Markel (MKL) 10-Q 6 Nov 13 2013 Q3 Quarterly report Financial data

Document_And_Entity_Informatio

Document And Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 30, 2013
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Entity Registrant Name	'MARKEL CORP	'
Entity Central Index Key	'0001096343	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	13,969,015

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
ASSETS	'	'
Fixed maturities (amortized cost of $10,242,410 in 2013 and $4,562,278 in 2012)	$10,312,939	$4,979,283
Equity securities (cost of $1,511,463 in 2013 and $1,387,305 in 2012)	2,988,614	2,406,951
Short-term investments (estimated fair value approximates cost)	1,159,457	973,330
Total Investments	14,461,010	8,359,564
Cash and cash equivalents	2,115,322	863,766
Restricted cash and cash equivalents	600,171	109,415
Receivables	1,253,781	413,883
Reinsurance recoverable on unpaid losses	1,910,023	778,774
Reinsurance recoverable on paid losses	105,279	51,145
Deferred policy acquisition costs	260,230	157,465
Prepaid reinsurance premiums	437,435	110,332
Goodwill	1,008,205	674,930
Intangible assets	560,836	374,295
Other assets	1,096,285	663,019
Total Assets	23,808,577	12,556,588
LIABILITIES AND EQUITY	'	'
Unpaid losses and loss adjustment expenses	10,172,692	5,371,426
Life and annuity benefits	1,482,998	0
Unearned premiums	2,307,266	1,000,261
Payables to insurance and reinsurance companies	385,734	103,212
Senior long-term debt and other debt (estimated fair value of $2,361,000 in 2013 and $1,688,000 in 2012)	2,250,479	1,492,550
Other liabilities	675,580	613,897
Total Liabilities	17,274,749	8,581,346
Redeemable noncontrolling interests	71,488	86,225
Commitments and contingencies	'	'
Shareholders' equity:	'	'
Common stock	3,277,488	908,980
Retained earnings	2,197,816	2,068,340
Accumulated other comprehensive income	982,119	911,337
Total Shareholders' Equity	6,457,423	3,888,657
Noncontrolling interests	4,917	360
Total Equity	6,462,340	3,889,017
Total Liabilities and Equity	$23,808,577	$12,556,588

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Statement of Financial Position [Abstract]	'	'
Fixed maturities, amortized cost	$10,242,410	$4,562,278
Equity securities, cost	1,511,463	1,387,305
Senior long-term debt and other debt, estimated fair value	$2,361,000	$1,688,000

Consolidated_Statements_Of_Inc

Consolidated Statements Of Income And Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
OPERATING REVENUES	'	'	'	'
Earned premiums	$919,723	$530,537	$2,269,129	$1,573,189
Net investment income	86,192	64,438	228,788	207,834
Other-than-temporary impairment losses	0	-3,159	-4,589	-4,151
Net realized investment gains, excluding other-than-temporary impairment losses	11,238	8,390	45,290	29,507
Net realized investment gains	11,238	5,231	40,701	25,356
Other revenues	174,512	165,569	504,680	385,778
Total Operating Revenues	1,191,665	765,775	3,043,298	2,192,157
OPERATING EXPENSES	'	'	'	'
Losses and loss adjustment expenses	533,372	303,459	1,263,674	813,074
Underwriting, acquisition and insurance expenses	352,126	231,841	943,894	695,322
Amortization of intangible assets	16,848	7,959	37,755	25,078
Other expenses	166,566	145,339	459,642	343,462
Total Operating Expenses	1,068,912	688,598	2,704,965	1,876,936
Operating Income	122,753	77,177	338,333	315,221
Interest expense	30,619	24,692	82,754	69,068
Income Before Income Taxes	92,134	52,485	255,579	246,153
Income tax expense	25,167	811	70,673	45,998
Net Income	66,967	51,674	184,906	200,155
Net income attributable to noncontrolling interests	1,368	2,021	2,649	3,562
Net Income to Shareholders	65,599	49,653	182,257	196,593
OTHER COMPREHENSIVE INCOME	'	'	'	'
Net holding gains arising during the period	84,564	100,630	107,473	246,056
Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	-219	-128	-40	-136
Reclassification adjustments for net gains included in net income	-7,654	-6,484	-27,866	-20,154
Change in net unrealized gains on investments, net of taxes	76,691	94,018	79,567	225,766
Change in foreign currency translation adjustments, net of taxes	1,709	3,266	-9,931	2,927
Change in net actuarial pension loss, net of taxes	410	522	1,146	1,487
Total Other Comprehensive Income	78,810	97,806	70,782	230,180
Comprehensive Income	145,777	149,480	255,688	430,335
Comprehensive income attributable to noncontrolling interests	1,368	2,026	2,649	3,520
Comprehensive Income to Shareholders	$144,409	$147,454	$253,039	$426,815
NET INCOME PER SHARE	'	'	'	'
Basic (in dollars per share)	$4.69	$5.33	$15.38	$19.72
Diluted (in dollars per share)	$4.67	$5.32	$15.33	$19.67

Consolidated_Statements_Of_Cha

Consolidated Statements Of Changes In Equity (USD $)	Total	Total Equity [Member]	Total Shareholders' Equity [Member]	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Noncontrolling Interests [Member]	Redeemable Noncontrolling Interests [Member]
In Thousands
Balance at Dec. 31, 2011	'	$3,388,115	$3,387,513	$891,507	$1,835,086	$660,920	$602	'
Balance, redeemable noncontrolling interests at Dec. 31, 2011	'	'	'	'	'	'	'	74,231
Net income (loss)	200,155	195,851	196,593	'	196,593	0	-742	4,304
Other comprehensive income (loss)	230,180	230,222	230,222	'	0	230,222	0	-42
Comprehensive Income	430,335	426,073	426,815	'	'	'	-742	4,262
Issuance of common stock	'	8,803	8,803	8,803	0	0	0	0
Repurchase of common stock	'	-16,062	-16,062	0	-16,062	0	0	0
Restricted stock units expensed	'	5,184	5,184	5,184	0	0	0	0
Acquisitions	'	0	0	0	0	0	0	7,896
Adjustment of redeemable noncontrolling interests	'	-6,484	-6,484	0	-6,484	0	0	6,484
Other	'	290	347	347	0	0	-57	-5,135
Balance, redeemable noncontrolling interests at Sep. 30, 2012	'	'	'	'	'	'	'	87,738
Balance at Sep. 30, 2012	'	3,805,919	3,806,116	905,841	2,009,133	891,142	-197	'
Balance at Dec. 31, 2012	3,889,017	3,889,017	3,888,657	908,980	2,068,340	911,337	360	'
Balance, redeemable noncontrolling interests at Dec. 31, 2012	86,225	'	'	'	'	'	'	86,225
Net income (loss)	184,906	181,814	182,257	'	182,257	0	-443	3,092
Other comprehensive income (loss)	70,782	70,782	70,782	'	0	70,782	0	0
Comprehensive Income	255,688	252,596	253,039	'	'	'	-443	3,092
Issuance of common stock	'	18,663	18,663	18,663	0	0	0	0
Repurchase of common stock	'	-56,862	-56,862	0	-56,862	0	0	0
Restricted stock units expensed	'	20,182	20,182	20,182	0	0	0	0
Acquisition of Alterra	'	2,330,199	2,330,199	2,330,199	0	0	0	0
Adjustment of redeemable noncontrolling interests	'	3,101	3,101	0	3,101	0	0	-3,101
Purchase of noncontrolling interest	'	-136	-136	-136	0	0	0	-11,716
Other	'	5,580	580	-400	980	0	5,000	-3,012
Balance, redeemable noncontrolling interests at Sep. 30, 2013	71,488	'	'	'	'	'	'	71,488
Balance at Sep. 30, 2013	$6,462,340	$6,462,340	$6,457,423	$3,277,488	$2,197,816	$982,119	$4,917	'

Condensed_Consolidated_Stateme

Condensed Consolidated Statements Of Cash Flows (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
OPERATING ACTIVITIES	'	'
Net income	$184,906	$200,155
Adjustments to reconcile net income to net cash provided by operating activities	357,551	39,390
Net Cash Provided By Operating Activities	542,457	239,545
INVESTING ACTIVITIES	'	'
Proceeds from sales of fixed maturities and equity securities	497,364	193,514
Proceeds from maturities, calls and prepayments of fixed maturities	851,194	354,489
Cost of fixed maturities and equity securities purchased	-737,515	-390,927
Net change in short-term investments	-177,311	-183,748
Proceeds from sales of equity method investments	228,001	0
Cost of equity method investments	-24,878	-38,650
Change in restricted cash and cash equivalents	-76,259	-23,271
Additions to property and equipment	-36,677	-33,489
Acquisitions, net of cash acquired	-13,354	-246,182
Other	2,912	-8,996
Net Cash Provided (Used) By Investing Activities	513,477	-377,260
FINANCING ACTIVITIES	'	'
Additions to senior long-term debt and other debt	563,913	461,360
Repayment and retirement of senior long-term debt and other debt	-320,094	-263,120
Repurchases of common stock	-56,862	-16,062
Issuance of common stock	18,663	8,803
Purchase of redeemable noncontrolling interests	-11,852	-320
Distributions to noncontrolling interests	-3,983	-4,821
Other	-246	-15,213
Net Cash Provided By Financing Activities	189,539	170,627
Effect of foreign currency rate changes on cash and cash equivalents	6,083	3,613
Increase in cash and cash equivalents	1,251,556	36,525
Cash and cash equivalents at beginning of period	863,766	703,259
CASH AND CASH EQUIVALENTS AT END OF PERIOD	$2,115,322	$739,784

Basis_Of_Presentation

Basis Of Presentation	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Basis Of Presentation	'
	Basis of Presentation
	Markel Corporation is a diverse financial holding company serving a variety of niche markets. Markel Corporation's principal business markets and underwrites specialty insurance products and programs. Markel Corporation also owns interests in various industrial and service businesses that operate outside of the specialty insurance marketplace.
	On May 1, 2013 (the Acquisition Date), Markel Corporation completed the acquisition of 100% of the issued and outstanding common stock of Alterra Capital Holdings Limited (Alterra) pursuant to an agreement dated December 18, 2012 which provided for the merger of Alterra with one of Markel Corporation's subsidiaries. Total purchase consideration was $3.3 billion. Alterra was a Bermuda-headquartered global enterprise providing diversified specialty insurance and reinsurance products to corporations, public entities and other property and casualty insurers.
	The consolidated balance sheet as of September 30, 2013, the related consolidated statements of income and comprehensive income for the quarters and nine months ended September 30, 2013 and 2012, and the consolidated statements of changes in equity and cash flows for the nine months ended September 30, 2013 and 2012 are unaudited. In the opinion of management, all adjustments necessary for fair presentation of such consolidated financial statements have been included. Such adjustments consist only of normal, recurring items. Interim results are not necessarily indicative of results of operations for the entire year. The consolidated balance sheet as of December 31, 2012 was derived from Markel Corporation's audited annual consolidated financial statements.
	The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (U.S. GAAP) and include the accounts of Markel Corporation and its subsidiaries (the Company). All significant intercompany balances and transactions have been eliminated in consolidation. The consolidated financial statements include the results of operations and cash flows of Alterra from the Acquisition Date to September 30, 2013 and not in any prior periods, except with respect to the Supplemental Pro Forma Information included in note 3. The Company consolidates the results of its non-insurance subsidiaries on a one-month lag. Certain prior year amounts have been reclassified to conform to the current presentation.
	The preparation of financial statements in accordance with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities. Actual results may differ materially from the estimates and assumptions used in preparing the consolidated financial statements.
	The consolidated financial statements and notes are presented as permitted by Form 10-Q and do not contain certain information included in the Company's annual consolidated financial statements and notes. Readers are urged to review the Company's 2012 Annual Report on Form 10-K for a more complete description of the Company's business and accounting policies.

