	Acquisition costs	(105,914		)	(18,222		)	(170,986		)	(295,122		)
	General and administrative expenses	(219,665		)	(68,106		)	(77,965		)	(365,736		)
	Underwriting income	101,749			15,181			205,780			322,710
	Other insurance-related revenue	2,064			—			—			2,064
	Other insurance-related expenses	(1,854		)	(6,724		)	—			(8,578		)
	Segment income	101,959			8,457			205,780			316,196
	Net investment income										176,874
	Net realized investment gains										88,957
	Amortization of intangible assets										(2,533		)
	Interest expense										(57,755		)
	Foreign exchange loss										(957		)
	Income before income taxes										$	520,782
	Loss and loss expense ratio	61.5		%	37.6		%	50		%	54.9		%
	Acquisition cost ratio	9.5		%	11.2		%	18.8		%	13.5		%
	General and administrative expense ratio	19.8		%	41.9		%	8.6		%	16.8		%
	Expense ratio	29.3		%	53.1		%	27.4		%	30.3		%
	Combined ratio	90.8		%	90.7		%	77.4		%	85.2		%
	Year Ended December 31, 2013	North American  Insurance			Global Markets			Reinsurance			Total
					Insurance
	Gross premiums written	$	1,572,355		$	232,555		$	933,754		$	2,738,664
	Net premiums written	1,082,441			145,002			893,038			2,120,481
	Net premiums earned	1,022,979			126,008			856,846			2,005,833
	Net losses and loss expenses	(651,267		)	(50,395		)	(421,580		)	(1,123,242		)
	Acquisition costs	(94,869		)	(10,051		)	(147,753		)	(252,673		)
	General and administrative expenses	(209,042		)	(63,164		)	(80,079		)	(352,285		)
	Underwriting income	67,801			2,398			207,434			277,633
	Other insurance-related revenue	—			—			—			—
	Other insurance-related expenses	—			—			—			—
	Segment income	67,801			2,398			207,434			277,633
	Net investment income										157,564
	Net realized investment gains										59,525
	Amortization of intangible assets										(2,533		)
	Interest expense										(56,510		)
	Foreign exchange loss										(8,019		)
	Income before income taxes										$	427,660
	Loss and loss expense ratio	63.7		%	40		%	49.2		%	56		%
	Acquisition cost ratio	9.3		%	8		%	17.2		%	12.6		%
	General and administrative expense ratio	20.4		%	50.1		%	9.3		%	17.6		%
	Expense ratio	29.7		%	58.1		%	26.5		%	30.2		%
	Combined ratio	93.4		%	98.1		%	75.7		%	86.2		%
	Year Ended December 31, 2012	North American  Insurance			Global Markets			Reinsurance			Total
					Insurance
	Gross premiums written	$	1,376,040		$	192,981		$	760,250		$	2,329,271
	Net premiums written	964,199			125,577			748,047			1,837,823
	Net premiums earned	892,721			115,660			740,517			1,748,898
	Net losses and loss expenses	(608,762		)	(19,536		)	(510,966		)	(1,139,264		)
	Acquisition costs	(77,701		)	(8,483		)	(119,538		)	(205,722		)
	General and administrative expenses	(185,231		)	(48,365		)	(73,725		)	(307,321		)
	Underwriting income	21,027			39,276			36,288			96,591
	Other insurance-related revenue	—			—			—			—
	Other insurance-related expenses	—			—			—			—
	Segment income	21,027			39,276			36,288			96,591
	Net investment income										167,141
	Net realized investment gains										306,436
	Amortization of intangible assets										(2,533		)
	Interest expense										(55,405		)
	Foreign exchange loss										(783		)
Income before income taxes										$	511,447
Loss and loss expense ratio	68.2		%	16.9		%	69		%	65.1		%
Acquisition cost ratio	8.7		%	7.3		%	16.1		%	11.8		%
General and administrative expense ratio	20.7		%	41.8		%	10		%	17.6		%
Expense ratio	29.4		%	49.1		%	26.1		%	29.4		%
Combined ratio	97.6		%	66		%	95.1		%	94.5		%
Analysis of gross premiums written by geographic location	The following table shows an analysis of the Company’s gross premiums written by geographic location of the Company’s subsidiaries. All intercompany premiums have been eliminated.
		Year Ended December 31,
		2014			2013			2012
	United States	$	1,795,564		$	1,636,010		$	1,360,190
	Bermuda	640,948			676,174			611,382
	Europe	318,626			264,867			228,791
	Asia Pacific	167,334			161,613			128,908
	Canada	12,953			—			—
	Total gross premiums written	$	2,935,425		$	2,738,664		$	2,329,271
Premiums earned by segment and line of business	The following table shows the Company’s net premiums earned by line of business for each segment for the years ended December 31, 2014, 2013 and 2012.
		Year Ended December 31,
		2014			2013			2012
	North American Insurance:
	General casualty	$	334,603		$	269,114		$	211,238
	Professional liability	276,105			259,215			245,583
	Healthcare	168,476			194,344			187,053
	General property	126,646			141,896			137,002
	Programs	119,756			107,870			88,180
	Inland marine	43,666			25,939			10,612
	Environmental	19,507			17,017			12,627
	Other	22,405			7,584			426
	Total	1,111,164			1,022,979			892,721
	Global Markets Insurance:
	Professional liability	57,176			45,967			43,621
	General property	33,957			31,364			36,751
	Aviation	22,342			9,521			—
	Trade credit	17,623			14,099			9,471
	General casualty	15,266			13,903			16,762
	Healthcare	11,963			11,154			9,055
	Other	4,239			—			—
	Total	162,566			126,008			115,660
	Reinsurance:
	Property	411,968			336,009			273,167
	Specialty	321,538			349,044			283,415
	Casualty	175,522			171,793			183,935
	Total	909,028			856,846			740,517
	Total net premiums earned	$	2,182,758		$	2,005,833		$	1,748,898