Recent_Accounting_Pronouncemen

Recent Accounting Pronouncements	9 Months Ended
	Sep. 30, 2013
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]	'
Recent Accounting Pronouncements	'
	Recent Accounting Pronouncements
	Effective January 1, 2013, the Company adopted Financial Accounting Standards Board (FASB) Accounting Standards Update (ASU) No. 2013-02, Reporting Amounts Reclassified Out of Accumulated Other Comprehensive Income, which amends FASB Accounting Standards Codification (ASC) 220, Comprehensive Income, to improve the reporting of reclassifications out of accumulated other comprehensive income. ASU No. 2013-02 requires information about reclassifications out of accumulated other comprehensive income to be reported in one place, by component. The guidance also requires disclosure of the effect of significant reclassifications out of accumulated other comprehensive income on the respective line items in net income if the amount being reclassified is required under U.S. GAAP to be reclassified in its entirety to net income. For other amounts that are not required to be reclassified in their entirety to net income, an entity is required to cross-reference to other disclosures required under U.S. GAAP that provide additional detail about those amounts. The adoption of this guidance did not have an impact on the Company's financial position, results of operations or cash flows. The Company has included the additional disclosures required by ASU No. 2013-02 in note 14.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2013
Acquisitions [Abstract]	'
Acquisitions	'
	Acquisitions
	Acquisition of Alterra
	a)Overview. On May 1, 2013, the Company completed the acquisition of 100% of the issued and outstanding common stock of Alterra pursuant to an agreement dated December 18, 2012 (the Merger Agreement) which provided for the merger of Alterra with one of the Company's subsidiaries. Alterra was a Bermuda-headquartered global enterprise providing diversified specialty property and casualty insurance and reinsurance products to corporations, public entities and other property and casualty insurers. The acquisition of Alterra creates additional size and scale, providing additional insurance and investment opportunities for the Company. As a result of the acquisition of Alterra, the Company formed a new operating segment, the Alterra segment. Results attributable to Alterra's property and casualty insurance and reinsurance business are included in the Alterra segment. Previously, Alterra also offered life and annuity reinsurance products. In 2010, Alterra ceased writing life and annuity reinsurance contracts and placed this business into run-off. Results attributable to the run-off of Alterra's life and annuity reinsurance business are included in the Company's Other Insurance (Discontinued Lines) segment. See note 6 for further discussion of the Company's operating segments.
	Pursuant to the terms of the Merger Agreement, on the Acquisition Date, equity holders of Alterra received, in exchange for each share of Alterra common stock held (other than restricted shares that did not vest in connection with the transaction), (1) 0.04315 shares of the Company's common stock and (2) $10.00 in cash. In total, equity holders of Alterra received total consideration of $3.3 billion, consisting of cash consideration of $964.3 million and stock consideration of 4.3 million shares of the Company's common stock.
	Following the acquisition, the Company's board of directors consists of all 10 members from its pre-acquisition board of directors and two additional members who were designated by Alterra and approved by the Company's Nominating/Corporate Governance Committee.
	b)Purchase Price. The Company's total purchase price for Alterra as of the Acquisition Date was calculated as follows:
	(in thousands, except per share amounts)
	Shares of Alterra common stock outstanding as of the Acquisition Date	96,433
	Exchange ratio per the Merger Agreement	0.04315
	Markel share issuance to Alterra shareholders	4,161
	Shares of Alterra restricted stock outstanding as of the Acquisition Date	2,239
	Incentive award ratio per the Merger Agreement	0.06252
	Markel restricted stock issuance to Alterra restricted stock holders	140
	Multiplied by Markel's weighted average stock price on April 30, 2013 (1)	$	529.59
	Markel share and restricted stock issuance consideration, net of taxes	$	2,267,648
	Alterra common shares outstanding as of the Acquisition Date that received cash consideration	96,433
	Multiplied by cash price per share component per the Merger Agreement	$	10
	Markel cash consideration	$	964,330
	Fair value of Markel warrant issuance to Alterra warrant holders as of the Acquisition Date	$	73,685
	Fair value of Markel stock option issuance to Alterra stock option holders as of the Acquisition Date, net of taxes	$	12,335
	Fair value of partially vested Markel restricted stock unit issuance as of the Acquisition Date, net of taxes	$	6,867
	Unrecognized compensation on unvested restricted stock and restricted stock units	$	(20,572	)
	Total acquisition consideration	$	3,304,293
	(1)	The fair value of the shares issued by the Company was calculated as the weighted average price of the Company's stock on April 30, 2013, the day preceding the Acquisition Date.
	As part of the consideration, the Company issued replacement warrants, options and restricted stock awards to holders of Alterra warrants, options and restricted stock awards. The acquisition consideration related to the options, restricted stock awards and partially vested restricted stock units issued was net of income taxes of $1.9 million, $10.1 million and $0.7 million, respectively. See note 13 for additional information about the equity awards issued in connection with the acquisition.
	c)Fair Value of Net Assets Acquired and Liabilities Assumed. The purchase price was allocated to the acquired assets and liabilities of Alterra based on estimated fair values at the Acquisition Date. The Company recognized goodwill of $337.0 million, all of which was preliminarily recorded within the Alterra segment and is primarily attributable to Alterra's assembled workforce and synergies that are expected to result upon integration of Alterra into the Company's insurance operations and investing activities. The Company estimates that none of the goodwill that was recorded will be deductible for income tax purposes. The Company also recognized indefinite lived intangible assets of $37.5 million and other intangible assets of $150 million, which will be amortized over a weighted average period of 17 years.
	The Company has not completed the process of determining the fair value of the intangible assets acquired, unpaid losses and loss adjustment expenses and deferred taxes. These valuations will be completed within the measurement period, which cannot exceed 12 months from the Acquisition Date. As a result, the fair value recorded for these items is a provisional estimate and may be subject to adjustment. Once completed, any adjustments resulting from the valuations may impact the individual amounts recorded for assets acquired and liabilities assumed, as well as the residual goodwill.
	The following table summarizes the provisional fair values of the assets acquired and liabilities assumed at the Acquisition Date.
	(dollars in thousands)
	ASSETS
	Investments	$	6,407,841
	Cash and cash equivalents	1,036,274
	Restricted cash and cash equivalents	414,497
	Receivables	866,388
	Reinsurance recoverable on unpaid losses	1,169,084
	Reinsurance recoverable on paid losses	80,672
	Prepaid reinsurance premiums	317,445
	Other assets	834,024
	LIABILITIES
	Unpaid losses and loss adjustment expenses	4,719,461
	Life and annuity benefits	1,477,482
	Unearned premiums	1,075,610
	Payables to insurance and reinsurance companies	342,858
	Senior long-term debt	512,463
	Other liabilities	218,523
	Net assets	2,779,828
	Goodwill	336,965
	Intangible assets	187,500
	Acquisition date fair value	$	3,304,293
	An explanation of the significant adjustments for fair value and the related impact on amortization is as follows:
	•	Investments - Investments acquired include a net increase of $223.1 million to adjust the historical carrying amount of Alterra's investments to their estimated fair value as of the Acquisition Date. The difference in the par value and estimated fair value of Alterra's fixed maturity securities investments as of the Acquisition Date, $552.4 million, will be amortized to net investment income over the term of the underlying securities.
	•	Intangible assets - Establish the provisional estimated fair value of intangible assets related to Alterra (see below for further detail).
	•	Unearned Premiums - Unearned premiums acquired include a decrease of $176.3 million to adjust the carrying value of Alterra's historical unearned premiums to fair value as of the Acquisition Date. The adjustment consists of the present value of the expected underwriting profit within the unearned premiums liability less costs to service the related policies and a risk premium. This adjustment will be amortized to underwriting, acquisition and insurance expenses over a weighted average period of approximately one year, as the contracts for business in-force as of the Acquisition Date expire.
	•	Unpaid losses and loss adjustment expenses - Unpaid losses and loss adjustment expenses acquired include a provisional increase of $120.8 million to adjust the carrying value of Alterra's historical unpaid losses and loss adjustment expenses, net of related reinsurance recoverable, to fair value as of the Acquisition Date. The estimated fair value consists of the present value of the expected net loss and loss adjustment expense payments plus a risk premium. This adjustment, plus the $26.5 million unamortized fair value adjustment included in Alterra's historical unpaid losses and loss adjustment expenses, will be amortized to losses and loss adjustment expenses over a weighted average period of approximately five years, based on the estimated payout pattern of net reserves as of the Acquisition Date.
	•	Life and Annuity Benefits - Life and annuity benefits acquired include an increase of $329.6 million to adjust the carrying value of Alterra's historical life and annuity benefits to fair value as of the Acquisition Date. The estimated fair value consists of the present value of the expected net life and annuity benefit payments plus a risk premium. See note 8 for detail regarding accounting for life and annuity benefits.
	•	Senior long-term debt - Senior long-term debt acquired includes an increase of $71.9 million to adjust the carrying value of Alterra's senior long-term debt to its estimated fair value based on prevailing interest rates and other factors as of the Acquisition Date. This adjustment will be amortized to interest expense over the term of the notes. See note 9.
	The following table summarizes the provisional intangible assets recorded in connection with the acquisition, and as of September 30, 2013.
	(dollars in thousands)	Amount			Economic
					Useful Life
	Customer relationships	$	113,000		18 years
	Broker relationships	18,000			18 years
	Technology	18,000			10 years
	Trade names	1,000			One year
Lloyd's syndicate capacity	12,000			Indefinite
Insurance licenses	25,500			Indefinite
Intangible assets, before amortization, as of the Acquisition Date	187,500
Amortization (from the Acquisition Date through September 30, 2013)	4,246
Net intangible assets as of September 30, 2013	$	183,254
Customer relationships represent policyholder relationships and the network of insurance companies through which Alterra conducts its operations. The fair value of customer relationships and broker relationships was estimated using the income approach. Critical inputs into the valuation model for customer relationships and broker relationships include estimates of expected premium and attrition rates, and discounting at a weighted average cost of capital. Technology represents the intangible asset related to Alterra's internally developed software and was valued using the income approach.
The fair value of Lloyd's syndicate capacity and insurance licenses was estimated using the market approach. Lloyd's syndicate capacity represents Alterra's authorized premium income limit to write insurance business in the Lloyd's market. Alterra's proportionate share of Syndicates 1400 and 2526, the syndicates through which it conducts its Lloyd's operations, is 100% and approximately 20%, respectively. The capacity is renewed annually at no cost to the Company but may be freely purchased or sold, subject to Lloyd's approval. The ability to write insurance business within the syndicate capacity is indefinite with the premium income limit being set annually by the Company, subject to Lloyd's approval.
d)Income Taxes. As a result of the acquisition, Alterra and its non-U.S. subsidiaries became controlled foreign corporations subject to U.S. income tax at a statutory rate of 35%. The acquisition was taxable to U.S. shareholders of Alterra, and Markel has elected to treat it as an asset acquisition under section 338(g) of the U.S. Internal Revenue Code of 1986 (IRC), as amended.
Effective May 1, 2013, the Company made an IRC section 953(d) election with respect to Markel Bermuda Limited (Markel Bermuda, formerly known as Alterra Bermuda Limited), a wholly-owned subsidiary of Alterra. As a result of the 953(d) election, Markel Bermuda is treated as a domestic corporation for U.S. tax purposes and, accordingly, was required to record deferred taxes at the 35% statutory U.S. rate.
As part of the provisional allocation of the purchase price, the Company recorded net deferred tax assets of $299.0 million. Of this amount, $320.0 million represents deferred tax assets related to accrued losses and loss adjustment expenses and life and annuity benefits, which were partially offset by deferred tax liabilities of $56.8 million related to the estimated fair value of the intangible assets recorded. Other net deferred tax assets recorded primarily relate to differences between financial reporting and tax bases of the acquired assets and liabilities as of the Acquisition Date. At September 30, 2013, earnings of Alterra's foreign subsidiaries are considered reinvested indefinitely, consistent with the Company's other foreign subsidiaries, and no provision for deferred U.S. income tax has been recorded. As of September 30, 2013, the provisional allocation of the purchase price reflects an increase of $43.2 million in the amount recorded for net deferred tax assets from the allocation reported at June 30, 2013 and a corresponding reduction in the amount recorded for goodwill.
e)Transaction and Acquisition-Related Costs. The following table summarizes transaction and acquisition-related costs incurred by the Company in connection with the acquisition, all of which were included in underwriting, acquisition and insurance expenses in the consolidated statements of income and comprehensive income.
(dollars in thousands)	Quarter Ended			Nine Months Ended
	30-Sep-13			30-Sep-13
Transaction costs	$	—		$	15,981
Acquisition-related costs:
Severance costs	3,519			31,734
Stay bonuses	3,906			9,981
Acceleration of Alterra long-term incentive compensation awards and restricted stock awards	1,083			12,621
Total transaction and acquisition-related costs	$	8,508		$	70,317
Transaction costs primarily consist of due diligence, legal and investment banking costs. Per the terms of the Merger Agreement, transaction costs attributable to Alterra were recorded and paid by Alterra prior to the Acquisition Date ($23.0 million) and are not included within the Company's consolidated statements of income and comprehensive income.
In connection with the acquisition, Alterra instituted a retention plan for certain employees under which Alterra committed to the payment of stay bonuses to such employees one year from the Acquisition Date, provided they remain employed with the Company through that date. Payments may be accelerated for certain qualifying employment terminations.
Prior to its acquisition by the Company, Alterra granted long term incentive awards to certain employees to be paid in the form of cash on March 1, 2016, provided they remain employed with the Company on that date. Payments may be accelerated prior to March 1, 2016 for certain qualifying employment terminations. Additionally, as part of the purchase consideration, the Company issued replacement restricted stock awards to holders of Alterra restricted stock awards. As a result of separations made in connection with the acquisition, the Company recognized expense totaling $1.1 million and $12.6 million related to the acceleration of certain of these awards during the quarter and nine months ended September 30, 2013, respectively.
f)Financial Results. The following table summarizes the results of Alterra since the Acquisition Date that have been included within the Company's consolidated statements of income and comprehensive income.
(dollars in thousands)	Quarter Ended September 30, 2013			Nine Months Ended September 30, 2013
Operating revenues	$	314,200		$	557,446
Net loss to shareholders	$	(11,312	)	$	(67,055	)
g)Supplemental Pro Forma Information. Alterra's results have been included in the Company's Consolidated Financial Statements from the Acquisition Date to September 30, 2013. The following table presents unaudited pro forma consolidated information for the quarter and nine months ended September 30, 2013 and 2012 and assumes the Company's acquisition of Alterra occurred on January 1, 2012. The pro forma financial information is presented for informational purposes only and does not necessarily reflect the results that would have occurred had the acquisition taken place on January 1, 2012, nor is it necessarily indicative of future results. Significant adjustments used to determine pro forma results include amortization of intangible assets and amortization of fair value adjustments discussed in c) above, and the corresponding income tax effects. The Company also excluded certain charges from the pro forma results, including transaction costs incurred by the Company ($16.0 million) and Alterra ($23.0 million) totaling $39.0 million for the nine months ended September 30, 2013, severance costs attributable to the acquisition totaling $3.5 million and $31.7 million for the quarter and nine months ended September 30, 2013, respectively, and stay bonuses of $3.9 million and $10.0 million for the quarter and nine months ended September 30, 2013, respectively. The acceleration of compensation expense during the quarter and nine months ended September 30, 2013 related to Alterra's long-term incentive compensation awards and restricted stock awards was attributable to the acquisition; however, the incremental expense recognized during the period only represents a timing difference in the recognition of expense. Therefore, it was not excluded from the pro forma underwriting results.
	Pro Forma						Pro Forma