Condensed_Consolidated_Guarant1

Condensed Consolidated Guarantor Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule of condensed consolidating balance sheet	Condensed Consolidating Balance Sheet:
	As of December 31, 2014	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	ASSETS:
	Investments	$	—		$	—		$	7,868,682		$	—		$	7,868,682
	Cash and cash equivalents	32,579			1,734			555,026			—			589,339
	Insurance balances receivable	—			—			664,815			—			664,815
	Funds held	—			—			724,021			—			724,021
	Reinsurance recoverable	—			—			1,340,256			—			1,340,256
	Reinsurance recoverable on paid losses	—			—			86,075						86,075
	Net deferred acquisition costs	—			—			151,546			—			151,546
	Goodwill and intangible assets	—			—			324,556			—			324,556
	Balances receivable on sale of investments	—			—			47,149			—			47,149
	Investments in subsidiaries	3,629,301			4,218,028			—			(7,847,329		)	—
	Due from subsidiaries	147,072			19,190			14,396			(180,658		)	—
	Other assets	1,470			3,192			620,462			—			625,124
	Total assets	$	3,810,422		$	4,242,144		$	12,396,984		$	(8,027,987	)	$	12,421,563
	LIABILITIES:
	Reserve for losses and loss expenses	$	—		$	—		$	5,881,165		$	—		$	5,881,165
	Unearned premiums	—			—			1,555,313			—			1,555,313
	Reinsurance balances payable	—			—			180,060			—			180,060
	Balances due on purchases of investments	—						5,428			—			5,428
	Senior notes	—			798,802			—			—			798,802
	Other long-term debt	—			—			19,213			—			19,213
	Due to subsidiaries	7,599			6,797			166,262			(180,658		)	—
	Other liabilities	24,532			19,618			159,141			—			203,291
	Total liabilities	32,131			825,217			7,966,582			(180,658		)	8,643,272
	Total shareholders’ equity	3,778,291			3,416,927			4,430,402			(7,847,329		)	3,778,291
	Total liabilities and shareholders’ equity	$	3,810,422		$	4,242,144		$	12,396,984		$	(8,027,987	)	$	12,421,563
	As of December 31, 2013	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	ASSETS:
	Investments	$	—		$	—		$	7,712,036		$	—		$	7,712,036
	Cash and cash equivalents	10,790			2,775			518,371			—			531,936
	Insurance balances receivable	—			—			588,632			—			588,632
	Funds held	—			—			632,430			—			632,430
	Reinsurance recoverable	—			—			1,234,504			—			1,234,504
	Reinsurance recoverable on paid losses	—			—			76,099			—			76,099
	Net deferred acquisition costs	—			—			126,661			—			126,661
	Goodwill and intangible assets	—			—			317,207			—			317,207
	Balances receivable on sale of investments	—			—			76,544			—			76,544
	Investments in subsidiaries	3,413,001			4,018,619			—			(7,431,620		)	—
	Due from subsidiaries	111,172			122,846			123,479			(357,497		)	—
	Other assets	1,757			4,671			643,353			—			649,781
	Total assets	$	3,536,720		$	4,148,911		$	12,049,316		$	(7,789,117	)	$	11,945,830
	LIABILITIES:
	Reserve for losses and loss expenses	$	—		$	—		$	5,766,529		$	—		$	5,766,529
	Unearned premiums	—			—			1,396,256			—			1,396,256
	Reinsurance balances payable	—			—			173,023			—			173,023
	Balances due on purchases of investments	—			—			104,740			—			104,740
	Senior notes	—			798,499			—			—			798,499
	Due to subsidiaries	12,945			110,534			234,018			(357,497		)	—
	Other liabilities	3,949			17,797			165,211			—			186,957
	Total liabilities	16,894			926,830			7,839,777			(357,497		)	8,426,004
	Total shareholders’ equity	3,519,826			3,222,081			4,209,539			(7,431,620		)	3,519,826
	Total liabilities and shareholders’ equity	$	3,536,720		$	4,148,911		$	12,049,316		$	(7,789,117	)	$	11,945,830
Schedule of condensed consolidating income statement	Condensed Consolidating Income Statement:
	Year Ended December 31, 2014	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	Net premiums earned	$	—		$	—		$	2,182,758		$	—		$	2,182,758
	Net investment income	50			—			176,824			—			176,874
	Net realized investment gains	—			—			88,957			—			88,957
	Other income	—			—			2,064			—			2,064
	Net losses and loss expenses	—			—			(1,199,190		)	—			(1,199,190		)
	Acquisition costs	—			—			(295,122		)	—			(295,122		)
	General and administrative expenses	(35,596		)	(3,088		)	(327,052		)	—			(365,736		)
	Other expense	—			—			(8,578		)	—			(8,578		)
	Amortization of intangible assets	—			—			(2,533		)	—			(2,533		)
	Interest expense	—			(55,418		)	(2,337		)	—			(57,755		)
	Foreign exchange gain (loss)	20			104			(1,081		)	—			(957		)
	Income tax (expense) benefit	(89		)	—			(30,434		)	—			(30,523		)
	Equity in earnings of consolidated subsidiaries	525,874			564,648			—			(1,090,522		)	—
	NET INCOME (LOSS)	$	490,259		$	506,246		$	584,276		$	(1,090,522	)	$	490,259
	Other comprehensive income	—			—			—			—			—
	COMPREHENSIVE INCOME (LOSS)	$	490,259		$	506,246		$	584,276		$	(1,090,522	)	$	490,259
	Year Ended December 31, 2013	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	Net premiums earned	$	—		$	—		$	2,005,833		$	—		$	2,005,833
	Net investment income	13			4			157,547			—			157,564
	Net realized investment gains	—			—			59,525			—			59,525
	Other income	—			—			—			—			—
	Net losses and loss expenses	—			—			(1,123,242		)	—			(1,123,242		)
	Acquisition costs	—			—			(252,673		)	—			(252,673		)
	General and administrative expenses	(42,122		)	(1,320		)	(308,843		)	—			(352,285		)
	Other expense	—			—			—			—			—
	Amortization of intangible assets	—			—			(2,533		)	—			(2,533		)
	Interest expense	—			(55,347		)	(1,163		)	—			(56,510		)
	Foreign exchange gain (loss)	263			(1,045		)	(7,237		)	—			(8,019		)
	Income tax (expense) benefit	98			—			(9,878		)	—			(9,780		)
	Equity in earnings of consolidated subsidiaries	459,628			515,156			—			(974,784		)	—
	NET INCOME (LOSS)	$	417,880		$	457,448		$	517,336		$	(974,784	)	$	417,880
	Other comprehensive income	—			—			—			—			—
	COMPREHENSIVE INCOME (LOSS)	$	417,880		$	457,448		$	517,336		$	(974,784	)	$	417,880
	Year Ended December 31, 2012	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	Net premiums earned	$	—		$	—		$	1,748,898		$	—		$	1,748,898
	Net investment income	26			20			167,095			—			167,141
	Net realized investment gains (loss)	—			—			306,436			—			306,436
	Other income	—			—			—			—			—
	Net losses and loss expenses	—			—			(1,139,264		)	—			(1,139,264		)
	Acquisition costs	—			—			(205,722		)	—			(205,722		)
	General and administrative expenses	(35,258		)	(2,973		)	(269,090		)	—			(307,321		)
	Other expense	—			—			—			—			—
	Amortization of intangible assets	—			—			(2,533		)	—			(2,533		)
	Interest expense	—			(55,405		)	—			—			(55,405		)
	Foreign exchange gain (loss)	159			(319		)	(623		)	—			(783		)
	Income tax (expense) benefit	358			—			(18,798		)	—			(18,440		)
	Equity in earnings of consolidated subsidiaries	527,722			573,256			—			(1,100,978		)	—
	NET INCOME (LOSS)	$	493,007		$	514,579		$	586,399		$	(1,100,978	)	$	493,007
	Unrealized gains on investments arising during the year net of applicable deferred income tax benefit of $265	(493		)	—			(493		)	493			(493		)
	Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax	(13,991		)	—			(13,991		)	13,991			(13,991		)
	Other comprehensive loss	(14,484		)	—			(14,484		)	14,484			(14,484		)
	COMPREHENSIVE INCOME (LOSS)	$	478,523		$	514,579		$	571,915		$	(1,086,494	)	$	478,523
Schedule of condensed consolidating statement of cash flows	Condensed Consolidating Statement of Cash Flows:
	Year Ended December 31, 2014	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES	$	264,422		$	292,959		$	493,659		$	(643,250	)	$	407,790
	CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
	Purchases trading securities	—			—			(7,630,034		)	—			(7,630,034		)
	Purchases of other invested assets	—			—			(307,955		)	—			(307,955		)
	Sales of trading securities	—			—			7,536,936			—			7,536,936
	Sales of other invested assets	—			—			267,885			—			267,885
	Other	—			—			6,201			—			6,201
	Net cash provided by (used in) investing activities	—			—			(126,967		)	—			(126,967		)
	CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
	Dividends paid	(76,750		)	—			—			—			(76,750		)
	Intercompany dividends paid	—			(294,000		)	(349,250		)	643,250			—
	Proceeds from the exercise of stock options	10,027			—			—			—			10,027
	Proceeds from other long-term debt	—			—			19,213			—			19,213
	Share repurchases	(175,910		)	—			—			—			(175,910		)
	Net cash provided by (used in) financing activities	(242,633		)	(294,000		)	(330,037		)	643,250			(223,420		)
	NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	21,789			(1,041		)	36,655			—			57,403
	CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	10,790			2,775			518,371			—			531,936
	CASH AND CASH EQUIVALENTS, END OF YEAR	$	32,579		$	1,734		$	555,026		$	—		$	589,339
	Year Ended December 31, 2013	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES	$	198,702		$	265,951		$	237,956		$	(596,500	)	$	106,109
	CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
	Purchases of trading securities	—			—			(7,527,670		)	—			(7,527,670		)
	Purchases of other invested assets	—			—			(276,926		)	—			(276,926		)
	Sales of trading securities	—			—			7,540,194			—			7,540,194
	Sales of other invested assets	—			—			187,477			—			187,477
	Other	—			—			28,782			—			28,782
	Net cash provided by (used in) investing activities	—			—			(48,143		)	—			(48,143		)
	CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
	Partial par value reduction	(12,981		)	—			—			—			(12,981		)
	Dividends paid	(34,059		)	—			—			—			(34,059		)
	Intercompany dividends paid	—			(274,500		)	(322,000		)	596,500			—
	Proceeds from the exercise of stock options	12,137			—			—			—			12,137
	Share repurchases	(173,006		)	—			—			—			(173,006		)
	Net cash provided by (used in) financing activities	(207,909		)	(274,500		)	(322,000		)	596,500			(207,909		)
	NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(9,207		)	(8,549		)	(132,187		)	—			(149,943		)
	CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	19,997			11,324			650,558			—			681,879
	CASH AND CASH EQUIVALENTS, END OF YEAR	$	10,790		$	2,775		$	518,371		$	—		$	531,936
	Year Ended December 31, 2012	Allied World			Allied World			Other Allied			Consolidating			Allied World
		Switzerland			Bermuda			World			Adjustments			Switzerland
		(Parent			(Subsidiary			Subsidiaries						Consolidated
		Guarantor)			Issuer)
	CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$	338,565		$	357,438		$	702,853		$	(770,000	)	$	628,856
	CASH FLOWS PROVIDED BY (USED IN) INVESTING ACTIVITIES:
	Purchases of trading securities	—			—			(8,006,237		)	—			(8,006,237		)
	Purchases of other invested assets	—			—			(333,031		)	—			(333,031		)
	Sales of available for sale securities	—			—			236,978			—			236,978
	Sales of trading securities	—			—			7,892,907			—			7,892,907
	Sales of other invested assets	—			—			53,741			—			53,741
	Capital contribution to subsidiary	(110,000		)	—			—			110,000			—
	Other	—			—			(104,091		)	—			(104,091		)
	Net cash provided by (used in) investing activities	(110,000		)	—			(259,733		)	110,000			(259,733		)
	CASH FLOWS PROVIDED BY (USED IN) FINANCING ACTIVITIES:
	Partial par value reduction	(67,825		)	—			—			—			(67,825		)
	Intercompany dividends paid	—			(355,000		)	(415,000		)	770,000			—
	Proceeds from the exercise of stock options	10,527			—			—			—			10,527
	Share repurchases	(263,942		)	—			—			—			(263,942		)
Capital contribution from parent	—			—			110,000			(110,000		)	—
Net cash provided by (used in) financing activities	(321,240		)	(355,000		)	(305,000		)	660,000			(321,240		)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(92,675		)	2,438			138,120			—			47,883
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	112,672			8,886			512,438			—			633,996
CASH AND CASH EQUIVALENTS, END OF YEAR	$	19,997		$	11,324		$	650,558		$	—		$	681,879

Unaudited_Quarterly_Financial_1

Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of unaudited consolidated statements of income by quarter	The following are the unaudited consolidated statements of income by quarter for the years ended December 31, 2014 and 2013:
		Quarter Ended
		December 31,			September 30,			June 30,			March 31,
		2014			2014			2014			2014
	REVENUES:
	Gross premiums written	$	565,743		$	707,884		$	760,405		$	901,393
	Premiums ceded	(137,948		)	(139,142		)	(206,481		)	(129,779		)
	Net premiums written	427,795			568,742			553,924			771,614
	Change in unearned premiums	145,694			(27,005		)	(16,677		)	(241,329		)
	Net premiums earned	573,489			541,737			537,247			530,285
	Net investment income	49,050			43,412			36,793			47,619
	Net realized investment (losses) gains	(15,329		)	(35,136		)	85,217			54,205
	Other income	1,032			1,032			—			—
		608,242			551,045			659,257			632,109
	EXPENSES:
	Net losses and loss expenses	272,959			336,090			314,855			275,286
	Acquisition costs	80,718			72,403			74,279			67,722
	General and administrative expenses	100,914			88,294			96,188			80,340
	Other expense	2,003			6,575			—			—
	Amortization of intangible assets	633			633			634			633
	Interest expense	14,304			14,325			14,592			14,534
	Foreign exchange (gain) loss	(21		)	278			651			49
		471,510			518,598			501,199			438,564
	Income before income taxes	136,732			32,447			158,058			193,545
	Income tax expense	6,223			1,532			6,195			16,573
	NET INCOME	$	130,509		$	30,915		$	151,863		$	176,972
	PER SHARE DATA
	Basic earnings per share	$	1.35		$	0.32		$	1.55		$	1.78
	Diluted earnings per share	$	1.33		$	0.31		$	1.52		$	1.74
	Weighted average common shares outstanding	96,386,796			96,458,231			97,809,639			99,545,187
	Weighted average common shares and common share equivalents outstanding	98,394,432			98,444,238			99,724,802			101,584,662
		Quarter Ended
		December 31,			September 30,			June 30,			March 31,
		2013			2013			2013			2013
	REVENUES:
	Gross premiums written	$	555,490		$	580,893		$	765,200		$	837,081
	Premiums ceded	(164,360		)	(127,816		)	(183,978		)	(142,029		)
	Net premiums written	391,130			453,077			581,222			695,052
	Change in unearned premiums	133,431			57,696			(73,951		)	(231,824		)
	Net premiums earned	524,561			510,773			507,271			463,228
	Net investment income	47,270			39,271			37,635			33,388
	Net realized investment gains (losses)	67,599			27,487			(115,198		)	79,637
		639,430			577,531			429,708			576,253
	EXPENSES:
	Net losses and loss expenses	315,966			276,970			275,128			255,178
	Acquisition costs	66,257			65,114			64,617			56,685
	General and administrative expenses	100,467			88,553			80,585			82,680
	Amortization of intangible assets	633			633			634			633
	Interest expense	14,094			14,094			14,188			14,134
	Foreign exchange loss	658			4,353			490			2,518
		498,075			449,717			435,642			411,828
	Income (loss) before income taxes	141,355			127,814			(5,934		)	164,425
	Income tax expense (benefit)	3,448			4,971			(4,072		)	5,433
	NET INCOME (LOSS)	$	137,907		$	122,843		$	(1,862	)	$	158,992
	PER SHARE DATA
	Basic earnings (loss) per share	1.37			1.2			(0.02		)	1.53
	Diluted earnings (loss) per share	1.34			1.18			(0.02		)	1.5
	Weighted average common shares outstanding	100,814,955			101,974,077			103,267,659			103,840,818
	Weighted average common shares and common share equivalents outstanding	103,129,812			104,184,579			103,267,659			106,295,529

General_Details

General (Details) (USD $)	0 Months Ended
In Thousands, unless otherwise specified	1-May-14	Dec. 31, 2014
Business Acquisition [Line Items]
Stockholders' Equity Note, Stock Split, Conversion Ratio	3
Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc
Business Acquisition [Line Items]
Business Acquisition Preliminary Purchase Price		$201,761
Business Acquisition Expected Additional Capitalization		$90,000

Investments_Narrative_Details

Investments (Narrative) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014
Other invested assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Income (loss) from equity method investments	$9,348
Minimum | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption restriction, lock-up period	1 year
Minimum | Investments without redemption restrictions | Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	30 days
Gate trigger, percentage	15.00%
Maximum | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption restriction, lock-up period	3 years
Maximum | Investments without redemption restrictions | Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Gate trigger, percentage	25.00%

Investments_Schedule_Of_Tradin

Investments (Schedule Of Trading Securities) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Equity securities and other invested assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	$1,656,706	$1,463,927
Trading securities, Amortized Cost	1,516,275	1,305,984
Equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	844,163	699,846
Trading securities, Amortized Cost	791,206	647,301
Other invested assets
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	812,543	764,081
Trading securities, Amortized Cost	725,069	658,683
Fixed Maturities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	6,069,010	6,100,798
Trading securities, Amortized Cost	6,035,240	6,065,350
U.S. Government and Government Agencies
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,610,502	1,676,788
Trading securities, Amortized Cost	1,610,880	1,684,832
Non-U.S. Government and Government Agencies
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	188,199	191,776
Trading securities, Amortized Cost	196,332	197,082
States, municipalities and political subdivisions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	170,567	231,555
Trading securities, Amortized Cost	165,615	234,406
Corporate debt: financial institutions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,024,667	958,794
Trading securities, Amortized Cost	1,018,777	943,518
Corporate debt: Industrials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,029,729	1,174,047
Trading securities, Amortized Cost	1,037,820	1,165,448
Corporate debt: utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	110,997	69,426
Trading securities, Amortized Cost	111,599	69,658
Mortgage-backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,263,517	1,292,502
Trading securities, Amortized Cost	1,219,712	1,267,863
Asset-backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	670,832	505,910
Trading securities, Amortized Cost	$674,505	$502,543

Investments_Schedule_Of_Other_

Investments (Schedule Of Other Investments) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	955,509		911,392
Unfunded commitments	483,214		467,524
Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	527,519		368,052
Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	427,990		543,340
Private equity (primary and secondary) | Investments with redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period	2 years		2 years
Private equity (primary and secondary) | Investments with redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period	8 years		9 years
Mezzanine debt | Investments with redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period	5 years		8 years
Mezzanine debt | Investments with redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period	9 years		9 years
Distressed | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period	3 years		4 years
Real estate | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period	9 years
Hedge funds | Investments without redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	30 days
Distressed funds | Investments with redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period			1 year
Distressed funds | Investments with redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Estimated remaining restriction period			2 years
Distressed funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	60 days	[1]
Equity long/short | Investments without redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	30 days	[1]	30 days	[1]
Equity long/short | Investments without redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	60 days	[1]	60 days	[1]
Multi-strategy | Investments without redemption restrictions | Minimum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	45 days	[1]	45 days	[1]
Multi-strategy | Investments without redemption restrictions | Maximum
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	90 days	[1]	90 days	[1]
Event driven | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period			60 days	[1]
Relative value credit | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	60 days	[1]	60 days	[1]
High yield loan fund | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption notice period	30 days	[1]	30 days	[1]
Total Fair Value
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	812,543		764,081
Unfunded commitments	483,214		467,524
Total Fair Value | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	527,519		368,052
Total Fair Value | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	285,024		396,029
Total Fair Value | Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	357,350		216,825
Unfunded commitments	483,214		467,524
Total Fair Value | Private equity funds | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	357,350		216,825
Total Fair Value | Private equity funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Private equity (primary and secondary)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	184,576		144,422
Unfunded commitments	223,802		263,519
Total Fair Value | Private equity (primary and secondary) | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	184,576		144,422
Total Fair Value | Private equity (primary and secondary) | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Mezzanine debt
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	166,905		64,627
Unfunded commitments	204,232		198,756
Total Fair Value | Mezzanine debt | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	166,905		64,627
Total Fair Value | Mezzanine debt | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Distressed
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	5,869		7,776
Unfunded commitments	5,180		5,249
Total Fair Value | Distressed | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	5,869		7,776
Total Fair Value | Distressed | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0
Unfunded commitments	50,000
Total Fair Value | Real estate | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0
Total Fair Value | Real estate | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0
Total Fair Value | Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	425,030		515,298
Unfunded commitments	0		0
Total Fair Value | Hedge funds | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	170,169		151,227
Total Fair Value | Hedge funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	254,861		364,071
Total Fair Value | Distressed funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	170,169		151,227
Unfunded commitments	0		0
Total Fair Value | Distressed funds | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	170,169		151,227
Total Fair Value | Distressed funds | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Redemption frequency	Based on net asset value	[1]
Total Fair Value | Equity long/short
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	84,198		99,365
Unfunded commitments	0		0
Total Fair Value | Equity long/short | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Equity long/short | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	84,198		99,365
Redemption frequency	Quarterly	[1]	Quarterly	[1]
Total Fair Value | Multi-strategy
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	51,507		136,958
Unfunded commitments	0		0
Total Fair Value | Multi-strategy | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Multi-strategy | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	51,507		136,958
Redemption frequency	Quarterly	[1]	Quarterly	[1]
Total Fair Value | Event driven
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets			14,018
Unfunded commitments			0
Total Fair Value | Event driven | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets			0
Total Fair Value | Event driven | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets			14,018
Redemption frequency			Annual	[1]
Total Fair Value | Relative value credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	119,156		113,730
Unfunded commitments	0		0
Total Fair Value | Relative value credit | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | Relative value credit | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	119,156		113,730
Redemption frequency	Quarterly	[1]	Quarterly	[1]
Total Fair Value | High yield loan fund
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	30,163		31,958
Unfunded commitments	0		0
Total Fair Value | High yield loan fund | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Total Fair Value | High yield loan fund | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	30,163		31,958
Redemption frequency	Monthly	[1]	Monthly	[1]
Carrying Value [Member] | Other private securities
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	142,966		147,311
Unfunded commitments	0		0
Carrying Value [Member] | Other private securities | Investments with redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	0		0
Carrying Value [Member] | Other private securities | Investments without redemption restrictions
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Other invested assets	142,966		147,311
[1]	The redemption frequency and notice periods only apply to the investments without redemption restrictions.

Investments_Schedule_Of_Net_In

Investments (Schedule Of Net Investment Income) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income	$49,050	$43,412	$36,793	$47,619	$47,270	$39,271	$37,635	$33,388	$176,874	$157,564	$167,141
Fixed maturity investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income									149,449	130,380	157,437
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income									17,581	19,076	17,080
Other invested assets: hedge funds and private equity
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income									12,564	8,226	6,772
Other invested assets: other private securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income									13,305	14,711	0
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income									2,134	2,010	2,319
Expenses
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Net investment income									($18,159)	($16,839)	($16,467)

Investments_Components_Of_Real

Investments (Components Of Realized Gains And Losses) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Gross realized gains on sale of invested assets									$195,390	$213,618	$183,592
Gross realized losses on sale of invested assets									-48,239	-106,321	-51,702
Net realized and unrealized losses on derivatives									-39,007	9,485	-3,608
Net realized investment gains	-15,329	-35,136	85,217	54,205	67,599	27,487	-115,198	79,637	88,957	59,525	306,436
Proceeds from sale of available for sale securities									0	0	236,336
Debt securities
Gain (Loss) on Investments [Line Items]
Mark-to-market changes									-1,675	-117,577	106,328
Equity securities
Gain (Loss) on Investments [Line Items]
Mark-to-market changes									413	4,288	33,018
Other invested assets
Gain (Loss) on Investments [Line Items]
Mark-to-market changes									($17,925)	$56,032	$38,808

Investments_Pledged_Assets_Det

Investments (Pledged Assets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
In favor of ceding companies, other counterparties, or government authorities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Restricted cash, cash equivalents and investments	$3,585,792	$2,894,401
Pledged as collateral for letter of credity facility
Schedule of Trading Securities and Other Trading Assets [Line Items]
Restricted cash, cash equivalents and investments	$571,750	$1,053,632

Derivative_Instruments_Locatio

Derivative Instruments (Location and Amounts of Derivative Fair Values On The Condensed Consolidated Balance Sheets) (Details) (Not designated as hedging instruments, USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Other assets
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	$33,875	$786,188
Asset Derivative Fair Value	1,274	13,083
Accounts payable and accrued liabilities
Derivatives, Fair Value [Line Items]
Liability Derivative Notional Amount	738,876	162,439
Liability Derivative Fair Value	1,674	5,390
Foreign exchange contracts | Other assets
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	33,875	294,788
Asset Derivative Fair Value	1,274	6,254
Foreign exchange contracts | Accounts payable and accrued liabilities
Derivatives, Fair Value [Line Items]
Liability Derivative Notional Amount	167,376	122,439
Liability Derivative Fair Value	991	1,176
Interest rate swaps | Other assets
Derivatives, Fair Value [Line Items]
Asset Derivative Notional Amount	0	491,400
Asset Derivative Fair Value	0	6,829
Interest rate swaps | Accounts payable and accrued liabilities
Derivatives, Fair Value [Line Items]
Liability Derivative Notional Amount	571,500	40,000
Liability Derivative Fair Value	$683	$4,214

Derivative_Instruments_Locatio1

Derivative Instruments (Location and Amounts of Unrealized And Realized Gains (Losses) On Derivatives Recorded In The Condensed Consolidated Income Statements) (Details) (Not designated as hedging instruments, USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	($38,110)	$10,759	($4,357)
Foreign exchange gain (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	897	1,274	-749
Foreign exchange gain (loss) | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	897	1,274	-749
Net realized investment gains (losses)
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	-39,007	9,485	-3,608
Net realized investment gains (losses) | Put options
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	494	-3,822	-2,222
Net realized investment gains (losses) | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	3,538	3,054	-1,024
Net realized investment gains (losses) | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	-45,427	5,591	0
Net realized investment gains (losses) | Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	$2,388	$4,662	($362)

Fair_Value_Of_Financial_Instru2

Fair Value Of Financial Instruments (Schedule Of Fair Value Of Financial Instruments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	$6,069,010	$6,100,798
Equity securities	844,163	699,846
Other invested assets	955,509	911,392
Total investments	7,868,682	7,712,036
Senior notes	798,802	798,499
Other long-term debt	19,213	0
Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	812,543	764,081
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	6,069,010	6,100,798
Equity securities	844,163	699,846
Other invested assets	812,543	764,081
Total investments	7,725,716	7,564,725
Senior notes	798,802	798,499
Other long-term debt	19,213
Fair Value, Measurements, Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,274	6,254
Derivative liabilities	991	1,176
Fair Value, Measurements, Recurring | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		6,829
Derivative liabilities	683	4,214
Fair Value, Measurements, Recurring | U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,610,502	1,676,788
Fair Value, Measurements, Recurring | Non-U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	188,199	191,776
Fair Value, Measurements, Recurring | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	170,567	231,555
Fair Value, Measurements, Recurring | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,165,393	2,202,267
Fair Value, Measurements, Recurring | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,263,517	1,292,502
Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	670,832	505,910
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,499,347	1,370,088
Equity securities	800,833	625,942
Other invested assets	0	0
Total investments	2,300,180	1,996,030
Senior notes	0	0
Other long-term debt	0
Fair Value, Measurements, Recurring | Level 1 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,499,347	1,370,088
Fair Value, Measurements, Recurring | Level 1 | Non-U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 1 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 1 | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 1 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	4,332,467	4,489,959
Equity securities	0	0
Other invested assets	0	0
Total investments	4,332,467	4,489,959
Senior notes	879,317	897,601
Other long-term debt	22,583
Fair Value, Measurements, Recurring | Level 2 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,274	6,254
Derivative liabilities	991	1,176
Fair Value, Measurements, Recurring | Level 2 | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		6,829
Derivative liabilities	683	4,214
Fair Value, Measurements, Recurring | Level 2 | U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	111,155	306,700
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	188,199	191,776
Fair Value, Measurements, Recurring | Level 2 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	170,567	231,555
Fair Value, Measurements, Recurring | Level 2 | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,165,393	2,202,267
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,081,734	1,145,164
Fair Value, Measurements, Recurring | Level 2 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	615,419	412,497
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	237,196	240,751
Equity securities	43,330	73,904
Other invested assets	812,543	764,081
Total investments	1,093,069	1,078,736
Senior notes	0	0
Other long-term debt	0
Fair Value, Measurements, Recurring | Level 3 | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 3 | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Derivative liabilities	0	0
Fair Value, Measurements, Recurring | Level 3 | U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 3 | Non-U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 3 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	0	0
Fair Value, Measurements, Recurring | Level 3 | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	181,783	147,338
Fair Value, Measurements, Recurring | Level 3 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	55,413	93,413
Fair Value, Measurements, Recurring | Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	6,069,010	6,100,798
Equity securities	844,163	699,846
Other invested assets	812,543	764,081
Total investments	7,725,716	7,564,725
Senior notes	879,317	897,601
Other long-term debt	22,583
Fair Value, Measurements, Recurring | Total Fair Value | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1,274	6,254
Derivative liabilities	991	1,176
Fair Value, Measurements, Recurring | Total Fair Value | Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		6,829
Derivative liabilities	683	4,214
Fair Value, Measurements, Recurring | Total Fair Value | U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,610,502	1,676,788
Fair Value, Measurements, Recurring | Total Fair Value | Non-U.S. Government and Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	188,199	191,776
Fair Value, Measurements, Recurring | Total Fair Value | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	170,567	231,555
Fair Value, Measurements, Recurring | Total Fair Value | Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	2,165,393	2,202,267
Fair Value, Measurements, Recurring | Total Fair Value | Mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	1,263,517	1,292,502
Fair Value, Measurements, Recurring | Total Fair Value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading fixed maturity investments	$670,832	$505,910