	Quarter Ended September 30,						Nine Months Ended September 30,
(in thousands, except per share amounts)	2013			2012			2013			2012
Earned premiums	$	919,723		$	862,534		$	2,717,732		$	2,592,818
Operating revenues	1,234,380			1,146,994			3,629,659			3,365,605
Net income to shareholders	90,910			60,090			298,970			265,664
U.S. GAAP combined ratio (1)	95		%	102		%	95		%	96		%
Basic net income per share	$	6.49		$	4.29		$	21.33		$	18.96
Diluted net income per share	$	6.47		$	4.27		$	21.24		$	18.9
Weighted average common shares outstanding:
Basic	14,001			14,015			14,014			14,012
Diluted	14,057			14,067			14,075			14,060
-1	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
Acquisition of Essentia
On January 1, 2013, the Company completed its acquisition of 100% of the outstanding shares of Essentia Insurance Company, a company that underwrites insurance exclusively for Hagerty Insurance Agency and Hagerty Classic Marine Insurance Agency (collectively, Hagerty) throughout the United States. Hagerty offers insurance for classic cars, vintage boats, motorcycles and related automotive collectibles. The Company recognized intangible assets of $35.4 million associated with this acquisition, which includes $25.0 million of customer relationships to be amortized over a weighted average period of six years. Results attributable to this acquisition are included in the Specialty Admitted segment.
Offer to Acquire Abbey Protection
On October 9, 2013, the Company made an offer to purchase the entire issued and to be issued share capital of Abbey Protection plc (Abbey), an integrated specialist insurance and consultancy group, for a total purchase price of approximately £116.5 million (approximately $187.4 million as of October 9, 2013). The acquisition is subject to a number of conditions, including shareholder and regulatory approvals in the United Kingdom. If approved, the transaction is expected to close in January 2014.  Abbey's business is focused on the underwriting and sale of insurance products to small and medium enterprises and affinity groups in the United Kingdom providing protection against legal expenses and professional fees incurred as a result of legal actions or investigations by tax authorities, as well as providing a range of complementary legal, professional and reinsurance services.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
Investments, Debt and Equity Securities [Abstract]	'
Investments	'
	Investments
	a)The following tables summarize the Company's available-for-sale investments.
		September 30, 2013
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized			Estimated
		Cost			Unrealized			Unrealized			Other-Than-			Fair
					Holding			Holding			Temporary			Value
					Gains			Losses			Impairment
											Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	1,257,855		$	10,933		$	(24,660	)	$	—		$	1,244,128
	Obligations of states, municipalities and political subdivisions	2,739,483			135,139			(23,561		)	—			2,851,061
	Foreign governments	1,435,249			34,323			(45,888		)	—			1,423,684
	Commercial mortgage-backed securities	401,036			116			(10,443		)	—			390,709
	Residential mortgage-backed securities	927,880			12,364			(12,012		)	(2,258		)	925,974
	Asset-backed securities	217,394			261			(1,779		)	—			215,876
	Corporate bonds	3,263,513			67,385			(62,519		)	(6,872		)	3,261,507
	Total fixed maturities	10,242,410			260,521			(180,862		)	(9,130		)	10,312,939
	Equity securities:
	Insurance, banks and other financial institutions	404,923			540,749			(241		)	—			945,431
	Industrial, consumer and all other	1,106,540			939,612			(2,969		)	—			2,043,183
	Total equity securities	1,511,463			1,480,361			(3,210		)	—			2,988,614
	Short-term investments	1,159,460			3			(6		)	—			1,159,457
	Investments, available-for-sale	$	12,913,333		$	1,740,885		$	(184,078	)	$	(9,130	)	$	14,461,010
		December 31, 2012
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized			Estimated
		Cost			Unrealized			Unrealized			Other-Than-			Fair
					Holding			Holding			Temporary			Value
					Gains			Losses			Impairment
											Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	297,663		$	19,844		$	—		$	—		$	317,507
	Obligations of states, municipalities and political subdivisions	2,586,867			245,057			(362		)	—			2,831,562
	Foreign governments	503,844			52,764			—			—			556,608
	Residential mortgage-backed securities	202,644			14,996			(5		)	(2,258		)	215,377
	Asset-backed securities	13,828			517			—			—			14,345
	Corporate bonds	957,432			93,395			(121		)	(6,822		)	1,043,884
	Total fixed maturities	4,562,278			426,573			(488		)	(9,080		)	4,979,283
	Equity securities:
	Insurance, banks and other financial institutions	508,771			389,434			(138		)	—			898,067
	Industrial, consumer and all other	878,534			637,783			(7,433		)	—			1,508,884
	Total equity securities	1,387,305			1,027,217			(7,571		)	—			2,406,951
	Short-term investments	973,318			26			(14		)	—			973,330
	Investments, available-for-sale	$	6,922,901		$	1,453,816		$	(8,073	)	$	(9,080	)	$	8,359,564
	b)The following tables summarize gross unrealized investment losses by the length of time that securities have continuously been in an unrealized loss position.
		September 30, 2013
		Less than 12 months						12 months or longer						Total
	(dollars in thousands)	Estimated			Gross			Estimated			Gross			Estimated		Gross
		Fair			Unrealized			Fair			Unrealized			Fair		Unrealized
		Value			Holding and			Value			Holding and			Value		Holding and
					Other-Than-						Other-Than-					Other-Than-
					Temporary						Temporary					Temporary
					Impairment						Impairment					Impairment
					Losses						Losses					Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	652,752		$	(24,660	)	$	—		$	—		$	652,752	$	(24,660	)
	Obligations of states, municipalities and political subdivisions	307,119			(23,439		)	3,196			(122		)	310,315		(23,561		)
	Foreign governments	907,456			(45,888		)	—			—			907,456		(45,888		)
	Commercial mortgage-backed securities	378,001			(10,443		)	—			—			378,001		(10,443		)
	Residential mortgage-backed securities	455,942			(14,269		)	166			(1		)	456,108		(14,270		)
	Asset-backed securities	189,070			(1,779		)	—			—			189,070		(1,779		)
	Corporate bonds	2,127,583			(69,391		)	—			—			2,127,583		(69,391		)
	Total fixed maturities	5,017,923			(189,869		)	3,362			(123		)	5,021,285		(189,992		)
	Equity securities:
Insurance, banks and other financial institutions	11,518			(241		)	—			—			11,518		(241		)
Industrial, consumer and all other	51,261			(2,957		)	125			(12		)	51,386		(2,969		)
Total equity securities	62,779			(3,198		)	125			(12		)	62,904		(3,210		)
Short-term investments	525,986			(6		)	—			—			525,986		(6		)
Total	$	5,606,688		$	(193,073	)	$	3,487		$	(135	)	$	5,610,175	$	(193,208	)
At September 30, 2013, the Company held 1,569 securities with a total estimated fair value of $5.6 billion and gross unrealized losses of $193.2 million. Of these 1,569 securities, 12 securities had been in a continuous unrealized loss position for one year or longer and had a total estimated fair value of $3.5 million and gross unrealized losses of $0.1 million. Of these securities, 10 securities were fixed maturities and two were equity securities. The Company does not intend to sell or believe it will be required to sell these fixed maturities before recovery of their amortized cost. The Company has the ability and intent to hold these equity securities for a period of time sufficient to allow for the anticipated recovery of their fair value.
	December 31, 2012
	Less than 12 months						12 months or longer						Total
(dollars in thousands)	Estimated			Gross			Estimated			Gross			Estimated		Gross
	Fair			Unrealized			Fair			Unrealized			Fair		Unrealized
	Value			Holding and			Value			Holding and			Value		Holding and
				Other-Than-						Other-Than-					Other-Than-
				Temporary						Temporary					Temporary
				Impairment						Impairment					Impairment
				Losses						Losses					Losses
Fixed maturities:
Obligations of states, municipalities and political subdivisions	$	2,833		$	(46	)	$	3,616		$	(316	)	$	6,449	$	(362	)
Residential mortgage-backed securities	364			(2,260		)	201			(3		)	565		(2,263		)
Corporate bonds	—			(6,822		)	3,860			(121		)	3,860		(6,943		)
Total fixed maturities	3,197			(9,128		)	7,677			(440		)	10,874		(9,568		)
Equity securities:
Insurance, banks and other financial institutions	2,431			(138		)	—			—			2,431		(138		)
Industrial, consumer and all other	82,109			(7,310		)	1,983			(123		)	84,092		(7,433		)
Total equity securities	84,540			(7,448		)	1,983			(123		)	86,523		(7,571		)
Short-term investments	228,820			(14		)	—			—			228,820		(14		)
Total	$	316,557		$	(16,590	)	$	9,660		$	(563	)	$	326,217	$	(17,153	)
At December 31, 2012, the Company held 35 securities with a total estimated fair value of $326.2 million and gross unrealized losses of $17.2 million. Of these 35 securities, 10 securities had been in a continuous unrealized loss position for one year or longer and had a total estimated fair value of $9.7 million and gross unrealized losses of $0.6 million. Of these securities, eight securities were fixed maturities and two were equity securities.
The Company completes a detailed analysis each quarter to assess whether the decline in the fair value of any investment below its cost basis is deemed other-than-temporary. All securities with unrealized losses are reviewed. The Company considers many factors in completing its quarterly review of securities with unrealized losses for other-than-temporary impairment, including the length of time and the extent to which fair value has been below cost and the financial condition and near-term prospects of the issuer. For equity securities, the ability and intent to hold the security for a period of time sufficient to allow for anticipated recovery is considered. For fixed maturities, the Company considers whether it intends to sell the security or if it is more likely than not that it will be required to sell the security before recovery, the implied yield-to-maturity, the credit quality of the issuer and the ability to recover all amounts outstanding when contractually due.
For equity securities, a decline in fair value that is considered to be other-than-temporary is recognized in net income based on the fair value of the security at the time of assessment, resulting in a new cost basis for the security. For fixed maturities where the Company intends to sell the security or it is more likely than not that the Company will be required to sell the security before recovery of its amortized cost, a decline in fair value is considered to be other-than-temporary and is recognized in net income based on the fair value of the security at the time of assessment, resulting in a new cost basis for the security. If the decline in fair value of a fixed maturity below its amortized cost is considered to be other-than-temporary based upon other considerations, the Company compares the estimated present value of the cash flows expected to be collected to the amortized cost of the security. The extent to which the estimated present value of the cash flows expected to be collected is less than the amortized cost of the security represents the credit-related portion of the other-than-temporary impairment, which is recognized in net income, resulting in a new cost basis for the security. Any remaining decline in fair value represents the non-credit portion of the other-than-temporary impairment, which is recognized in other comprehensive income. The discount rate used to calculate the estimated present value of the cash flows expected to be collected is the effective interest rate implicit for the security at the date of purchase.
When assessing whether it intends to sell a fixed maturity or if it is likely to be required to sell a fixed maturity before recovery of its amortized cost, the Company evaluates facts and circumstances including decisions to reposition the investment portfolio, potential sales of investments to meet cash flow needs and, ultimately, current market prices. Additional information on the methodology and significant inputs, by security type, that the Company used to determine the amount of credit loss recognized on fixed maturities with declines in fair value below amortized cost that were considered to be other-than-temporary is provided below.
Residential / Commercial mortgage-backed securities. For mortgage-backed securities, credit impairment is assessed by estimating future cash flows from the underlying mortgage loans and interest payments. The cash flow estimate incorporates actual cash flows from the mortgage-backed securities through the current period and then projects the remaining cash flows using a number of assumptions, including prepayment rates, default rates, recovery rates on foreclosed properties and loss severity assumptions. Management develops specific assumptions using market data and internal estimates, as well as estimates from rating agencies and other third party sources. Default rates are estimated by considering current underlying mortgage loan performance and expectations of future performance. Estimates of future cash flows are discounted to present value. If the present value of expected cash flows is less than the amortized cost, the Company recognizes the estimated credit loss in net income.
Corporate bonds and obligations of states, municipalities and political subdivisions. For corporate bonds and obligations of states, municipalities and political subdivisions, credit impairment is assessed by evaluating the underlying issuer. As part of this assessment, the Company analyzes various factors, including the following:
•	fundamentals of the issuer, including current and projected operating results, current liquidity position and ability to raise capital;
•	fundamentals of the industry in which the issuer operates;
•	expectations of defaults and recovery rates;
•	changes in ratings by rating agencies;
•	other relevant market considerations; and
•	receipt of interest payments.
Default probabilities and recovery rates from rating agencies are key factors used in calculating the credit loss. Additional research of the industry and issuer is completed to determine if there is any current information that may affect the fixed maturity or its issuer in a negative manner and require an adjustment to the cash flow assumptions.
c)The amortized cost and estimated fair value of fixed maturities at September 30, 2013 are shown below by contractual maturity.
(dollars in thousands)	Amortized			Estimated
	Cost			Fair Value
Due in one year or less	$	900,263		$	905,564
Due after one year through five years	3,151,362			3,220,106
Due after five years through ten years	2,227,954			2,277,491
Due after ten years	2,416,521			2,377,219
	8,696,100			8,780,380
Commercial mortgage-backed securities	401,036			390,709
Residential mortgage-backed securities	927,880			925,974
Asset-backed securities	217,394			215,876
Total fixed maturities	$	10,242,410		$	10,312,939
d)The following table presents the components of net investment income.
	Quarter Ended September 30,						Nine Months Ended September 30,
(dollars in thousands)	2013			2012			2013			2012
Interest:
Municipal bonds (tax-exempt)	$	20,347		$	22,508		$	62,307		$	68,287
Municipal bonds (taxable)	7,452			5,670			19,467			17,055
Other taxable bonds	38,211			25,335			93,565			82,930
Short-term investments, including overnight deposits	1,042			721			2,316			1,995
Dividends on equity securities	12,059			10,128			35,600			30,586
Change in fair value of credit default swap	1,660			1,480			8,860			13,681
Other	9,004			875			16,036			(356		)
	89,775			66,717			238,151			214,178
Investment expenses	(3,583		)	(2,279		)	(9,363		)	(6,344		)
Net investment income	$	86,192		$	64,438		$	228,788		$	207,834
e)Cumulative credit losses recognized in net income on fixed maturities where other-than-temporary impairment was identified and a portion of the other-than-temporary impairment was included in other comprehensive income were $21,370 as of September 30, 2013 and 2012. There were no changes in cumulative credit losses for the quarter and nine months ended September 30, 2013 or 2012.
f)The following table presents net realized investment gains and the change in net unrealized gains on investments.
	Quarter Ended September 30,						Nine Months Ended September 30,
(dollars in thousands)	2013			2012			2013			2012
Realized gains:
Sales of fixed maturities	$	4,933		$	2,576		$	6,031		$	7,863
Sales of equity securities	24,648			10,403			58,563			25,865
Other	398			—			451			655
Total realized gains	29,979			12,979			65,045			34,383
Realized losses:
Sales of fixed maturities	(18,741		)	(118		)	(19,585		)	(457		)
Sales of equity securities	—			(383		)	(170		)	(331		)
Other-than-temporary impairments	—			(3,159		)	(4,589		)	(4,151		)
Other	—			(4,088		)	—			(4,088		)
Total realized losses	(18,741		)	(7,748		)	(24,344		)	(9,027		)
Net realized investment gains	$	11,238		$	5,231		$	40,701		$	25,356
Change in net unrealized gains on investments:
Fixed maturities	$	(8,679	)	$	37,488		$	(346,476	)	$	82,078
Equity securities	117,899			98,964			457,505			248,214
Short-term investments	(13		)	18			(15		)	13
Net increase	$	109,207		$	136,470		$	111,014		$	330,305
g)The following table presents other-than-temporary impairment losses recognized in net income and included in net realized investment gains by investment type.
	Quarter Ended September 30,						Nine Months Ended September 30,
(dollars in thousands)	2013			2012			2013			2012
Fixed maturities:
Obligations of states, municipalities and political subdivisions	$	—		$	—		$	(1,242	)	$	—
Residential mortgage-backed securities	—			—			(523		)	—
Total fixed maturities	—			—			(1,765		)	—
Equity securities:
Insurance, banks and other financial institutions	—			(1,615		)	—			(2,441		)
Industrial, consumer and all other	—			(1,544		)	(2,824		)	(1,710		)
Total equity securities	—			(3,159		)	(2,824		)	(4,151		)
Total	$	—		$	(3,159	)	$	(4,589	)	$	(4,151	)
h)As of September 30, 2013 and December 31, 2012, the Company had restricted assets totaling $5.8 billion and $1.4 billion, respectively. Restricted assets held in trust or on deposit for the benefit of policyholders or ceding companies or to support underwriting activities were $5.1 billion and $1.3 billion as of September 30, 2013 and December 31, 2012, respectively, of which $3.6 billion at September 30, 2013 is attributable to Alterra. Additionally, the Company had pledged investments and cash and cash equivalents totaling $706.8 million and $23.2 million as of September 30, 2013 and December 31, 2012, respectively, as security for letters of credit that have been issued by various banks on behalf of the Company, of which $671.0 million at September 30, 2013 is attributable to Alterra.
Total restricted assets are included on the Company's consolidated balance sheets as follows.
(dollars in thousands)	September 30, 2013			December 31, 2012
Investments, available-for-sale	$	5,121,181		$	1,262,755
Restricted cash and cash equivalents	600,171			109,415
Other assets	38,435			—
Total	$	5,759,787		$	1,372,170