Fair_Value_Of_Financial_Instru3

Fair Value Of Financial Instruments (Schedule Of Fair Value Measurement Using Significant Unobservable Inputs Level Three) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	$764,081		$655,888
Realized and unrealized gains (losses) included in net income	64,566		83,621
Purchases	331,099		287,977
Sales	-347,203		-263,405
Transfers into Level 3 from Level 2	0		0
Transfers out of Level 3 to Level 2	0	[1]	0	[1]
Ending balance	812,543		764,081
Realized and unrealized gains (losses) included in net income for investments still held as of balance sheet date	-17,925		62,104
Mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	147,338		167,825
Realized and unrealized gains (losses) included in net income	5,505		-7,658
Purchases	100,019		83,455
Sales	-71,141		-92,985
Transfers into Level 3 from Level 2	1,505		5,393
Transfers out of Level 3 to Level 2	-1,443	[1]	-8,692	[1]
Ending balance	181,783		147,338
Realized and unrealized gains (losses) included in net income for investments still held as of balance sheet date	4,341		-4,914
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	93,413		62,246
Realized and unrealized gains (losses) included in net income	-2,068		-1,176
Purchases	4,628		50,669
Sales	-24,463		-28,393
Transfers into Level 3 from Level 2	28,813		10,067
Transfers out of Level 3 to Level 2	-44,910	[1]	0	[1]
Ending balance	55,413		93,413
Realized and unrealized gains (losses) included in net income for investments still held as of balance sheet date	-116		-360
Equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Opening balance	73,904		54,680
Realized and unrealized gains (losses) included in net income	-1,277		9,224
Purchases	0		10,000
Sales	-29,297		0
Transfers into Level 3 from Level 2	0		0
Transfers out of Level 3 to Level 2	0	[1]	0	[1]
Ending balance	43,330		73,904
Realized and unrealized gains (losses) included in net income for investments still held as of balance sheet date	($1,277)		$9,225
[1]	Transfers out of Level 3 are primarily attributable to the availability of market observable information.

Reserve_For_Losses_And_Loss_Ex2

Reserve For Losses And Loss Expenses (Schedule Of Reserve For Losses And Loss Expenses) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Insurance Loss Reserves [Abstract]
Outstanding loss reserves	$1,514,051	$1,520,867
Reserves for losses incurred but not reported	4,367,114	4,245,662
Reserve for losses and loss expenses	$5,881,165	$5,766,529	$5,645,549	$5,225,143

Reserve_For_Losses_And_Loss_Ex3

Reserve For Losses And Loss Expenses (Schedule Of Reconciliation Of Unpaid Losses And Loss Expenses) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Insurance Loss Reserves [Abstract]
Gross liability at beginning of year	$5,766,529	$5,645,549	$5,225,143
Reinsurance recoverable at beginning of year	-1,234,504	-1,141,110	-1,002,919
Net liability at beginning of year	4,532,025	4,504,439	4,222,224
Net losses incurred related to:
Current year	1,411,797	1,303,573	1,309,613
Prior years	-212,607	-180,331	-170,349
Total incurred	1,199,190	1,123,242	1,139,264
Net paid losses related to:
Current year	171,824	115,669	117,123
Prior years	1,001,537	973,987	743,844
Total paid	1,173,361	1,089,656	860,967
Foreign exchange revaluation	-16,945	-6,000	3,918
Net liability at end of year	4,540,909	4,532,025	4,504,439
Reinsurance recoverable at end of year	1,340,256	1,234,504	1,141,110
Gross liability at end of year	$5,881,165	$5,766,529	$5,645,549

Ceded_Reinsurance_Narrative_De

Ceded Reinsurance (Narrative) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	reinsurers
Concentration Risk [Line Items]
Percentage of reinsurance recoverable	100.00%	100.00%
Credit concentration risk | Premiums ceded
Concentration Risk [Line Items]
Number of ceded reinsurers	4
Concentration risk, percentage	42.00%	45.00%	48.00%
Credit concentration risk | Reinsurance recoverable | A- or better rating
Concentration Risk [Line Items]
Percentage of reinsurance recoverable	99.00%

Ceded_Reinsurance_Components_o

Ceded Reinsurance (Components of Reinsurance) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Reinsurance Disclosures [Abstract]
OSLR recoverable	$232,545	$225,796
IBNR recoverable	1,107,711	1,008,708
Reinsurance recoverable	1,340,256	1,234,504	1,141,110	1,002,919
Reinsurance recoverable on paid losses	$86,075	$76,099

Ceded_Reinsurance_Effects_of_R

Ceded Reinsurance (Effects of Reinsurance) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Disclosures [Abstract]
Direct premiums written									$1,996,831	$1,804,910	$1,569,021
Assumed premiums written									938,594	933,754	760,250
Ceded premiums written	-137,948	-139,142	-206,481	-129,779	-164,360	-127,816	-183,978	-142,029	-613,350	-618,183	-491,448
Net premiums written	427,795	568,742	553,924	771,614	391,130	453,077	581,222	695,052	2,322,075	2,120,481	1,837,823
Direct premiums earned									1,835,186	1,664,343	1,446,086
Assumed premiums earned									941,182	896,088	743,575
Ceded premiums earned									-593,610	-554,598	-440,763
Net premiums earned	573,489	541,737	537,247	530,285	524,561	510,773	507,271	463,228	2,182,758	2,005,833	1,748,898
Direct losses and loss expenses									1,031,284	936,084	877,961
Assumed losses and loss expenses									464,728	430,627	513,871
Ceded losses and loss expenses									-296,822	-243,469	-252,568
Net losses and loss expenses	$272,959	$336,090	$314,855	$275,286	$315,966	$276,970	$275,128	$255,178	$1,199,190	$1,123,242	$1,139,264

Ceded_Reinsurance_Credit_Risk_

Ceded Reinsurance (Credit Risk by Reinsurer) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Ceded Credit Risk [Line Items]
Reinsurance recoverable	$1,340,256	$1,234,504	$1,141,110	$1,002,919
Percentage of total	100.00%	100.00%
Top five reinsurers
Ceded Credit Risk [Line Items]
Reinsurance recoverable	727,682	655,489
Percentage of total	54.30%	53.10%
Top five reinsurers | A plus | Munich Re
Ceded Credit Risk [Line Items]
Reinsurance recoverable	285,918	214,070
Percentage of total	21.30%	17.30%
Top five reinsurers | A plus | Axis Capital
Ceded Credit Risk [Line Items]
Reinsurance recoverable	148,371	143,063
Percentage of total	11.10%	11.60%
Top five reinsurers | A plus | Arch Re
Ceded Credit Risk [Line Items]
Reinsurance recoverable	128,421	143,037
Percentage of total	9.60%	11.60%
Top five reinsurers | A | Markel
Ceded Credit Risk [Line Items]
Reinsurance recoverable	86,595	86,462
Percentage of total	6.50%	7.00%
Top five reinsurers | A | XL Group
Ceded Credit Risk [Line Items]
Reinsurance recoverable	78,377	68,857
Percentage of total	5.80%	5.60%
Other reinsurers' balances
Ceded Credit Risk [Line Items]
Reinsurance recoverable	$612,574	$579,015
Percentage of total	45.70%	46.90%

Funds_Held_Details

Funds Held (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effects of Reinsurance [Line Items]
Funds held	$724,021	$632,430
Assumed premiums written	938,594	933,754	760,250
Aeolus Capital Management [Member]
Effects of Reinsurance [Line Items]
Funds held	708,047	621,567
Assumed premiums written	$87,275	$83,300	$34,597

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, beginning of year	$268,376	$268,376
Additions	9,882
Goodwill, end of year	278,258	268,376
Indefinite-lived intangible assets, net carrying value	23,920	23,920	23,920
Finite-Lived Intangible Assets, Net [Abstract]
Gross carrying value	41,334	41,334	41,334
Accumulated amortization	-18,956	-16,423	-13,890
Net carrying value	22,378	24,911	27,444
Amortization of intangible assets	-2,533	-2,533
Goodwill, accumulated impairment loss			156
Intangible assets, accumulated impairment loss			6,866
Expected remaining useful life	8 years 9 months 18 days
North American Insurance
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, end of year	274,341
Reinsurance
Schedule Of Goodwill And Intangible Assets [Line Items]
Goodwill, end of year	$3,917

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets (Future amortization expense) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2015	$2,533
2016	2,533
2017	2,533
2018	2,533
2019	2,533
2020 and thereafter	9,713
Net carrying value	$22,378	$24,911	$27,444

Debt_and_Financing_Arrangement2

Debt and Financing Arrangements (Financing Structure) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
Components of financing structure:
Outstanding, total	$1,316,545	[1]	$1,594,017	[1]
Balance, debt	818,015	[2]	798,499	[2]
7.5% Senior notes due 2016
Components of financing structure:
Outstanding, debt	500,000	[1]	500,000	[1]
Balance, debt	499,690	[2]	499,512	[2]
5.5% Senior notes due 2020
Components of financing structure:
Outstanding, debt	300,000	[1]	300,000	[1]
Balance, debt	299,112	[2]	298,987	[2]
Swiss office building mortgage
Components of financing structure:
Outstanding, debt	14,157	[1]
Balance, debt	14,157	[2]
Swiss office building credit facility
Components of financing structure:
Outstanding, credit facility	5,056	[1]
Balance, credit facility	5,056	[1]
$1,000,000 secured letter of credit facility - uncommitted
Components of financing structure:
Outstanding, credit facility	497,332	[1]	707,915	[1]
Credit facility, maximum borrowing capacity	1,000,000		1,000,000
$150,000 secured letter of credit facility - committed
Components of financing structure:
Outstanding, credit facility	0	[1],[3]	86,102	[1]
Credit facility, maximum borrowing capacity	$150,000		$450,000
[1]	Indicates utilization of commitment amount, not drawn borrowings.
[2]	Represents the principal amount borrowed, net of unamortized discount.
[3]	The commitment was reduced from $450,000 to $150,000 during 2014.

Debt_and_Financing_Arrangement3

Debt and Financing Arrangements (Debt Maturities) (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Maturities of Long-term Debt [Abstract]
2015	$228
2016	500,303
2017	5,359
2018	303
2019	303
2020 and thereafter	312,717
Total	$819,213

Debt_and_Financing_Arrangement4

Debt and Financing Arrangements (Narrative) (Details)

12 Months Ended

12 Months Ended

12 Months Ended

In Thousands, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

USD ($)

USD ($)

USD ($)

Allied World Bermuda (Subsidiary Issuer)

7.5% Senior notes due 2016

5.5% Senior notes due 2020

$1,000,000 secured letter of credit facility - uncommitted

$1,000,000 secured letter of credit facility - uncommitted

$1,000,000 secured letter of credit facility - uncommitted

$1,000,000 secured letter of credit facility - uncommitted

$800,000 secured letter of credit facility [Member]

$400,000 secured letter of credit facility (pre-amendment)

$400,000 unsecured letter of credit facility

$150,000 secured letter of credit facility

$150,000 secured letter of credit facility

$150,000 secured letter of credit facility

Switzerland

Switzerland

Switzerland

Switzerland

Switzerland

USD ($)

Allied World Bermuda (Subsidiary Issuer)

Allied World Bermuda (Subsidiary Issuer)

USD ($)

USD ($)

Allied World Bermuda (Subsidiary Issuer)

Allied World Bermuda (Subsidiary Issuer)

Allied World Bermuda (Subsidiary Issuer)

Allied World Bermuda (Subsidiary Issuer)

Allied World Bermuda (Subsidiary Issuer)

USD ($)

USD ($)

Allied World Bermuda (Subsidiary Issuer)

Swiss office building mortgage

Swiss office building mortgage

Swiss office building credit facility

Swiss office building credit facility

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

CHF

USD ($)

CHF

Debt Instrument [Line Items]

Senior Notes, face amount

$500,000

$300,000

Debt Instrument, interest rate

7.50%

5.50%

3.20%

3.20%

2.50%

2.50%

Senior Notes, maturity date

1-Aug-16

15-Nov-20

Senior Notes, frequency of periodic payment

semi-annually

semi-annually

Senior Notes, date of first required payment

1-Feb-07

15-May-11

Proceeds from other long-term debt

19,213

0

0

0

14,157

14,000

Additional borrowing capacity

4,044

4,000

Annual principal payment

303

300

Number of annual principal payments required

19 years

Line of Credit Facility, Initiation Date

27-Nov-07

7-Jun-12

Line of Credit Facility, Maximum Borrowing Capacity

1,000,000

1,000,000

1,000,000

1,000,000

800,000

400,000

400,000

150,000

450,000

450,000

5,056

5,000

Debt instrument term

5 years

4 years

20 years

20 years

3 years

3 years

Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases

150,000

Additional commitments subject to lender approval

$150,000

Income_Taxes_Components_of_Inc

Income Taxes (Components of Income Taxes) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current income tax expense									$26,768	$21,669	$18,239
Deferred income tax expense (benefit)									3,755	-11,889	201
Income tax expense	$6,223	$1,532	$6,195	$16,573	$3,448	$4,971	($4,072)	$5,433	$30,523	$9,780	$18,440

Income_Taxes_Sources_of_Income

Income Taxes (Sources of Income) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes	$136,732	$32,447	$158,058	$193,545	$141,355	$127,814	($5,934)	$164,425	$520,782	$427,660	$511,447
Non-U.S.
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes									447,875	401,778	474,412
United States
Income Before Income Tax Domestic And Foreign [Line Items]
Income before income taxes									$72,907	$25,882	$37,035

Income_Taxes_Deferred_Tax_Comp

Income Taxes (Deferred Tax Components) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Increase in valuation allowance	$1,221
Deferred tax assets:
Reserve for losses and loss expenses	21,644	22,567
Equity compensation	15,316	15,828
Unearned premium	16,921	14,299
Deferred acquisition costs	11,184	10,604
Other-than-temporary impairments	0	407
Net loss carryforward	7,958	7,642
Total deferred tax assets	73,023	71,347
Deferred tax liabilities:
Intangible assets	-13,856	-14,743
Realized gains	-12,819	-6,980
Depreciation	-2,513	-2,942
Unrealized appreciation and timing differences on investments	-94	-56
Market discount on bonds	-740	-271
Other	-2,965	-1,244
Total deferred tax liabilities	-32,987	-26,236
Net deferred taxes before valuation allowance	40,036	45,111
Valuation allowance	-6,421	-7,642
Net deferred tax assets	$33,615	$37,469

Income_Taxes_Current_Taxes_Det

Income Taxes (Current Taxes) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Income Tax Disclosure [Abstract]
Current tax receivable	$8,640	$11,684
Current tax payable	$829	$1,452

Income_Taxes_Reconciliation_of

Income Taxes (Reconciliation of Effective Tax Rate) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Expected tax rate	7.80%	7.80%	7.80%
Income not subject to income tax	-7.10%	-8.20%	-7.30%
Foreign taxes at local expected tax rates	4.40%	1.40%	2.10%
Disallowed expenses and capital allowances	0.20%	1.00%	0.70%
Prior year refunds and adjustments	1.20%	0.80%	0.40%
Other	-0.60%	-0.50%	-0.10%
Effective tax rate	5.90%	2.30%	3.60%

Shareholders_Equity_Narrative_

Shareholders' Equity (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 09, 2013	1-May-14	2-May-13
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	2,702,196
Treasury Stock, Shares, Acquired	4,906,785	5,544,432	10,967,877
Common stock, shares outstanding	96,195,482	100,253,646
Shares cancelled, shares	2,702,196	5,632,152
Maximum percentage of registered capital in treasury shares	10.00%
Allied World Switzerland (Parent Guarantor)
Class of Stock [Line Items]
Threshold for dividend restrictions	20.00%
Minimum Annual Profit Percentage Required	5.00%
Nonvoting common stock
Class of Stock [Line Items]
Shares cancelled, shares				87,720
Common shares repurchased and constructively retired
Class of Stock [Line Items]
Shares cancelled, shares				4,616,058
Common shares
Class of Stock [Line Items]
Dividends declared per share					$0.23	$0.17

Shareholders_Equity_Issued_Sha

Shareholders' Equity (Issued Share Capital) (Details)	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified	USD ($)	CHF	USD ($)	CHF
Stockholders' Equity Note [Abstract]
Common shares issued and fully paid, 2014 and 2013: CHF 4.10 per share	100,775,256	100,775,256	103,477,452	103,477,452	109,109,604
Common shares, par value		4.1		4.1
Total share capital	$408,020		$418,988

Shareholders_Equity_Schedule_O

Shareholders' Equity (Schedule Of Common Stock Outstanding Roll Forward) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares issued at beginning of year	103,477,452	109,109,604
Shares cancelled, shares	-2,702,196	-5,632,152
Shares issued at end of year	100,775,256	103,477,452	109,109,604
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Treasury shares issued at beginning of year	3,223,806	4,716,261
Shares repurchased, shares	4,906,785	5,544,432	10,967,877
Shares issued out of treasury	-848,621	-1,404,735
Shares cancelled, shares	-2,702,196	-5,632,152
Total treasury shares at end of year	4,579,774	3,223,806	4,716,261
Total shares outstanding at end of year	96,195,482	100,253,646

Shareholders_Equity_Schedule_O1

Shareholders' Equity (Schedule Of Dividends Paid) (Details)

12 Months Ended

0 Months Ended

In Thousands, except Per Share data, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Oct. 02, 2014

Jul. 02, 2014

Apr. 03, 2014

Jan. 02, 2014

Oct. 03, 2013

Jul. 03, 2013

Mar. 12, 2013

Mar. 12, 2013

Dec. 18, 2012

Dec. 18, 2012

Sep. 25, 2012

Sep. 25, 2012

Aug. 06, 2012

Aug. 06, 2012

Apr. 06, 2012

Apr. 06, 2012

Jan. 06, 2012

Jan. 06, 2012

USD ($)

USD ($)

USD ($)

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

Common shares

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

Par Value Reduction

USD ($)

CHF

USD ($)

CHF

USD ($)

CHF

USD ($)

CHF

USD ($)

CHF

USD ($)

CHF

Dividends, Common Stock [Abstract]

Dividends paid per common share

$0.78

$0.46

$0.63

$0.23

$0.23

$0.17

$0.17

$0.17

$0.17

$0.13

0.11

$0.13

0.11

$0.13

0.12

$0.13

0.12

$0.13

0.11

$0.13

0.12

Payment of quarterly dividends

$21,686

$21,870

$16,495

$16,732

$16,952

$17,117

$12,981

$13,010

$13,324

$13,394

$13,795

$14,302

Shareholders_Equity_Schedule_O2

Shareholders' Equity (Schedule Of Share Repurchases) (Details) (USD $)	0 Months Ended	12 Months Ended
In Thousands, except Share data, unless otherwise specified	1-May-14	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholders' Equity Note [Abstract]
Stock repurchase program, authorized amount	$500,000
Treasury shares reserved for issuance under stock compensation plans	3,000,000
Shares repurchased, shares		4,906,785	5,544,432	10,967,877
Cost of shares repurchased		$175,373	$174,686	$263,942
Shares repurchased, average price per share		$35.74	$31.51	$24.07

Employee_Benefit_Plans_Options

Employee Benefit Plans (Options) (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning of year	2,928,312
Granted	0	0	0
Exercised	-479,831	-674,187	-801,810
Forfeited	-15,096
Expired	-6,711
Outstanding, end of year	2,426,674	2,928,312
Exercisable, end of year	2,151,790
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding, beginning of year - weighted average exercise price	$16.07
Granted - weighted average exercise price	$0
Exercised - weighted average exercise price	($14.25)
Forfeited - weighted average exercise price	($20.50)
Expired - weighted average exercise price	($16.35)
Outstanding, end of year - weighted average exercise price	$16.40	$16.07
Exercisable, end of year - weighted average exercise price	$15.88
Additional disclosures:
Outstanding, end of year, weighted average contractual term	4 years 8 months 12 days
Exercisable, end of year, weighted average contractual term	4 years 6 months 0 days
Outstanding, end of year, aggregate intrinsic value	$52,212
Exercisable, end of year, aggregate intrinsic value	47,424
Weighted average grant date fair value	$0	$0	$0
Total intrinsic value of options exercised	10,313	12,212	9,650
Proceeds from option exercises	10,027	12,137	10,527
Stock options
Additional disclosures:
Total compensation cost not yet recognized	$299
Compensation cost not yet recognized, period for recognition	0 years 1 month 22 days
Life of award	10 years
Vesting period	4 years

Employee_Benefit_Plans_Restric

Employee Benefit Plans (Restricted Stock Units) (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
RSUs and performance-based equity awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Aggregate intrinsic value of vested awards	$14,366	$28,836	$13,947
Vesting period	4 years
Rsus Other Than Performance Based Rsus
Activity:
Outstanding, beginning of year	143,697
Granted, shares	454,176
Forfeited, shares	-16,386
Fully vested, shares	-78,981
Outstanding, end of year	502,506
Weighted average grant date fair value:
Outstanding, beginning of year - weighted average grant date fair value	$21.69
Granted - weighted average grant date fair value	$33.56
Forfeited - weighted average grant date fair value	($31.22)
Vested - weighted average grant date fair value	($21.76)
Outstanding, end of year - weighted average grant date fair value	$32.10
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Aggregate intrinsic value	19,055
Total compensation cost not yet recognized	12,057
Compensation cost not yet recognized, period for recognition	1 year 11 months 8 days
Performance Based Rsus
Activity:
Outstanding, beginning of year	804,519
Granted, shares	166,302
Additional awards granted due to achievement of performance criteria, shares	104,895
Forfeited, shares	-4,635
Fully vested, shares	-454,440
Outstanding, end of year	616,641	804,519
Weighted average grant date fair value:
Outstanding, beginning of year - weighted average grant date fair value	$23.21
Granted - weighted average grant date fair value	$33.56
Additional awards granted due to achievement of performance criteria - weighted average grant date fair value	$20.50
Forfeited - weighted average grant date fair value	($25.20)
Vested - weighted average grant date fair value	($20.50)
Outstanding, end of year - weighted average grant date fair value	$27.52	$23.21
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Aggregate intrinsic value	23,383
Total compensation cost not yet recognized	$5,814
Compensation cost not yet recognized, period for recognition	1 year 7 months 20 days
The rate at which performance awards are expensed	100.00%	110.00%	135.00%

Employee_Benefit_Plans_CashEqu

Employee Benefit Plans (Cash-Equivalent Stock Awards) (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Accounts payable and accrued liabilities	$181,622	$170,225
Cash-equivalent stock awards
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Accounts payable and accrued liabilities	$51,343
Cash Equivalent Stock Awards Rsus [Member]
Activity:
Outstanding, beginning of year	2,049,084
Granted, shares	438,162
Additional awards granted due to achievement of performance criteria, shares	0
Forfeited, shares	-77,322
Fully vested, shares	-755,860
Outstanding, end of year	1,654,064
Weighted average grant date fair value:
Outstanding, beginning of year - weighted average grant date fair value	$24.69
Granted - weighted average grant date fair value	$33.56
Additional awards granted due to achievement of performance criteria - weighted average grant date fair value	$0
Forfeited - weighted average grant date fair value	($27.12)
Vested - weighted average grant date fair value	($22.51)
Outstanding, end of year - weighted average grant date fair value	$27.99
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Vesting period	4 years
Cash Equivalent Stock Awards Performance Based [Member]
Activity:
Outstanding, beginning of year	1,031,961
Granted, shares	249,438
Additional awards granted due to achievement of performance criteria, shares	104,895
Forfeited, shares	-6,948
Fully vested, shares	-454,440
Outstanding, end of year	924,906
Weighted average grant date fair value:
Outstanding, beginning of year - weighted average grant date fair value	$23.67
Granted - weighted average grant date fair value	$33.56
Additional awards granted due to achievement of performance criteria - weighted average grant date fair value	$20.50
Forfeited - weighted average grant date fair value	($25.20)
Vested - weighted average grant date fair value	($20.50)
Outstanding, end of year - weighted average grant date fair value	$27.52
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Vesting period	3 years