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]	'
Fair Value Measurements	'
	Fair Value Measurements
	FASB ASC 820-10, Fair Value Measurements and Disclosures, establishes a three-level hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets or liabilities (Level 1) and the lowest priority to unobservable inputs (Level 3). If the inputs used to measure the assets or liabilities fall within different levels of the hierarchy, the classification is based on the lowest level input that is significant to the fair value measurement of the asset or liability.
	Classification of assets and liabilities within the hierarchy considers the markets in which the assets and liabilities are traded and the reliability and transparency of the assumptions used to determine fair value. The hierarchy requires the use of observable market data when available. The levels of the hierarchy are defined as follows:
	Level 1 – Inputs to the valuation methodology are quoted prices (unadjusted) for identical assets or liabilities traded in active markets.
	Level 2 – Inputs to the valuation methodology include quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, inputs other than quoted prices that are observable for the asset or liability and market-corroborated inputs.
	Level 3 – Inputs to the valuation methodology are unobservable for the asset or liability and are significant to the fair value measurement.
	In accordance with FASB ASC 820, the Company determines fair value based on 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. In determining fair value, the Company uses various methods, including the market, income and cost approaches. The Company uses valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. The following section describes the valuation methodologies used by the Company to measure assets and liabilities at fair value, including an indication of the level within the fair value hierarchy in which each asset or liability is generally classified.
	Investments available-for-sale. Investments available-for-sale are recorded at fair value on a recurring basis and include fixed maturities, equity securities and short-term investments. Short-term investments include certificates of deposit, commercial paper, discount notes and treasury bills with original maturities of one year or less. Fair value for investments available-for-sale is determined by the Company after considering various sources of information, including information provided by a third party pricing service. The pricing service provides prices for substantially all of the Company's fixed maturities and equity securities. In determining fair value, the Company generally does not adjust the prices obtained from the pricing service. The Company obtains an understanding of the pricing service's valuation methodologies and related inputs, which include, but are not limited to, reported trades, benchmark yields, issuer spreads, bids, offers, duration, credit ratings, estimated cash flows and prepayment speeds. The Company validates prices provided by the pricing service by reviewing prices from other pricing sources and analyzing pricing data in certain instances.
	The Company has evaluated the various types of securities in its investment portfolio to determine an appropriate fair value hierarchy level based upon trading activity and the observability of market inputs. Level 1 investments include those traded on an active exchange, such as the New York Stock Exchange. Level 2 investments include U.S. Treasury securities and obligations of U.S. government agencies, municipal bonds, foreign government bonds, residential mortgage-backed securities and corporate debt securities.
	Fair value for investments available-for-sale is measured based upon quoted prices in active markets, if available. Due to variations in trading volumes and the lack of quoted market prices, fixed maturities are classified as Level 2 investments. The fair value of fixed maturities is normally derived through recent reported trades for identical or similar securities, making adjustments through the reporting date based upon available market observable data described above. If there are no recent reported trades, the fair value of fixed maturities may be derived through the use of matrix pricing or model processes, where future cash flow expectations are developed based upon collateral performance and discounted at an estimated market rate. Significant inputs used to determine the fair value of obligations of states, municipalities and political subdivisions, corporate bonds and obligations of foreign governments include reported trades, benchmark yields, issuer spreads, bids, offers, credit information and estimated cash flows. Significant inputs used to determine the fair value of residential mortgage-backed securities include the type of underlying mortgage loans, benchmark yields, prepayment speeds, collateral information, tranche type and volatility, estimated cash flows, credit information, default rates, recovery rates, issuer spreads and the year of issue.
	Derivatives. Derivatives are recorded at fair value on a recurring basis and include a credit default swap. The fair value of the credit default swap is measured by the Company using an external valuation model. See note 10 for a discussion of the valuation model for the credit default swap, including the key inputs and assumptions used in the model and a description of the valuation processes used by the Company. Due to the significance of unobservable inputs required in measuring the fair value of the credit default swap, the credit default swap has been classified as Level 3 within the fair value hierarchy.
	Senior long-term debt and other debt. Senior long-term debt and other debt is carried at amortized cost with the estimated fair value disclosed on the consolidated balance sheets. Senior long-term debt and other debt is classified as Level 2 within the fair value hierarchy due to variations in trading volumes and the lack of quoted market prices. Fair value for senior long-term debt and other debt is generally derived through recent reported trades for identical securities, making adjustments through the reporting date, if necessary, based upon available market observable data including U.S. Treasury securities and implied credit spreads. Significant inputs used to determine the fair value of senior long-term debt and other debt include reported trades, benchmark yields, issuer spreads, bids and offers.
	The following tables present the balances of assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy.
		September 30, 2013
	(dollars in thousands)	Level 1			Level 2			Level 3			Total
	Assets:
	Investments available-for-sale:
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	—		$	1,244,128		$	—		$	1,244,128
	Obligations of states, municipalities and political subdivisions	—			2,851,061			—			2,851,061
	Foreign governments	—			1,423,684			—			1,423,684
	Commercial mortgage-backed securities	—			390,709			—			390,709
	Residential mortgage-backed securities	—			925,974			—			925,974
	Asset-backed securities	—			215,876			—			215,876
	Corporate bonds	—			3,261,507			—			3,261,507
	Total fixed maturities	—			10,312,939			—			10,312,939
	Equity securities:
	Insurance, banks and other financial institutions	945,431			—			—			945,431
	Industrial, consumer and all other	2,043,183			—			—			2,043,183
	Total equity securities	2,988,614			—			—			2,988,614
	Short-term investments	1,016,185			143,272			—			1,159,457
	Total investments available-for-sale	$	4,004,799		$	10,456,211		$	—		$	14,461,010
	Liabilities:
	Derivative contracts	$	—		$	—		$	3,830		$	3,830
		December 31, 2012
	(dollars in thousands)	Level 1			Level 2			Level 3			Total
	Assets:
	Investments available-for-sale:
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	—		$	317,507		$	—		$	317,507
	Obligations of states, municipalities and political subdivisions	—			2,831,562			—			2,831,562
	Foreign governments	—			556,608			—			556,608
	Residential mortgage-backed securities	—			215,377			—			215,377
	Asset-backed securities	—			14,345			—			14,345
	Corporate bonds	—			1,043,884			—			1,043,884
	Total fixed maturities	—			4,979,283			—			4,979,283
	Equity securities:
	Insurance, banks and other financial institutions	898,067			—			—			898,067
	Industrial, consumer and all other	1,508,884			—			—			1,508,884
	Total equity securities	2,406,951			—			—			2,406,951
	Short-term investments	888,758			84,572			—			973,330
	Total investments available-for-sale	$	3,295,709		$	5,063,855		$	—		$	8,359,564
	Liabilities:
	Derivative contracts	$	—		$	—		$	12,690		$	12,690
	The following table summarizes changes in Level 3 liabilities measured at fair value on a recurring basis.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Derivatives, beginning of period	$	5,490		$	17,130		$	12,690		$	29,331
	Total gains included in:
	Net income	(1,660		)	(1,480		)	(8,860		)	(13,681		)
	Other comprehensive income	—			—			—			—
	Transfers into Level 3	—			—			—			—
	Transfers out of Level 3	—			—			—			—
Derivatives, end of period	$	3,830		$	15,650		$	3,830		$	15,650
Net unrealized gains included in net income relating to liabilities held at September 30, 2013 and 2012 (1)	$	1,660		$	1,480		$	8,860		$	13,681
(1)	Included in net investment income in the consolidated statements of income and comprehensive income.
There were no transfers into or out of Level 1 and Level 2 during the quarter and nine months ended September 30, 2013 and 2012. Except as disclosed in note 3, the Company did not have any assets or liabilities measured at fair value on a non-recurring basis during the nine months ended September 30, 2013 and 2012.