Employee_Benefit_Plans_Other_D

Employee Benefit Plans (Other Details) (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$52,034	$64,847	$50,402
Expense related to defined contribution arrangements	10,648	9,287	6,602
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	1,949	3,269	4,256
Vesting period	4 years
RSUs and performance-based equity awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	12,295	10,700	13,010
Vesting period	4 years
LTIP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	0	0	376
Cash-equivalent stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$37,790	$50,878	$32,760
2012 Omnibus Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	4,500,000
2012 Omnibus Plan | Awards other than performance-based
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
2012 Omnibus Plan | Performance-based awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Employee Share Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share purchase plan, discount from fair market value	15.00%
Number of shares authorized	3,000,000

Earnings_Per_Share_Narrative_D

Earnings Per Share (Narrative) (Details) (Employee stock options and RSUs)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee stock options and RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share	0	0	678,312

Earnings_Per_Share_Comparison_

Earnings Per Share (Comparison Of Basic And Diluted Earnings Per Share) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic earnings per share:
Net income	$130,509	$30,915	$151,863	$176,972	$137,907	$122,843	($1,862)	$158,992	$490,259	$417,880	$493,007
Weighted average common shares outstanding	96,386,796	96,458,231	97,809,639	99,545,187	100,814,955	101,974,077	103,267,659	103,840,818	97,538,319	102,464,715	108,171,435
Basic earnings per share	$1.35	$0.32	$1.55	$1.78	$1.37	$1.20	($0.02)	$1.53	$5.03	$4.08	$4.56
Diluted earnings per share:
Net income	$130,509	$30,915	$151,863	$176,972	$137,907	$122,843	($1,862)	$158,992	$490,259	$417,880	$493,007
Weighted average common shares outstanding	96,386,796	96,458,231	97,809,639	99,545,187	100,814,955	101,974,077	103,267,659	103,840,818	97,538,319	102,464,715	108,171,435
Share equivalents:
Warrants and options									1,432,960	1,536,939	1,306,002
RSUs and LTIP awards									605,808	864,180	1,727,265
Employee share purchase plan									14,686	0	4,953
Weighted average common shares and common share equivalents outstanding - diluted	98,394,432	98,444,238	99,724,802	101,584,662	103,129,812	104,184,579	103,267,659	106,295,529	99,591,773	104,865,834	111,209,655
Diluted earnings per share	$1.33	$0.31	$1.52	$1.74	$1.34	$1.18	($0.02)	$1.50	$4.92	$3.98	$4.43

Commitments_And_Contingencies_1

Commitments And Contingencies (Operating Leases) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	$12,568
2016	16,903
2017	20,596
2018	20,096
2019	19,013
2020 and thereafter	138,602
Total	227,778
Other details:
Total rent expense	18,950	11,392	11,929
Accelerated rent expense due to early termination of lease	$963

Commitments_And_Contingencies_2

Commitments And Contingencies (Credit Concentration Disclosure) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Concentration Risk [Line Items]
Number of custodians	1	1
Gross written premium | Credit concentration risk | Marsh & McLennan Companies, Inc.
Concentration Risk [Line Items]
Concentration risk, percentage	25.00%	26.00%	25.00%
Gross written premium | Credit concentration risk | Aon Corporation
Concentration Risk [Line Items]
Concentration risk, percentage	17.00%	18.00%	19.00%
Gross written premium | Credit concentration risk | Willis Group Holdings
Concentration Risk [Line Items]
Concentration risk, percentage	10.00%	12.00%	11.00%

Statutory_Capital_and_Surplus_1

Statutory Capital and Surplus (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statutory Accounting Practices [Line Items]
Amount of restricted net assets for the Company's consolidated subsidiaries	$2,953,535
Bermuda
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	3,273,730	3,208,776
Minimum Required Statutory Capital and Surplus	876,235	806,012
Statutory Net Income (Loss)	487,051	467,283	528,712
Statutory Amount Available for Dividend Payments	818,433
United States
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	1,220,299	1,187,023
Minimum Required Statutory Capital and Surplus	226,876	205,699
Statutory Net Income (Loss)	45,445	36,744	25,390
Statutory Amount Available for Dividend Payments	102,283
Ireland
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	410,841	407,600
Minimum Required Statutory Capital and Surplus	29,204	27,098
Statutory Net Income (Loss)	3,241	-4,880	9,325
Statutory Amount Available for Dividend Payments	21,637
Switzerland
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	172,306	167,840
Minimum Required Statutory Capital and Surplus	19,943	22,905
Statutory Net Income (Loss)	7,408	-6,418	10,657
Statutory Amount Available for Dividend Payments	6,323
United Kingdom
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	231,659	229,745
Minimum Required Statutory Capital and Surplus	203,742	229,689
Statutory Net Income (Loss)	-7,819	9,362	1,800
Statutory Amount Available for Dividend Payments	$1,601

Segment_Information_Summary_Of

Segment Information (Summary Of Segment Results) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
									segments
Segment Reporting Information [Line Items]
Gross premiums written	$565,743	$707,884	$760,405	$901,393	$555,490	$580,893	$765,200	$837,081	$2,935,425	$2,738,664	$2,329,271
Net premiums written	427,795	568,742	553,924	771,614	391,130	453,077	581,222	695,052	2,322,075	2,120,481	1,837,823
Net premiums earned	573,489	541,737	537,247	530,285	524,561	510,773	507,271	463,228	2,182,758	2,005,833	1,748,898
Net losses and loss expenses	-272,959	-336,090	-314,855	-275,286	-315,966	-276,970	-275,128	-255,178	-1,199,190	-1,123,242	-1,139,264
Acquisition costs	-80,718	-72,403	-74,279	-67,722	-66,257	-65,114	-64,617	-56,685	-295,122	-252,673	-205,722
General and administrative expenses	-100,914	-88,294	-96,188	-80,340	-100,467	-88,553	-80,585	-82,680	-365,736	-352,285	-307,321
Underwriting income									322,710	277,633	96,591
Other insurance-related revenue	1,032	1,032	0	0					2,064	0	0
Other insurance-related expenses	-2,003	-6,575	0	0					-8,578	0	0
Segment income									316,196	277,633	96,591
Net investment income	49,050	43,412	36,793	47,619	47,270	39,271	37,635	33,388	176,874	157,564	167,141
Net realized investment gains (losses)	-15,329	-35,136	85,217	54,205	67,599	27,487	-115,198	79,637	88,957	59,525	306,436
Amortization of intangible assets	-633	-633	-634	-633	-633	-633	-634	-633	-2,533	-2,533	-2,533
Interest expense	-14,304	-14,325	-14,592	-14,534	-14,094	-14,094	-14,188	-14,134	-57,755	-56,510	-55,405
Foreign exchange (loss) gain	21	-278	-651	-49	-658	-4,353	-490	-2,518	-957	-8,019	-783
Income before income taxes	136,732	32,447	158,058	193,545	141,355	127,814	-5,934	164,425	520,782	427,660	511,447
Loss and loss expense ratio									54.90%	56.00%	65.10%
Acquisition cost ratio									13.50%	12.60%	11.80%
General and administrative expense ratio									16.80%	17.60%	17.60%
Expense ratio									30.30%	30.20%	29.40%
Combined ratio									85.20%	86.20%	94.50%
Number of operating segments									3
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Gross premiums written									1,716,288	1,572,355	1,376,040
Net premiums written									1,230,839	1,082,441	964,199
Net premiums earned									1,111,164	1,022,979	892,721
Net losses and loss expenses									-683,836	-651,267	-608,762
Acquisition costs									-105,914	-94,869	-77,701
General and administrative expenses									-219,665	-209,042	-185,231
Underwriting income									101,749	67,801	21,027
Other insurance-related revenue									2,064	0	0
Other insurance-related expenses									-1,854	0	0
Segment income									101,959	67,801	21,027
Loss and loss expense ratio									61.50%	63.70%	68.20%
Acquisition cost ratio									9.50%	9.30%	8.70%
General and administrative expense ratio									19.80%	20.40%	20.70%
Expense ratio									29.30%	29.70%	29.40%
Combined ratio									90.80%	93.40%	97.60%
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Gross premiums written									280,543	232,555	192,981
Net premiums written									187,997	145,002	125,577
Net premiums earned									162,566	126,008	115,660
Net losses and loss expenses									-61,057	-50,395	-19,536
Acquisition costs									-18,222	-10,051	-8,483
General and administrative expenses									-68,106	-63,164	-48,365
Underwriting income									15,181	2,398	39,276
Other insurance-related revenue									0	0	0
Other insurance-related expenses									-6,724	0	0
Segment income									8,457	2,398	39,276
Loss and loss expense ratio									37.60%	40.00%	16.90%
Acquisition cost ratio									11.20%	8.00%	7.30%
General and administrative expense ratio									41.90%	50.10%	41.80%
Expense ratio									53.10%	58.10%	49.10%
Combined ratio									90.70%	98.10%	66.00%
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written									938,594	933,754	760,250
Net premiums written									903,239	893,038	748,047
Net premiums earned									909,028	856,846	740,517
Net losses and loss expenses									-454,297	-421,580	-510,966
Acquisition costs									-170,986	-147,753	-119,538
General and administrative expenses									-77,965	-80,079	-73,725
Underwriting income									205,780	207,434	36,288
Other insurance-related revenue									0	0	0
Other insurance-related expenses									0	0	0
Segment income									$205,780	$207,434	$36,288
Loss and loss expense ratio									50.00%	49.20%	69.00%
Acquisition cost ratio									18.80%	17.20%	16.10%
General and administrative expense ratio									8.60%	9.30%	10.00%
Expense ratio									27.40%	26.50%	26.10%
Combined ratio									77.40%	75.70%	95.10%

Segment_Information_Gross_Prem

Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$565,743	$707,884	$760,405	$901,393	$555,490	$580,893	$765,200	$837,081	$2,935,425	$2,738,664	$2,329,271
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,795,564	1,636,010	1,360,190
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									640,948	676,174	611,382
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									318,626	264,867	228,791
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									167,334	161,613	128,908
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									12,953	0	0
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$67,564	$61,042	$43,664

Segment_Information_Net_Premiu

Segment Information (Net Premiums Earned by Line of Business) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net premiums earned	$573,489	$541,737	$537,247	$530,285	$524,561	$510,773	$507,271	$463,228	$2,182,758	$2,005,833	$1,748,898
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Net premiums earned									1,111,164	1,022,979	892,721
Operating segments | North American Insurance | General casualty
Segment Reporting Information [Line Items]
Net premiums earned									334,603	269,114	211,238
Operating segments | North American Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned									276,105	259,215	245,583
Operating segments | North American Insurance | Healthcare
Segment Reporting Information [Line Items]
Net premiums earned									168,476	194,344	187,053
Operating segments | North American Insurance | General property
Segment Reporting Information [Line Items]
Net premiums earned									126,646	141,896	137,002
Operating segments | North American Insurance | Programs
Segment Reporting Information [Line Items]
Net premiums earned									119,756	107,870	88,180
Operating segments | North American Insurance | Inland marine
Segment Reporting Information [Line Items]
Net premiums earned									43,666	25,939	10,612
Operating segments | North American Insurance | Environmental
Segment Reporting Information [Line Items]
Net premiums earned									19,507	17,017	12,627
Operating segments | North American Insurance | Other
Segment Reporting Information [Line Items]
Net premiums earned									22,405	7,584	426
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Net premiums earned									162,566	126,008	115,660
Operating segments | Global Markets Insurance | General casualty
Segment Reporting Information [Line Items]
Net premiums earned									15,266	13,903	16,762
Operating segments | Global Markets Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned									57,176	45,967	43,621
Operating segments | Global Markets Insurance | Healthcare
Segment Reporting Information [Line Items]
Net premiums earned									11,963	11,154	9,055
Operating segments | Global Markets Insurance | General property
Segment Reporting Information [Line Items]
Net premiums earned									33,957	31,364	36,751
Operating segments | Global Markets Insurance | Other
Segment Reporting Information [Line Items]
Net premiums earned									4,239	0	0
Operating segments | Global Markets Insurance | Aviation
Segment Reporting Information [Line Items]
Net premiums earned									22,342	9,521	0
Operating segments | Global Markets Insurance | Trade credit
Segment Reporting Information [Line Items]
Net premiums earned									17,623	14,099	9,471
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									909,028	856,846	740,517
Operating segments | Reinsurance | Property
Segment Reporting Information [Line Items]
Net premiums earned									411,968	336,009	273,167
Operating segments | Reinsurance | Specialty
Segment Reporting Information [Line Items]
Net premiums earned									321,538	349,044	283,415
Operating segments | Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned									$175,522	$171,793	$183,935

Condensed_Consolidated_Guarant2

Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
In Thousands, unless otherwise specified
ASSETS:
Investments	$7,868,682	$7,712,036
Cash and cash equivalents	589,339	531,936	681,879	633,996	633,996
Insurance balances receivable	664,815	588,632
Funds held	724,021	632,430
Reinsurance recoverable	1,340,256	1,234,504	1,141,110	1,002,919
Reinsurance recoverable on paid losses	86,075	76,099
Net deferred acquisition costs	151,546	126,661
Goodwill and intangible assets	324,556	317,207
Balances receivable on sale of investments	47,149	76,544
Investments in subsidiaries	0	0
Due from subsidiaries	0	0
Other assets	625,124	649,781
Total assets	12,421,563	11,945,830
LIABILITIES:
Reserve for losses and loss expenses	5,881,165	5,766,529	5,645,549	5,225,143
Unearned premiums	1,555,313	1,396,256
Reinsurance balances payable	180,060	173,023
Balances due on purchases of investments	5,428	104,740
Senior notes	798,802	798,499
Other long-term debt	19,213	0
Due to subsidiaries	0	0
Other liabilities	203,291	186,957
Total liabilities	8,643,272	8,426,004
Total shareholders' equity	3,778,291	3,519,826	3,326,335	3,149,022
Total liabilities and shareholders' equity	12,421,563	11,945,830
Consolidating Adjustments
ASSETS:
Investments	0	0
Cash and cash equivalents	0	0	0	0
Insurance balances receivable	0	0
Funds held	0	0
Reinsurance recoverable	0	0
Reinsurance recoverable on paid losses		0
Net deferred acquisition costs	0	0
Goodwill and intangible assets	0	0
Balances receivable on sale of investments	0	0
Investments in subsidiaries	-7,847,329	-7,431,620
Due from subsidiaries	-180,658	-357,497
Other assets	0	0
Total assets	-8,027,987	-7,789,117
LIABILITIES:
Reserve for losses and loss expenses	0	0
Unearned premiums	0	0
Reinsurance balances payable	0	0
Balances due on purchases of investments	0	0
Senior notes	0	0
Other long-term debt	0
Due to subsidiaries	-180,658	-357,497
Other liabilities	0	0
Total liabilities	-180,658	-357,497
Total shareholders' equity	-7,847,329	-7,431,620
Total liabilities and shareholders' equity	-8,027,987	-7,789,117
Allied World Switzerland (Parent Guarantor)
ASSETS:
Investments	0	0
Cash and cash equivalents	32,579	10,790	19,997	112,672
Insurance balances receivable	0	0
Funds held	0	0
Reinsurance recoverable	0	0
Reinsurance recoverable on paid losses	0	0
Net deferred acquisition costs	0	0
Goodwill and intangible assets	0	0
Balances receivable on sale of investments	0	0
Investments in subsidiaries	3,629,301	3,413,001
Due from subsidiaries	147,072	111,172
Other assets	1,470	1,757
Total assets	3,810,422	3,536,720
LIABILITIES:
Reserve for losses and loss expenses	0	0
Unearned premiums	0	0
Reinsurance balances payable	0	0
Balances due on purchases of investments	0	0
Senior notes	0	0
Other long-term debt	0
Due to subsidiaries	7,599	12,945
Other liabilities	24,532	3,949
Total liabilities	32,131	16,894
Total shareholders' equity	3,778,291	3,519,826
Total liabilities and shareholders' equity	3,810,422	3,536,720
Allied World Bermuda (Subsidiary Issuer)
ASSETS:
Investments	0	0
Cash and cash equivalents	1,734	2,775	11,324	8,886
Insurance balances receivable	0	0
Funds held	0	0
Reinsurance recoverable	0	0
Reinsurance recoverable on paid losses	0	0
Net deferred acquisition costs	0	0
Goodwill and intangible assets	0	0
Balances receivable on sale of investments	0	0
Investments in subsidiaries	4,218,028	4,018,619
Due from subsidiaries	19,190	122,846
Other assets	3,192	4,671
Total assets	4,242,144	4,148,911
LIABILITIES:
Reserve for losses and loss expenses	0	0
Unearned premiums	0	0
Reinsurance balances payable	0	0
Balances due on purchases of investments		0
Senior notes	798,802	798,499
Other long-term debt	0
Due to subsidiaries	6,797	110,534
Other liabilities	19,618	17,797
Total liabilities	825,217	926,830
Total shareholders' equity	3,416,927	3,222,081
Total liabilities and shareholders' equity	4,242,144	4,148,911
Other Allied World Subsidiaries
ASSETS:
Investments	7,868,682	7,712,036
Cash and cash equivalents	555,026	518,371	650,558	512,438
Insurance balances receivable	664,815	588,632
Funds held	724,021	632,430
Reinsurance recoverable	1,340,256	1,234,504
Reinsurance recoverable on paid losses	86,075	76,099
Net deferred acquisition costs	151,546	126,661
Goodwill and intangible assets	324,556	317,207
Balances receivable on sale of investments	47,149	76,544
Investments in subsidiaries	0	0
Due from subsidiaries	14,396	123,479
Other assets	620,462	643,353
Total assets	12,396,984	12,049,316
LIABILITIES:
Reserve for losses and loss expenses	5,881,165	5,766,529
Unearned premiums	1,555,313	1,396,256
Reinsurance balances payable	180,060	173,023
Balances due on purchases of investments	5,428	104,740
Senior notes	0	0
Other long-term debt	19,213
Due to subsidiaries	166,262	234,018
Other liabilities	159,141	165,211
Total liabilities	7,966,582	7,839,777
Total shareholders' equity	4,430,402	4,209,539
Total liabilities and shareholders' equity	$12,396,984	$12,049,316

Condensed_Consolidated_Guarant3

Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$573,489	$541,737	$537,247	$530,285	$524,561	$510,773	$507,271	$463,228	$2,182,758	$2,005,833	$1,748,898
Net investment income	49,050	43,412	36,793	47,619	47,270	39,271	37,635	33,388	176,874	157,564	167,141
Net realized investment gains (losses)	-15,329	-35,136	85,217	54,205	67,599	27,487	-115,198	79,637	88,957	59,525	306,436
Other income	1,032	1,032	0	0					2,064	0	0
Net losses and loss expenses	-272,959	-336,090	-314,855	-275,286	-315,966	-276,970	-275,128	-255,178	-1,199,190	-1,123,242	-1,139,264
Acquisition costs	-80,718	-72,403	-74,279	-67,722	-66,257	-65,114	-64,617	-56,685	-295,122	-252,673	-205,722
General and administrative expenses	-100,914	-88,294	-96,188	-80,340	-100,467	-88,553	-80,585	-82,680	-365,736	-352,285	-307,321
Other expense	-2,003	-6,575	0	0					-8,578	0	0
Amortization of intangible assets	-633	-633	-634	-633	-633	-633	-634	-633	-2,533	-2,533	-2,533
Interest expense	-14,304	-14,325	-14,592	-14,534	-14,094	-14,094	-14,188	-14,134	-57,755	-56,510	-55,405
Foreign exchange (loss) gain	21	-278	-651	-49	-658	-4,353	-490	-2,518	-957	-8,019	-783
Income tax (expense) benefit	-6,223	-1,532	-6,195	-16,573	-3,448	-4,971	4,072	-5,433	-30,523	-9,780	-18,440
Equity in earnings of consolidated subsidiaries									0	0	0
NET INCOME	130,509	30,915	151,863	176,972	137,907	122,843	-1,862	158,992	490,259	417,880	493,007
Unrealized losses on investments arising during the year net of applicable deferred income tax benefit 2014: $0; 2013: $0; 2012: $(265)									0	0	-493
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax											-13,991
Other comprehensive loss									0	0	-14,484
COMPREHENSIVE INCOME (LOSS)									490,259	417,880	478,523
Parenthetical disclosures:
Deferred income tax (benefit) expense for unrealized gains on investments arising during the period									0	0	-265
Consolidating Adjustments
Condensed Income Statements, Captions [Line Items]
Net premiums earned									0	0	0
Net investment income									0	0	0
Net realized investment gains (losses)									0	0	0
Other income									0	0	0
Net losses and loss expenses									0	0	0
Acquisition costs									0	0	0
General and administrative expenses									0	0	0
Other expense									0	0	0
Amortization of intangible assets									0	0	0
Interest expense									0	0	0
Foreign exchange (loss) gain									0	0	0
Income tax (expense) benefit									0	0	0
Equity in earnings of consolidated subsidiaries									-1,090,522	-974,784	-1,100,978
NET INCOME									-1,090,522	-974,784	-1,100,978
Unrealized losses on investments arising during the year net of applicable deferred income tax benefit 2014: $0; 2013: $0; 2012: $(265)											493
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax											13,991
Other comprehensive loss									0	0	14,484
COMPREHENSIVE INCOME (LOSS)									-1,090,522	-974,784	-1,086,494
Allied World Switzerland (Parent Guarantor)
Condensed Income Statements, Captions [Line Items]
Net premiums earned									0	0	0
Net investment income									50	13	26
Net realized investment gains (losses)									0	0	0
Other income									0	0	0
Net losses and loss expenses									0	0	0
Acquisition costs									0	0	0
General and administrative expenses									-35,596	-42,122	-35,258
Other expense									0	0	0
Amortization of intangible assets									0	0	0
Interest expense									0	0	0
Foreign exchange (loss) gain									20	263	159
Income tax (expense) benefit									-89	98	358
Equity in earnings of consolidated subsidiaries									525,874	459,628	527,722
NET INCOME									490,259	417,880	493,007
Unrealized losses on investments arising during the year net of applicable deferred income tax benefit 2014: $0; 2013: $0; 2012: $(265)											-493
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax											-13,991
Other comprehensive loss									0	0	-14,484
COMPREHENSIVE INCOME (LOSS)									490,259	417,880	478,523
Allied World Bermuda (Subsidiary Issuer)
Condensed Income Statements, Captions [Line Items]
Net premiums earned									0	0	0
Net investment income									0	4	20
Net realized investment gains (losses)									0	0	0
Other income									0	0	0
Net losses and loss expenses									0	0	0
Acquisition costs									0	0	0
General and administrative expenses									-3,088	-1,320	-2,973
Other expense									0	0	0
Amortization of intangible assets									0	0	0
Interest expense									-55,418	-55,347	-55,405
Foreign exchange (loss) gain									104	-1,045	-319
Income tax (expense) benefit									0	0	0
Equity in earnings of consolidated subsidiaries									564,648	515,156	573,256
NET INCOME									506,246	457,448	514,579
Unrealized losses on investments arising during the year net of applicable deferred income tax benefit 2014: $0; 2013: $0; 2012: $(265)											0
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax											0
Other comprehensive loss									0	0	0
COMPREHENSIVE INCOME (LOSS)									506,246	457,448	514,579
Other Allied World Subsidiaries
Condensed Income Statements, Captions [Line Items]
Net premiums earned									2,182,758	2,005,833	1,748,898
Net investment income									176,824	157,547	167,095
Net realized investment gains (losses)									88,957	59,525	306,436
Other income									2,064	0	0
Net losses and loss expenses									-1,199,190	-1,123,242	-1,139,264
Acquisition costs									-295,122	-252,673	-205,722
General and administrative expenses									-327,052	-308,843	-269,090
Other expense									-8,578	0	0
Amortization of intangible assets									-2,533	-2,533	-2,533
Interest expense									-2,337	-1,163	0
Foreign exchange (loss) gain									-1,081	-7,237	-623
Income tax (expense) benefit									-30,434	-9,878	-18,798
Equity in earnings of consolidated subsidiaries									0	0	0
NET INCOME									584,276	517,336	586,399
Unrealized losses on investments arising during the year net of applicable deferred income tax benefit 2014: $0; 2013: $0; 2012: $(265)											-493
Reclassification adjustment for net realized investment gains included in net income, net of applicable income tax											-13,991
Other comprehensive loss									0	0	-14,484
COMPREHENSIVE INCOME (LOSS)									$584,276	$517,336	$571,915