Segment_Reporting_Disclosures

Segment Reporting Disclosures	9 Months Ended
	Sep. 30, 2013
Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract]	'
Segment Reporting Disclosures	'
	Segment Reporting Disclosures
	The Company historically operated in three segments of the specialty insurance marketplace: the Excess and Surplus Lines, the Specialty Admitted and the London Insurance Market segments. The Company considers many factors, including the nature of its insurance products, production sources, distribution strategies and regulatory environment in determining how to aggregate operating segments.
	As a result of the acquisition of Alterra, the Company formed a new operating segment, effective May 1, 2013. The Alterra segment is comprised of all of the active property and casualty underwriting operations of the former Alterra companies. The Alterra segment provides specialty insurance and reinsurance products worldwide from offices in the United States, Bermuda, the United Kingdom, Europe and Latin America. Results attributable to Alterra are being separately evaluated by management. The Company is in the process of integrating Alterra with its existing operations, which is not expected to be complete until 2014.
	For purposes of segment reporting, the Other Insurance (Discontinued Lines) segment includes lines of business that have been discontinued in conjunction with acquisitions. Prior to its acquisition by the Company, Alterra offered life and annuity reinsurance products. In 2010, Alterra ceased writing life and annuity reinsurance contracts and placed this business into run-off. Results attributable to the run-off of Alterra's life and annuity reinsurance business are included in the Company's Other Insurance (Discontinued Lines) segment.
	All investing activities related to our insurance operations are included in the Investing segment.
	The Company's non-insurance operations primarily consist of controlling interests in various industrial and service businesses. For purposes of segment reporting, the Company's non-insurance operations are not considered to be a reportable operating segment.
	Segment profit for the Investing segment is measured by net investment income and net realized investment gains or losses. Segment profit or loss for the Company's operating segments is primarily measured by underwriting profit or loss. The property and casualty insurance industry commonly defines underwriting profit or loss as earned premiums net of losses and loss adjustment expenses and underwriting, acquisition and insurance expenses. Underwriting profit or loss does not replace operating income or net income computed in accordance with U.S. GAAP as a measure of profitability. Underwriting profit or loss provides a basis for management to evaluate the Company's underwriting performance. Segment profit or loss for the Company's operating segments also includes other revenues and other expenses, primarily related to the run-off of managing general agent operations that were discontinued in conjunction with acquisitions. Other revenues and other expenses in the Other Insurance (Discontinued Lines) segment are comprised of the results attributable to the run-off of Alterra's life and annuity reinsurance business.
	For management reporting purposes, the Company allocates assets to its underwriting, investing and non-insurance operations. Underwriting assets are all assets not specifically allocated to the Investing segment or to the Company's non-insurance operations. Underwriting and investing assets are not allocated to the Excess and Surplus Lines, Specialty Admitted, London Insurance Market, Alterra or Other Insurance (Discontinued Lines) segments since the Company does not manage its assets by operating segment. The Company does not allocate capital expenditures for long-lived assets to any of its operating segments for management reporting purposes.
	a)	The following tables summarize the Company's segment disclosures.
		Quarter Ended September 30, 2013
	(dollars in thousands)	Excess and			Specialty			London			Alterra				Other				Investing			Consolidated
		Surplus			Admitted			Insurance							Insurance
		Lines						Market							(Discontinued
															Lines)
	Gross premium volume	$	273,757		$	230,450		$	199,575		$	372,516			$	—			$	—		$	1,076,298
	Net written premiums	232,396			219,955			174,203			216,378				(1		)		—			842,931
	Earned premiums	218,538			197,372			199,633			304,181				(1		)		—			919,723
	Losses and loss adjustment expenses:
	Current accident year	(147,204		)	(126,649		)	(135,098		)	(201,732		)		—				—			(610,683		)
	Prior accident years	40,799			28,410			40,562			—				(32,460		)		—			77,311
	Underwriting, acquisition and insurance expenses:
	Transaction costs and other acquisition-related expenses (1)	—			—			—			(8,508		)		—				—			(8,508		)
	All other expenses	(85,570		)	(80,023		)	(73,335		)	(104,608		)		(82		)		—			(343,618		)
	Underwriting profit (loss)	26,563			19,110			31,762			(10,667		)		(32,543		)		—			34,225
	Net investment income	—			—			—			—				—				86,192			86,192
	Net realized investment gains	—			—			—			—				—				11,238			11,238
	Other revenues (insurance)	—			1,402			311			59				160				—			1,932
	Other expenses (insurance)	—			(6,292		)	(1,167		)	—				(8,789		)		—			(16,248		)
	Segment profit (loss)	$	26,563		$	14,220		$	30,906		$	(10,608	)		$	(41,172	)		$	97,430		$	117,339
	Other revenues (non-insurance)																					172,580
	Other expenses (non-insurance)																					(150,318		)
	Amortization of intangible assets																					(16,848		)
	Interest expense																					(30,619		)
	Income before income taxes																					$	92,134
	U.S. GAAP combined ratio (2)	88		%	90		%	84		%	104		%		NM			(3)				96		%
	(1)	In connection with the acquisition of Alterra, the Company incurred acquisition-related costs totaling $8.5 million for the quarter ended September 30, 2013, which included severance costs of $3.5 million, stay bonuses of $3.9 million and other compensation costs of $1.1 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
	(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
	(3)	NM – Ratio is not meaningful.
		Quarter Ended September 30, 2012
	(dollars in thousands)	Excess and			Specialty			London			Other				Investing				Consolidated
		Surplus			Admitted			Insurance			Insurance
		Lines						Market			(Discontinued
											Lines)
	Gross premium volume	$	253,014		$	167,747		$	190,071		$	1			$	—			$	610,833
	Net written premiums	211,538			157,894			170,193			(1		)		—				539,624
	Earned premiums	195,478			153,009			182,052			(2		)		—				530,537
	Losses and loss adjustment expenses:
	Current accident year	(134,504		)	(116,044		)	(122,192		)	—				—				(372,740		)
	Prior accident years	51,310			11,504			32,744			(26,277		)		—				69,281
	Underwriting, acquisition and insurance expenses:
	Prospective adoption of ASU 2010-26 (1)	(2,125		)	(2,600		)	(1,809		)	—				—				(6,534		)
	All other expenses	(88,093		)	(60,008		)	(76,820		)	(386		)		—				(225,307		)
	Underwriting profit (loss)	22,066			(14,139		)	13,975			(26,665		)		—				(4,763		)
	Net investment income	—			—			—			—				64,438				64,438
	Net realized investment gains	—			—			—			—				5,231				5,231
	Other revenues (insurance)	—			11,536			223			—				—				11,759
	Other expenses (insurance)	—			(12,181		)	(970		)	—				—				(13,151		)
	Segment profit (loss)	$	22,066		$	(14,784	)	$	13,228		$	(26,665	)		$	69,669			$	63,514
	Other revenues (non-insurance)																		153,810
	Other expenses (non-insurance)																		(132,188		)
	Amortization of intangible assets																		(7,959		)
	Interest expense																		(24,692		)
	Income before income taxes																		$	52,485
U.S. GAAP combined ratio (2)	89		%	109		%	92		%	NM			(3)					101		%
(1)	Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included $43.1 million of costs that no longer met the criteria for deferral as of January 1, 2012 and were recognized into income during 2012, consistent with policy terms. The quarter ended September 30, 2012 included $6.5 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.
(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
(3)	NM – Ratio is not meaningful.
	Nine Months Ended September 30, 2013
(dollars in thousands)	Excess and			Specialty			London			Alterra				Other				Investing			Consolidated
	Surplus			Admitted			Insurance							Insurance
	Lines						Market							(Discontinued
														Lines)
Gross premium volume	$	792,877		$	687,669		$	725,153		$	714,844			$	35			$	—		$	2,920,578
Net written premiums	673,741			655,350			634,180			469,894				35				—			2,433,200
Earned premiums	629,288			533,037			577,101			529,668				35				—			2,269,129
Losses and loss adjustment expenses:
Current accident year	(424,398		)	(353,581		)	(386,617		)	(380,513		)		—				—			(1,545,109		)
Prior accident years	165,587			45,863			101,589			—				(31,604		)		—			281,435
Underwriting, acquisition and insurance expenses:
Transaction costs and other acquisition-related expenses (1)	—			—			—			(70,317		)		—				—			(70,317		)
All other expenses	(250,101		)	(229,462		)	(222,469		)	(171,762		)		217				—			(873,577		)
Underwriting profit (loss)	120,376			(4,143		)	69,604			(92,924		)		(31,352		)		—			61,561
Net investment income	—			—			—			—				—				228,788			228,788
Net realized investment gains	—			—			—			—				—				40,701			40,701
Other revenues (insurance)	—			11,945			4,778			1,256				479				—			18,458
Other expenses (insurance)	—			(14,447		)	(3,762		)	—				(13,039		)		—			(31,248		)
Segment profit (loss)	$	120,376		$	(6,645	)	$	70,620		$	(91,668	)		$	(43,912	)		$	269,489		$	318,260
Other revenues (non-insurance)																					486,222
Other expenses (non-insurance)																					(428,394		)
Amortization of intangible assets																					(37,755		)
Interest expense																					(82,754		)
Income before income taxes																					$	255,579
U.S. GAAP combined ratio (2)	81		%	101		%	88		%	118		%		NM			(3)				97		%
(1)	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the nine months ended September 30, 2013, which primarily consist of due diligence, legal and investment banking costs. Additionally, the Company incurred severance costs of $31.7 million, stay bonuses of $10.0 million and other compensation costs totaling $12.6 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
(3)	NM – Ratio is not meaningful.
	Nine Months Ended September 30, 2012
(dollars in thousands)	Excess and			Specialty			London			Other				Investing				Consolidated
	Surplus			Admitted			Insurance			Insurance
	Lines						Market			(Discontinued
										Lines)
Gross premium volume	$	705,849		$	496,019		$	704,511		$	(6	)		$	—			$	1,906,373
Net written premiums	597,742			467,722			621,947			(7		)		—				1,687,404
Earned premiums	584,524			431,179			557,493			(7		)		—				1,573,189
Losses and loss adjustment expenses:
Current accident year	(390,254		)	(310,115		)	(372,869		)	—				—				(1,073,238		)
Prior accident years	132,583			27,747			118,994			(19,160		)		—				260,164
Underwriting, acquisition and insurance expenses:
Prospective adoption of ASU 2010-26 (1)	(16,652		)	(12,863		)	(11,578		)	—				—				(41,093		)
All other expenses	(255,295		)	(171,111		)	(226,770		)	(1,053		)		—				(654,229		)
Underwriting profit (loss)	54,906			(35,163		)	65,270			(20,220		)		—				64,793
Net investment income	—			—			—			—				207,834				207,834
Net realized investment gains	—			—			—			—				25,356				25,356
Other revenues (insurance)	—			36,065			4,753			—				—				40,818
Other expenses (insurance)	—			(35,184		)	(2,722		)	—				—				(37,906		)
Segment profit (loss)	$	54,906		$	(34,282	)	$	67,301		$	(20,220	)		$	233,190			$	300,895
Other revenues (non-insurance)																		344,960
Other expenses (non-insurance)																		(305,556		)
Amortization of intangible assets																		(25,078		)
Interest expense																		(69,068		)
Income before income taxes																		$	246,153
U.S. GAAP combined ratio (2)	91		%	108		%	88		%	NM			(3)					96		%
(1)	Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included $43.1 million of costs that no longer met the criteria for deferral as of January 1, 2012 and were recognized into income during 2012, consistent with policy terms. The nine months ended September 30, 2012 included $41.1 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.
(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
(3)	NM – Ratio is not meaningful.
b)	The following table reconciles segment assets to the Company's consolidated balance sheets.
(dollars in thousands)	September 30, 2013			December 31, 2012
Segment assets:
Investing	$	17,125,848		$	9,277,697
Underwriting	5,778,934			2,387,305
Total segment assets	22,904,782			11,665,002
Non-insurance operations	903,795			891,586
Total assets	$	23,808,577		$	12,556,588

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2013
Reinsurance Disclosures [Abstract]	'
Reinsurance	'
	Reinsurance
	The following tables summarize the effect of reinsurance on premiums written and earned.
		Quarter Ended September 30,
		2013						2012
	(dollars in thousands)	Written			Earned			Written			Earned
	Direct	$	836,117		$	818,854		$	519,408		$	505,514
	Assumed	240,181			331,495			91,425			94,822
	Ceded	(233,367		)	(230,626		)	(71,209		)	(69,799		)
	Net premiums	$	842,931		$	919,723		$	539,624		$	530,537
		Nine Months Ended September 30,
		2013						2012
	(dollars in thousands)	Written			Earned			Written			Earned
	Direct	$	2,310,966		$	2,076,682		$	1,568,092		$	1,505,710
	Assumed	609,612			675,286			338,281			269,771
	Ceded	(487,378		)	(482,839		)	(218,969		)	(202,292		)
	Net premiums	$	2,433,200		$	2,269,129		$	1,687,404		$	1,573,189
	The percentage of ceded earned premiums to gross earned premiums was 20% and 12%, respectively, for the quarters ended September 30, 2013 and 2012 and 18% and 11%, respectively, for the nine months ended September 30, 2013 and 2012.
	The Company uses reinsurance to manage its net retention on individual risks and overall exposure to losses while providing it with the ability to offer policies with sufficient limits to meet policyholder needs. In a reinsurance transaction, an insurance company transfers, or cedes, all or part of its exposure in return for a portion of the premium. The ceding of insurance does not legally discharge the Company from its primary liability for the full amount of the policies, and the Company will be required to pay the loss and bear collection risk if the reinsurer fails to meet its obligations under the reinsurance agreement.
	A credit risk exists with reinsurance ceded to the extent that any reinsurer is unable to meet the obligations assumed under the reinsurance agreements. Allowances are established for amounts deemed uncollectible. The Company evaluates the financial condition of its reinsurers and monitors concentration of credit risk arising from its exposure to individual reinsurers. At September 30, 2013 and December 31, 2012, balances recoverable from the Company's ten largest reinsurers, by group, represented approximately 61% and 74%, respectively, of the reinsurance recoverable on paid and unpaid losses, before considering reinsurance allowances. At September 30, 2013, the Company's largest reinsurance balance was due from Fairfax Financial Group and represented 10% of the reinsurance recoverable on paid and unpaid losses, before considering reinsurance allowances.
	Incurred losses and loss adjustment expenses were net of reinsurance recoverables (ceded incurred losses and loss adjustment expenses) of $98.0 million and $15.3 million, respectively, for the quarters ended September 30, 2013 and 2012 and $179.9 million and $79.6 million, respectively, for the nine months ended September 30, 2013 and 2012.

Life_And_Annuity_Benefits

Life And Annuity Benefits	9 Months Ended
	Sep. 30, 2013
Liability for Future Policy Benefits [Abstract]	'
Life And Annuity Benefits	'
	Life and Annuity Benefits
	Prior to its acquisition by the Company, Alterra entered into long duration reinsurance contracts which subject the Company to mortality, longevity and morbidity risks. The Company's related life and annuity reinsurance benefit reserves are compiled on a reinsurance contract-by-contract basis and are discounted using standard actuarial techniques and cash flow models. Since the development of the life and annuity reinsurance reserves is based upon cash flow projection models, the Company must make estimates and assumptions based on cedant experience and industry mortality tables, longevity, expense and investment experience, including a provision for adverse deviation. The assumptions used to determine policy benefit reserves were determined at the Acquisition Date and are generally locked-in for the life of the contract unless a premium deficiency develops or another unlocking event occurs. As the experience on the reinsurance contracts emerges, the assumptions are reviewed by management, no less than annually, to determine whether the actual and anticipated experience indicates that existing policy reserves, together with the present value of future gross premiums and expected investment income earned thereon, are sufficient to cover the present value of future benefits, settlement and maintenance costs. If such a review produces reserves in excess of those currently held, then the lock-in assumptions are revised and a charge to earnings for life and annuity benefits is recognized at that time. Results attributable to the run-off of Alterra's life and annuity reinsurance business are included in other revenues and other expenses in the Company's consolidated statements of income and comprehensive income and as part of the Company's Other Insurance (Discontinued Lines) segment.
	Because of the many assumptions and estimates used in establishing reserves and the long-term nature of reinsurance contracts, the reserving process, while based on actuarial science, is inherently uncertain. Actual results could differ materially from these estimates. The average discount rate for the life and annuity benefit reserves was 2.3% as of September 30, 2013.
	The following table presents life and annuity benefits as of September 30, 2013:
		(dollars in thousands)
	Life	$	189,792
	Annuities	1,188,914
	Accident and health	104,292
		$	1,482,998

Senior_LongTerm_Debt_And_Other

Senior Long-Term Debt And Other Debt	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Senior Long-Term Debt And Other Debt	'
	Senior Long-Term Debt and Other Debt
	The following table summarizes the Company's senior long-term debt and other debt.
	(dollars in thousands)	September 30, 2013		December 31, 2012
	6.80% unsecured senior notes, due February 15, 2013, interest payable semi-annually, net of unamortized discount of $45 in 2012	$	—	$	246,619
	7.20% unsecured senior notes, due April 14, 2017, interest payable semi-annually, net of unamortized premium of $5,160 in 2013	95,789		—
	7.125% unsecured senior notes, due September 30, 2019, interest payable semi-annually, net of unamortized discount of $1,697 in 2013 and $1,909 in 2012	348,303		348,091
	6.25% unsecured senior notes, due September 30, 2020, interest payable semi-annually, net of unamortized premium of $64,049 in 2013	414,049		—
	5.35% unsecured senior notes, due June 1, 2021, interest payable semi-annually, net of unamortized discount of $1,583 in 2013 and $1,738 in 2012	248,417		248,262
	4.90% unsecured senior notes, due July 1, 2022, interest payable semi-annually, net of unamortized discount of $2,444 in 2013 and $2,653 in 2012	347,556		347,347
	3.625% unsecured senior notes, due March 30, 2023, interest payable semi-annually, net of unamortized discount of $1,910 in 2013	248,090		—
	7.35% unsecured senior notes, due August 15, 2034, interest payable semi-annually, net of unamortized discount of $2,211 in 2013 and $2,291 in 2012	197,789		197,710
	5.0% unsecured senior notes, due March 30, 2043, interest payable semi-annually, net of unamortized discount of $6,607 in 2013	243,393		—
	Other debt, at various interest rates ranging from 1.6% to 6.5%	107,093		104,521
	Senior long-term debt and other debt	$	2,250,479	$	1,492,550
	On July 2, 2012, the Company issued $350 million of 4.90% unsecured senior notes due July 1, 2022. Net proceeds to the Company were $347.2 million. On August 1, 2012, the Company used a portion of these proceeds to redeem its 7.50% unsecured senior debentures due August 22, 2046 at a redemption price equal to 100% of their principal amount, or $150 million. This redemption resulted in a loss of $4.1 million, which is reflected in net realized investment gains. The proceeds from the July 2012 issuance were also used to pre-fund the repayment of the Company's 6.80% unsecured senior notes due February 15, 2013.
	On February 15, 2013, the Company repaid its 6.80% unsecured senior notes ($246.7 million principal amount outstanding at December 31, 2012).
	On March 8, 2013, the Company issued $250 million of 3.625% unsecured senior notes due March 30, 2023 and $250 million of 5.0% unsecured senior notes due March 30, 2043. Net proceeds to the Company were approximately $491.2 million, to be used for general corporate purposes.
	On April 16, 2007, Alterra USA Holdings Limited (Alterra USA), a wholly-owned indirect subsidiary of Alterra, privately issued $100 million of 7.20% unsecured senior notes due April 14, 2017 (the 7.20% unsecured senior notes). The 7.20% unsecured senior notes are Alterra USA's senior unsecured obligations and rank equally in right of payment with all existing and future senior unsecured indebtedness of Alterra USA. The 7.20% unsecured senior notes are fully and unconditionally guaranteed by Alterra. The principal amount of the 7.20% unsecured senior notes outstanding as of the Acquisition Date was $90.6 million. As of the Acquisition Date, the 7.20% unsecured senior notes were recorded at their estimated fair value of $95.8 million.
	On September 27, 2010, Alterra Finance LLC, a wholly-owned indirect subsidiary of Alterra, issued $350 million of 6.25% unsecured senior notes due September 30, 2020 (the 6.25% unsecured senior notes). The 6.25% unsecured senior notes are Alterra Finance LLC's senior unsecured obligations and rank equally in right of payment with all of Alterra Finance LLC's future unsecured and unsubordinated indebtedness and rank senior to all of Alterra Finance LLC's future subordinated indebtedness. The 6.25% unsecured senior notes are fully and unconditionally guaranteed by Alterra on a senior unsecured basis. The guarantee ranks equally with all of Alterra's existing and future unsecured and unsubordinated indebtedness and ranks senior to all of Alterra's future subordinated indebtedness. As of the Acquisition Date, the 6.25% unsecured senior notes were recorded at their estimated fair value of $416.6 million.
	Alterra Finance LLC is a finance subsidiary and has no independent activities, assets or operations other than in connection with the 6.25% unsecured senior notes.
	The Company's 7.35% unsecured senior notes due August 15, 2034 are not redeemable. The Company's other unsecured senior notes are redeemable by the Company at any time, subject to payment of a make-whole premium to the noteholders. None of the Company's senior long-term debt is subject to any sinking fund requirements.
	The Company's other debt is primarily associated with its non-insurance operations and is non-recourse to the holding company. The debt of the Company's non-insurance subsidiaries generally is secured by the assets of those subsidiaries. ParkLand Ventures, Inc., a subsidiary of the Company, has formed subsidiaries for the purpose of acquiring and financing real estate (the real estate subsidiaries). The assets of the real estate subsidiaries, which are not material to the Company, are consolidated in accordance with U.S. GAAP but are not available to satisfy the debt and other obligations of the Company or any affiliates other than the real estate subsidiaries.
	The following table summarizes the future principal payments due at maturity on senior long-term debt and other debt as of September 30, 2013.
	Years Ending December 31,	(dollars in
		thousands)
	2013	$	4,498
	2014	17,857
	2015	5,090
	2016	12,092
	2017	104,366
	2018 and thereafter	2,053,819
	Total principal payments	$	2,197,722
	Net unamortized premium	52,757
	Senior long-term debt and other debt	$	2,250,479
	The Company also maintains a revolving credit facility for working capital and other general corporate purposes that expires September 2015. Effective July 12, 2013, the capacity of the Company's revolving credit facility was increased from $150 million to $300 million, as previously provided for by the terms of the revolving credit facility. The Company may select from two interest rate options for balances outstanding under the revolving credit facility and pays a commitment fee (0.25% at September 30, 2013) on the unused portion of the facility based on the Company's debt to equity leverage ratio as calculated under the agreement. At September 30, 2013 and December 31, 2012, the Company had no borrowings outstanding under the revolving credit facility.
	Alterra and Markel Bermuda are party to a $900 million secured credit facility (the senior credit facility), which expires on December 15, 2015. The senior credit facility provides for secured letters of credit to be issued for the account of Alterra, Markel Bermuda and certain other subsidiaries of Alterra and for loans to Alterra and Markel Bermuda. Loans under the senior credit facility are subject to a sublimit of $250 million. Subject to certain conditions and at the request of Alterra, the aggregate commitments of the lenders under the senior credit facility may be increased up to a total of $1.4 billion. At September 30, 2013, the Company had no borrowings outstanding under the senior credit facility. At September 30, 2013, $439.6 million of letters of credit were issued and outstanding under the senior credit facility.
	At September 30, 2013, the Company was in compliance with all covenants contained in its revolving credit facility and senior credit facility. To the extent that the Company is not in compliance with its covenants, the Company's access to these credit facilities could be restricted. While the Company believes this to be unlikely, the inability to access the revolving credit facility and senior credit facility could adversely affect the Company's liquidity.