Condensed_Consolidated_Guarant4

Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Cash Flows) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	$407,790	$106,109	$628,856
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	-7,630,034	-7,527,670	-8,006,237
Purchases of other invested assets	-307,955	-276,926	-333,031
Sales of available for sale securities	0	0	236,978
Sales of trading securities	7,536,936	7,540,194	7,892,907
Sales of other invested assets	267,885	187,477	53,741
Capital contribution to subsidiary			0
Other	6,201	28,782	-104,091
Net cash used in investing activities	-126,967	-48,143	-259,733
CASH FLOWS USED IN FINANCING ACTIVITIES:
Partial par value reduction	0	-12,981	-67,825
Dividends paid	-76,750	-34,059	0
Intercompany dividends paid	0	0	0
Proceeds from the exercise of stock options	10,027	12,137	10,527
Proceeds from other long-term debt	19,213	0	0
Share repurchases	-175,910	-173,006	-263,942
Capital contribution from parent			0
Net cash used in financing activities	-223,420	-207,909	-321,240
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	57,403	-149,943	47,883
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	531,936	681,879	633,996
CASH AND CASH EQUIVALENTS, END OF PERIOD	589,339	531,936	681,879
Consolidating Adjustments
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	-643,250	-596,500	-770,000
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	0	0	0
Purchases of other invested assets	0	0	0
Sales of available for sale securities			0
Sales of trading securities	0	0	0
Sales of other invested assets	0	0	0
Capital contribution to subsidiary			110,000
Other	0	0	0
Net cash used in investing activities	0	0	110,000
CASH FLOWS USED IN FINANCING ACTIVITIES:
Partial par value reduction		0	0
Dividends paid	0	0
Intercompany dividends paid	643,250	596,500	770,000
Proceeds from the exercise of stock options	0	0	0
Proceeds from other long-term debt	0
Share repurchases	0	0	0
Capital contribution from parent			-110,000
Net cash used in financing activities	643,250	596,500	660,000
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	0	0	0
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0	0	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	0	0	0
Allied World Switzerland (Parent Guarantor)
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	264,422	198,702	338,565
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	0	0	0
Purchases of other invested assets	0	0	0
Sales of available for sale securities			0
Sales of trading securities	0	0	0
Sales of other invested assets	0	0	0
Capital contribution to subsidiary			-110,000
Other	0	0	0
Net cash used in investing activities	0	0	-110,000
CASH FLOWS USED IN FINANCING ACTIVITIES:
Partial par value reduction		-12,981	-67,825
Dividends paid	-76,750	-34,059
Intercompany dividends paid	0	0	0
Proceeds from the exercise of stock options	10,027	12,137	10,527
Proceeds from other long-term debt	0
Share repurchases	-175,910	-173,006	-263,942
Capital contribution from parent			0
Net cash used in financing activities	-242,633	-207,909	-321,240
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	21,789	-9,207	-92,675
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	10,790	19,997	112,672
CASH AND CASH EQUIVALENTS, END OF PERIOD	32,579	10,790	19,997
Allied World Bermuda (Subsidiary Issuer)
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	292,959	265,951	357,438
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	0	0	0
Purchases of other invested assets	0	0	0
Sales of available for sale securities			0
Sales of trading securities	0	0	0
Sales of other invested assets	0	0	0
Capital contribution to subsidiary			0
Other	0	0	0
Net cash used in investing activities	0	0	0
CASH FLOWS USED IN FINANCING ACTIVITIES:
Partial par value reduction		0	0
Dividends paid	0	0
Intercompany dividends paid	-294,000	-274,500	-355,000
Proceeds from the exercise of stock options	0	0	0
Proceeds from other long-term debt	0
Share repurchases	0	0	0
Capital contribution from parent			0
Net cash used in financing activities	-294,000	-274,500	-355,000
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	-1,041	-8,549	2,438
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	2,775	11,324	8,886
CASH AND CASH EQUIVALENTS, END OF PERIOD	1,734	2,775	11,324
Other Allied World Subsidiaries
Condensed Cash Flow Statements, Captions [Line Items]
CASH FLOWS PROVIDED BY (USED IN) OPERATING ACTIVITIES:	493,659	237,956	702,853
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of trading securities	-7,630,034	-7,527,670	-8,006,237
Purchases of other invested assets	-307,955	-276,926	-333,031
Sales of available for sale securities			236,978
Sales of trading securities	7,536,936	7,540,194	7,892,907
Sales of other invested assets	267,885	187,477	53,741
Capital contribution to subsidiary			0
Other	6,201	28,782	-104,091
Net cash used in investing activities	-126,967	-48,143	-259,733
CASH FLOWS USED IN FINANCING ACTIVITIES:
Partial par value reduction		0	0
Dividends paid	0	0
Intercompany dividends paid	-349,250	-322,000	-415,000
Proceeds from the exercise of stock options	0	0	0
Proceeds from other long-term debt	19,213
Share repurchases	0	0	0
Capital contribution from parent			110,000
Net cash used in financing activities	-330,037	-322,000	-305,000
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	36,655	-132,187	138,120
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	518,371	650,558	512,438
CASH AND CASH EQUIVALENTS, END OF PERIOD	$555,026	$518,371	$650,558

Condensed_Consolidated_Guarant5

Condensed Consolidated Guarantor Financial Statements (Notes to Consolidated Guarantor Financial Statements) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Subsidiary Investee Percentage Ownership	100.00%
Allied World Switzerland (Parent Guarantor)
Condensed Financial Statements, Captions [Line Items]
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries	$294,000	$274,500	$355,000

Unaudited_Quarterly_Financial_2

Unaudited Quarterly Financial Data (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
REVENUES:
Gross premiums written	$565,743	$707,884	$760,405	$901,393	$555,490	$580,893	$765,200	$837,081	$2,935,425	$2,738,664	$2,329,271
Premiums ceded	-137,948	-139,142	-206,481	-129,779	-164,360	-127,816	-183,978	-142,029	-613,350	-618,183	-491,448
Net premiums written	427,795	568,742	553,924	771,614	391,130	453,077	581,222	695,052	2,322,075	2,120,481	1,837,823
Change in unearned premiums	145,694	-27,005	-16,677	-241,329	133,431	57,696	-73,951	-231,824	-139,317	-114,648	-88,925
Net premiums earned	573,489	541,737	537,247	530,285	524,561	510,773	507,271	463,228	2,182,758	2,005,833	1,748,898
Net investment income	49,050	43,412	36,793	47,619	47,270	39,271	37,635	33,388	176,874	157,564	167,141
Net realized investment gains	-15,329	-35,136	85,217	54,205	67,599	27,487	-115,198	79,637	88,957	59,525	306,436
Other income	1,032	1,032	0	0					2,064	0	0
Total revenues	608,242	551,045	659,257	632,109	639,430	577,531	429,708	576,253	2,450,653	2,222,922	2,222,475
EXPENSES:
Net losses and loss expenses	272,959	336,090	314,855	275,286	315,966	276,970	275,128	255,178	1,199,190	1,123,242	1,139,264
Acquisition costs	80,718	72,403	74,279	67,722	66,257	65,114	64,617	56,685	295,122	252,673	205,722
General and administrative expenses	100,914	88,294	96,188	80,340	100,467	88,553	80,585	82,680	365,736	352,285	307,321
Other expense	2,003	6,575	0	0					8,578	0	0
Amortization of intangible assets	633	633	634	633	633	633	634	633	2,533	2,533	2,533
Interest expense	14,304	14,325	14,592	14,534	14,094	14,094	14,188	14,134	57,755	56,510	55,405
Foreign exchange loss	-21	278	651	49	658	4,353	490	2,518	957	8,019	783
Total expenses	471,510	518,598	501,199	438,564	498,075	449,717	435,642	411,828	1,929,871	1,795,262	1,711,028
Income before income taxes	136,732	32,447	158,058	193,545	141,355	127,814	-5,934	164,425	520,782	427,660	511,447
Income tax expense	6,223	1,532	6,195	16,573	3,448	4,971	-4,072	5,433	30,523	9,780	18,440
Net income	$130,509	$30,915	$151,863	$176,972	$137,907	$122,843	($1,862)	$158,992	$490,259	$417,880	$493,007
PER SHARE DATA
Basic earnings (loss) per share	$1.35	$0.32	$1.55	$1.78	$1.37	$1.20	($0.02)	$1.53	$5.03	$4.08	$4.56
Diluted earnings (loss) per share	$1.33	$0.31	$1.52	$1.74	$1.34	$1.18	($0.02)	$1.50	$4.92	$3.98	$4.43
Weighted average common shares outstanding	96,386,796	96,458,231	97,809,639	99,545,187	100,814,955	101,974,077	103,267,659	103,840,818	97,538,319	102,464,715	108,171,435
Weighted average common shares and common share equivalents outstanding	98,394,432	98,444,238	99,724,802	101,584,662	103,129,812	104,184,579	103,267,659	106,295,529	99,591,773	104,865,834	111,209,655

Subsequent_Events_Details

Subsequent Events (Details) (Subsequent Event [Member], USD $)	0 Months Ended
	Jan. 02, 2015
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends declared per share	$0.23

Schedule_III_Supplementary_Ins1

Schedule III - Supplementary Insurance Information (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	$151,546	$126,661	$108,010
Reserves for losses and loss expenses	5,881,165	5,766,529	5,645,549
Unearned premiums	1,555,313	1,396,256	1,218,021
Net premiums earned	2,182,758	2,005,833	1,748,898
Net investment income	176,874	157,564	167,141
Losses and loss expenses	1,199,190	1,123,242	1,139,264
Amortization of deferred acquisition costs	295,122	252,673	205,722
Other operating costs	365,736	352,285	307,321
Net premiums written	2,322,075	2,120,481	1,837,823
North American Insurance
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	55,756	45,446	38,413
Reserves for losses and loss expenses	3,806,621	3,701,744	3,597,226
Unearned premiums	1,003,151	872,628	763,743
Net premiums earned	1,111,164	1,022,979	892,721
Net investment income	0	0	0
Losses and loss expenses	683,836	651,267	608,762
Amortization of deferred acquisition costs	105,914	94,869	77,701
Other operating costs	219,665	209,042	185,231
Net premiums written	1,230,839	1,082,441	964,199
Global Markets Insurance
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	23,336	9,828	8,326
Reserves for losses and loss expenses	568,126	568,675	567,675
Unearned premiums	223,402	164,792	131,130
Net premiums earned	162,566	126,008	115,660
Net investment income	0	0	0
Losses and loss expenses	61,057	50,395	19,536
Amortization of deferred acquisition costs	18,222	10,051	8,483
Other operating costs	68,106	63,164	48,365
Net premiums written	187,997	145,002	125,577
Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	72,454	71,387	61,271
Reserves for losses and loss expenses	1,506,418	1,496,110	1,480,648
Unearned premiums	328,760	358,836	323,148
Net premiums earned	909,028	856,846	740,517
Net investment income	0	0	0
Losses and loss expenses	454,297	421,580	510,966
Amortization of deferred acquisition costs	170,986	147,753	119,538
Other operating costs	77,965	80,079	73,725
Net premiums written	903,239	893,038	748,047
Corporate
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	0	0	0
Reserves for losses and loss expenses	0	0	0
Unearned premiums	0	0	0
Net premiums earned	0	0	0
Net investment income	176,874	157,564	167,141
Losses and loss expenses	0	0	0
Amortization of deferred acquisition costs	0	0	0
Other operating costs	0	0	0
Net premiums written	$0	$0	$0

Schedule_IV_Supplementary_Rein1

Schedule IV - Supplementary Reinsurance Information (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount [Abstract]
Gross									$1,996,831	$1,804,910	$1,569,021
Ceded to other companies	137,948	139,142	206,481	129,779	164,360	127,816	183,978	142,029	613,350	618,183	491,448
Assumed from other companies									938,594	933,754	760,250
Net amount	427,795	568,742	553,924	771,614	391,130	453,077	581,222	695,052	2,322,075	2,120,481	1,837,823
Property and casualty
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount [Abstract]
Gross									1,996,831	1,804,910	1,569,021
Ceded to other companies									613,350	618,183	491,448
Assumed from other companies									938,594	933,754	760,250
Net amount									$2,322,075	$2,120,481	$1,837,823
Percentage of amount assumed to net									40.00%	44.00%	41.00%