Derivatives

Derivatives	9 Months Ended
	Sep. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Derivatives	'
	Derivatives
	The Company is a party to a credit default swap agreement, under which third party credit risk is transferred from a counterparty to the Company. The Company entered into the credit default swap agreement for investment purposes. At both September 30, 2013 and December 31, 2012, the notional amount of the credit default swap was $33.1 million, which represented the Company's aggregate exposure to losses if specified credit events involving third party reference entities occur. These third party reference entities are specified under the terms of the agreement and represent a portfolio of names upon which the Company has assumed credit risk from the counterparty. The Company's exposure to loss from any one reference entity is limited to $20.0 million. The credit default swap has a scheduled termination date of December 2014.
	The credit default swap is accounted for as a derivative instrument and is recorded at fair value with any changes in fair value recorded in net investment income. At September 30, 2013 and December 31, 2012, the credit default swap had a fair value of $3.8 million and $12.7 million, respectively. The fair value of the credit default swap is included in other liabilities on the consolidated balance sheets. Net investment income for the quarter and nine months ended September 30, 2013 included favorable changes in the fair value of the credit default swap of $1.7 million and $8.9 million, respectively. Net investment income for the quarter and nine months ended September 30, 2012 included favorable changes in the fair value of the credit default swap of $1.5 million and $13.7 million, respectively.
	The fair value of the credit default swap is determined by the Company using a Gaussian copula valuation model, a market standard model for valuing credit default swaps. The fair value is dependent upon several inputs, including changes in interest rates, credit spreads, expected default rates, changes in credit quality, future expected recovery rates and other market factors. The significant unobservable inputs used in the fair value measurement of the credit default swap are expected default rates and future expected recovery rates. The Company determines these unobservable inputs based upon default rates and recovery rates used to price similar credit default swap indices. A significant increase in expected default rates in isolation results in a significantly higher fair value measurement, while a significant decrease in expected default rates results in a significantly lower fair value measurement. A significant increase in future expected recovery rates in isolation results in a significantly lower fair value measurement, while a significant decrease in future expected recovery rates results in a significantly higher fair value measurement. Generally, a change in the assumption used for expected default rates is accompanied by a directionally opposite change in future expected recovery rates. The fair value measurement of the credit default swap at September 30, 2013 included expected default rates ranging between less than 1% and 10%, with a weighted-average expected default rate of 1%, and future expected recovery rates ranging between 20% and 40%, with a weighted-average future expected recovery rate of 39%. The fair value measurement of the credit default swap at December 31, 2012 included expected default rates ranging between 1% and 43%, with a weighted-average expected default rate of 3%, and future expected recovery rates ranging between 20% and 40%, with a weighted-average future expected recovery rate of 39%.
	The Company's valuation policies and procedures for the credit default swap are determined by an internal investment manager with oversight provided by the Company's Chief Financial Officer and Chief Investment Officer. Fair value measurements are analyzed quarterly to ensure the change in fair value from prior periods is reasonable relative to recent market trends. Additionally, the reported fair value of the credit default swap is compared to results from similar valuation models.
	The Company had no other material derivative instruments at September 30, 2013.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	The Company's effective tax rate was 28% for the nine months ended September 30, 2013 compared to 19% for the same period in 2012. In both periods, the effective tax rate differs from the statutory tax rate of 35% primarily as a result of tax-exempt investment income. The increase in the effective tax rate in 2013 was primarily due to higher estimated earnings taxed at a 35% tax rate and anticipating a smaller tax benefit related to tax-exempt investment income, which resulted from having higher estimated income before income taxes in 2013 compared to 2012.

Employee_Benefit_Plans

Employee Benefit Plans	9 Months Ended
	Sep. 30, 2013
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'
Employee Benefit Plans	'
	Employee Benefit Plans
	a)Expenses relating to the Company's defined contribution plans, including the defined contribution plans of Alterra effective May 1, 2013, were $6.2 million and $16.7 million, respectively, for the quarter and nine months ended September 30, 2013 and $4.6 million and $13.9 million, respectively, for the same periods of 2012.
	b)The following table presents the components of net periodic benefit income for the Terra Nova Pension Plan, a defined benefit plan.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Service cost	$	—		$	90		$	—		$	269
	Interest cost	1,611			1,693			4,837			5,079
	Expected return on plan assets	(2,671		)	(2,432		)	(8,015		)	(7,295		)
	Amortization of net actuarial pension loss	477			643			1,432			1,930
	Net periodic benefit income	$	(583	)	$	(6	)	$	(1,746	)	$	(17	)
	The Company contributed $5.3 million to the Terra Nova Pension Plan during the nine months ended September 30, 2013 and does not expect to make any additional contributions in 2013.

Shareholders_Equity

Shareholders' Equity	9 Months Ended
	Sep. 30, 2013
Stockholders' Equity Note [Abstract]	'
Shareholders' Equity	'
	Shareholders' Equity
	a)     Net income per share was determined by dividing adjusted net income to shareholders by the applicable weighted average shares outstanding. Unvested share-based compensation awards that contain non-forfeitable rights to dividends or dividend equivalents (whether paid or unpaid) are considered participating awards and are included in the computation of net income per share. Non-participating unvested share-based compensation awards are excluded from the computation of net income per share. Diluted net income per share is computed by dividing adjusted net income to shareholders by the weighted average number of common shares and dilutive potential common shares outstanding during the year.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(in thousands, except per share amounts)	2013			2012			2013			2012
	Net income to shareholders	$	65,599		$	49,653		$	182,257		$	196,593
	Adjustment of redeemable noncontrolling interests	—			1,702			3,101			(6,484		)
	Adjusted net income to shareholders	$	65,599		$	51,355		$	185,358		$	190,109
	Basic common shares outstanding	14,001			9,632			12,050			9,641
	Dilutive potential common shares from conversion of options	16			4			11			7
	Dilutive potential common shares from conversion of non participating restricted stock	39			23			32			19
	Diluted shares outstanding	14,056			9,659			12,093			9,667
	Basic net income per share	$	4.69		$	5.33		$	15.38		$	19.72
	Diluted net income per share	$	4.67		$	5.32		$	15.33		$	19.67
	b)     The 2012 Equity Incentive Compensation Plan (2012 Compensation Plan) provides for grants and awards of restricted stock, restricted stock units, performance grants, and other stock based awards to employees and directors. The 2012 Compensation Plan is administered by the Compensation Committee of the Company's Board of Directors (Compensation Committee). In May 2013, the Compensation Committee awarded 31,669 restricted stock units to certain associates and executive officers to assist the Company in retaining the services of key employees. The restricted stock units had a grant-date fair value of $16.6 million. The awards vest over a three year period and entitle the recipient to receive one share of the Company's common stock for each vested restricted stock unit.
	c)     In May 2013, in connection with the acquisition of Alterra, the Company issued 101,875 replacement options and 154,103 restricted stock awards to holders of Alterra options and restricted stock awards. The replacement options and restricted stock awards were issued under the terms and conditions of the Alterra Capital Holdings Limited 2008 Stock Incentive Plan, the Alterra Capital Holdings Limited 2006 Equity Incentive Plan and the Alterra Capital Holdings Limited 2000 Stock Incentive Plan (collectively, the Alterra Equity Award Plans). No further options or restricted stock awards are available for issuance under the Alterra Equity Award Plans.
	The replacement options issued were fully vested and exercisable as of the Acquisition Date and had a weighted average exercise price of $398.96 and a grant-date fair value of $140.08, which was included in the acquisition consideration. The fair value of the options was estimated on the grant date using the Black-Scholes option pricing model. Assumptions used in the pricing model included an expected annual volatility of 19.04%, a risk-free rate of approximately 0.20% and an expected term of approximately two years. The expected annual volatility was based on the historical volatility of the Company's stock and other factors. The risk-free rate was based on the U.S. Treasury yield curve, with a remaining term equal to the expected term assumption at the grant date. The expected term of the options granted represents the period of time that the options were expected to be outstanding at the grant date.
	The following table summarizes additional information with respect to these options.
		Number of Shares		Weighted Average Exercise Price			Weighted Average Remaining Contractual Term (years)		Intrinsic Value (in millions)
	Outstanding and exercisable, May 1, 2013	101,875		$	398.96
	Exercised	49,763		$	401.83
	Outstanding and exercisable, September 30, 2013	52,112		$	396.22		2.6		$	6.7
	From the Acquisition Date through September 30, 2013, 49,763 options were exercised under the Alterra Equity Award Plans, resulting in cash proceeds of $18.5 million and a current tax benefit of $0.7 million. The intrinsic value of options exercised in 2013 was $6.4 million.
	The replacement restricted stock awards issued by the Company had a grant date fair value of $81.6 million, or $529.59 per share. The awards were partially vested as of the Acquisition Date and had a weighted average remaining service period of approximately one year. As a result, $61.0 million was recognized as part of the Acquisition Consideration and $20.6 million will be recognized as compensation expense over the remaining service period of the awards. The fair value of the replacement restricted stock awards was determined based on the weighted average price of the Company's stock on April 30, 2013, the day preceding the Acquisition Date. The following table summarizes activity related to these nonvested restricted stock awards.
		Number		Weighted Average
		of Awards		Grant-Date
				Fair Value
	Nonvested awards issued on May 1, 2013	154,103		$	529.59
	Vested	(76,287	)	529.59
	Forfeited	(7,476	)	529.59
	Nonvested awards at September 30, 2013	70,340		$	529.59
	The Company recognized compensation expense totaling $1.5 million and $10.9 million, respectively, during the quarter and nine months ended September 30, 2013 related to these restricted stock awards. As of September 30, 2013, unrecognized compensation cost related to the nonvested restricted stock awards was $6.5 million, which is expected to be recognized over a weighted average period of approximately one year. The fair value of the restricted stock awards that vested during 2013 was $40.4 million.
	d)     In accordance with the terms of the Merger Agreement, the Company also assumed outstanding warrants to purchase Alterra common stock.  Holders of the warrants had the option to surrender them for consideration specified in the Merger Agreement, which included cash and registered shares of the Company's common stock, or to have them remain outstanding as "Company Converted Warrants," in which case the holders would be entitled to a combination of cash and unregistered shares of the Company's common stock upon exercise of the warrants. The warrants of all warrant holders who elected to exercise in conjunction with the Acquisition Date were considered to have been exercised on May 1, 2013. The Company issued 225,529 Company Converted Warrants with a fair value of $73.7 million to Alterra warrant holders who elected not to surrender their warrants in connection with the merger. Three holders who elected to have their warrants remain outstanding subsequently exercised a total of 225,499 warrants and received a total of 100,115 shares of the Company's common stock under the cashless exercise provision of the warrants.  As of September 30, 2013, the Company had outstanding warrants to purchase 30 shares of its common stock.

Other_Comprehensive_Income

Other Comprehensive Income	9 Months Ended
	Sep. 30, 2013
Other Comprehensive Income (Loss), Tax [Abstract]	'
Other Comprehensive Income	'
	Other Comprehensive Income
	Other comprehensive income includes net holding gains arising during the period, changes in unrealized other-than-temporary impairment losses on fixed maturities arising during the period and reclassification adjustments for net gains included in net income. Other comprehensive income also includes changes in foreign currency translation adjustments and changes in net actuarial pension loss.
	The following table presents the change in accumulated other comprehensive income by component, net of taxes and noncontrolling interests, for the nine months ended September 30, 2013 and 2012.
	(dollars in thousands)	Unrealized Holding Gains on Available-for-Sale Securities			Foreign Currency			Net Actuarial Pension Loss			Total
	31-Dec-11	$	704,719		$	(2,614	)	$	(41,185	)	$	660,920
	Other comprehensive income before reclassifications	245,920			2,969			—			248,889
	Amounts reclassified from accumulated other comprehensive income	(20,154		)	—			1,487			(18,667		)
	Total other comprehensive income	225,766			2,969			1,487			230,222
	30-Sep-12	$	930,485		$	355		$	(39,698	)	$	891,142
	31-Dec-12	$	946,933		$	(1,075	)	$	(34,521	)	$	911,337
	Other comprehensive income before reclassifications	107,433			(9,931		)	—			97,502
	Amounts reclassified from accumulated other comprehensive income	(27,866		)	—			1,146			(26,720		)
	Total other comprehensive income	79,567			(9,931		)	1,146			70,782
	30-Sep-13	$	1,026,500		$	(11,006	)	$	(33,375	)	$	982,119
	The following table summarizes the deferred tax expense (benefit) associated with each component of other comprehensive income.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Change in net unrealized gains on investments:
	Net holding gains arising during the period	$	35,765		$	45,325		$	43,840		$	113,215
	Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	(63		)	(38		)	(9		)	(41		)
	Reclassification adjustments for net gains included in net income	(3,186		)	(2,835		)	(12,384		)	(8,635		)
	Change in net unrealized gains on investments	32,516			42,452			31,447			104,539
	Change in foreign currency translation adjustments	—			(409		)	38			(457		)
	Change in net actuarial pension loss	67			121			286			443
	Total	$	32,583		$	42,164		$	31,771		$	104,525
	The following table presents the details of amounts reclassified from accumulated other comprehensive income into income, by component.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Unrealized holding gains on available-for-sale securities:
	Net realized investment gains	$	10,840		$	9,319		$	40,250		$	28,789
	Income taxes	(3,186		)	(2,835		)	(12,384		)	(8,635		)
	Reclassification of unrealized holding gains, net of taxes	$	7,654		$	6,484		$	27,866		$	20,154
	Net actuarial pension loss:
	Underwriting, acquisition and insurance expenses	$	(477	)	$	(643	)	$	(1,432	)	$	(1,930	)
	Income taxes	67			121			286			443
	Reclassification of net actuarial pension loss, net of taxes	$	(410	)	$	(522	)	$	(1,146	)	$	(1,487	)

Commitments_And_Contingencies

Commitments And Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Commitments And Contingencies	'
	Commitments and Contingencies
	On August 8, 2013, Alterra Holdings Limited, a wholly-owned subsidiary of Alterra, and several other third-party investors, executed a subscription agreement with New Point VI Limited to purchase common shares of New Point VI Limited. As of September 30, 2013, no shares had been issued under the subscription agreement; however, Alterra Holdings Limited's commitment under the subscription agreement with New Point VI Limited is $75.0 million. New Point VI Limited is the holding company of New Point Re VI Limited, a Bermuda-domiciled reinsurance company that will offer fully-collateralized retrocessional reinsurance, incepting between January 1, 2014 and December 31, 2014, to the property reinsurance catastrophe market.
	Contingencies arise in the normal course of the Company's operations and are not expected to have a material impact on the Company's financial condition or results of operations.

Acquisitions_Tables

Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2013
Acquisitions [Abstract]	'
Calculation Of Total Purchase Price	'
	The Company's total purchase price for Alterra as of the Acquisition Date was calculated as follows:
	(in thousands, except per share amounts)
	Shares of Alterra common stock outstanding as of the Acquisition Date	96,433
	Exchange ratio per the Merger Agreement	0.04315
	Markel share issuance to Alterra shareholders	4,161
	Shares of Alterra restricted stock outstanding as of the Acquisition Date	2,239
	Incentive award ratio per the Merger Agreement	0.06252
	Markel restricted stock issuance to Alterra restricted stock holders	140
	Multiplied by Markel's weighted average stock price on April 30, 2013 (1)	$	529.59
	Markel share and restricted stock issuance consideration, net of taxes	$	2,267,648
	Alterra common shares outstanding as of the Acquisition Date that received cash consideration	96,433
	Multiplied by cash price per share component per the Merger Agreement	$	10
	Markel cash consideration	$	964,330
	Fair value of Markel warrant issuance to Alterra warrant holders as of the Acquisition Date	$	73,685
	Fair value of Markel stock option issuance to Alterra stock option holders as of the Acquisition Date, net of taxes	$	12,335
	Fair value of partially vested Markel restricted stock unit issuance as of the Acquisition Date, net of taxes	$	6,867
	Unrecognized compensation on unvested restricted stock and restricted stock units	$	(20,572	)
	Total acquisition consideration	$	3,304,293
	(1)	The fair value of the shares issued by the Company was calculated as the weighted average price of the Company's stock on April 30, 2013, the day preceding the Acquisition Date.
Summary Of Provisional Fair Value Of Assets Acquired And Liabilities Assumed	'
	The following table summarizes the provisional fair values of the assets acquired and liabilities assumed at the Acquisition Date.
	(dollars in thousands)
	ASSETS
	Investments	$	6,407,841
	Cash and cash equivalents	1,036,274
	Restricted cash and cash equivalents	414,497
	Receivables	866,388
	Reinsurance recoverable on unpaid losses	1,169,084
	Reinsurance recoverable on paid losses	80,672
	Prepaid reinsurance premiums	317,445
	Other assets	834,024
	LIABILITIES
	Unpaid losses and loss adjustment expenses	4,719,461
	Life and annuity benefits	1,477,482
	Unearned premiums	1,075,610
	Payables to insurance and reinsurance companies	342,858
	Senior long-term debt	512,463
	Other liabilities	218,523
	Net assets	2,779,828
	Goodwill	336,965
	Intangible assets	187,500
	Acquisition date fair value	$	3,304,293
Summary Of Provisional Net Intangible Assets Acquired	'
	The following table summarizes the provisional intangible assets recorded in connection with the acquisition, and as of September 30, 2013.
	(dollars in thousands)	Amount			Economic
					Useful Life
	Customer relationships	$	113,000		18 years
	Broker relationships	18,000			18 years
	Technology	18,000			10 years
	Trade names	1,000			One year
	Lloyd's syndicate capacity	12,000			Indefinite
	Insurance licenses	25,500			Indefinite
	Intangible assets, before amortization, as of the Acquisition Date	187,500
	Amortization (from the Acquisition Date through September 30, 2013)	4,246
	Net intangible assets as of September 30, 2013	$	183,254
Summary Of Transaction And Acquisition-Related Costs	'
	The following table summarizes transaction and acquisition-related costs incurred by the Company in connection with the acquisition, all of which were included in underwriting, acquisition and insurance expenses in the consolidated statements of income and comprehensive income.
	(dollars in thousands)	Quarter Ended			Nine Months Ended
		30-Sep-13			30-Sep-13
	Transaction costs	$	—		$	15,981
	Acquisition-related costs:
	Severance costs	3,519			31,734
	Stay bonuses	3,906			9,981
	Acceleration of Alterra long-term incentive compensation awards and restricted stock awards	1,083			12,621
	Total transaction and acquisition-related costs	$	8,508		$	70,317
Summary Of Results Of Operations	'
	The following table summarizes the results of Alterra since the Acquisition Date that have been included within the Company's consolidated statements of income and comprehensive income.
	(dollars in thousands)	Quarter Ended September 30, 2013			Nine Months Ended September 30, 2013
	Operating revenues	$	314,200		$	557,446
	Net loss to shareholders	$	(11,312	)	$	(67,055	)
Unaudited Pro Forma Consolidated Financial Information	'
	The following table presents unaudited pro forma consolidated information for the quarter and nine months ended September 30, 2013 and 2012 and assumes the Company's acquisition of Alterra occurred on January 1, 2012. The pro forma financial information is presented for informational purposes only and does not necessarily reflect the results that would have occurred had the acquisition taken place on January 1, 2012, nor is it necessarily indicative of future results. Significant adjustments used to determine pro forma results include amortization of intangible assets and amortization of fair value adjustments discussed in c) above, and the corresponding income tax effects. The Company also excluded certain charges from the pro forma results, including transaction costs incurred by the Company ($16.0 million) and Alterra ($23.0 million) totaling $39.0 million for the nine months ended September 30, 2013, severance costs attributable to the acquisition totaling $3.5 million and $31.7 million for the quarter and nine months ended September 30, 2013, respectively, and stay bonuses of $3.9 million and $10.0 million for the quarter and nine months ended September 30, 2013, respectively. The acceleration of compensation expense during the quarter and nine months ended September 30, 2013 related to Alterra's long-term incentive compensation awards and restricted stock awards was attributable to the acquisition; however, the incremental expense recognized during the period only represents a timing difference in the recognition of expense. Therefore, it was not excluded from the pro forma underwriting results.
		Pro Forma						Pro Forma
		Quarter Ended September 30,						Nine Months Ended September 30,
	(in thousands, except per share amounts)	2013			2012			2013			2012
	Earned premiums	$	919,723		$	862,534		$	2,717,732		$	2,592,818
	Operating revenues	1,234,380			1,146,994			3,629,659			3,365,605
	Net income to shareholders	90,910			60,090			298,970			265,664
	U.S. GAAP combined ratio (1)	95		%	102		%	95		%	96		%
	Basic net income per share	$	6.49		$	4.29		$	21.33		$	18.96
	Diluted net income per share	$	6.47		$	4.27		$	21.24		$	18.9
	Weighted average common shares outstanding:
	Basic	14,001			14,015			14,014			14,012
	Diluted	14,057			14,067			14,075			14,060
	-1	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
Investments, Debt and Equity Securities [Abstract]	'
Available-For-Sale Investments	'
	The following tables summarize the Company's available-for-sale investments.
		September 30, 2013
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized			Estimated
		Cost			Unrealized			Unrealized			Other-Than-			Fair
					Holding			Holding			Temporary			Value
					Gains			Losses			Impairment
											Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	1,257,855		$	10,933		$	(24,660	)	$	—		$	1,244,128
	Obligations of states, municipalities and political subdivisions	2,739,483			135,139			(23,561		)	—			2,851,061
	Foreign governments	1,435,249			34,323			(45,888		)	—			1,423,684
	Commercial mortgage-backed securities	401,036			116			(10,443		)	—			390,709
	Residential mortgage-backed securities	927,880			12,364			(12,012		)	(2,258		)	925,974
	Asset-backed securities	217,394			261			(1,779		)	—			215,876
	Corporate bonds	3,263,513			67,385			(62,519		)	(6,872		)	3,261,507
	Total fixed maturities	10,242,410			260,521			(180,862		)	(9,130		)	10,312,939
	Equity securities:
	Insurance, banks and other financial institutions	404,923			540,749			(241		)	—			945,431
	Industrial, consumer and all other	1,106,540			939,612			(2,969		)	—			2,043,183
	Total equity securities	1,511,463			1,480,361			(3,210		)	—			2,988,614
	Short-term investments	1,159,460			3			(6		)	—			1,159,457
	Investments, available-for-sale	$	12,913,333		$	1,740,885		$	(184,078	)	$	(9,130	)	$	14,461,010
		December 31, 2012
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized			Estimated
		Cost			Unrealized			Unrealized			Other-Than-			Fair
					Holding			Holding			Temporary			Value
					Gains			Losses			Impairment
											Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	297,663		$	19,844		$	—		$	—		$	317,507
	Obligations of states, municipalities and political subdivisions	2,586,867			245,057			(362		)	—			2,831,562
	Foreign governments	503,844			52,764			—			—			556,608
	Residential mortgage-backed securities	202,644			14,996			(5		)	(2,258		)	215,377
	Asset-backed securities	13,828			517			—			—			14,345
	Corporate bonds	957,432			93,395			(121		)	(6,822		)	1,043,884
	Total fixed maturities	4,562,278			426,573			(488		)	(9,080		)	4,979,283
	Equity securities:
	Insurance, banks and other financial institutions	508,771			389,434			(138		)	—			898,067
	Industrial, consumer and all other	878,534			637,783			(7,433		)	—			1,508,884
	Total equity securities	1,387,305			1,027,217			(7,571		)	—			2,406,951
	Short-term investments	973,318			26			(14		)	—			973,330
	Investments, available-for-sale	$	6,922,901		$	1,453,816		$	(8,073	)	$	(9,080	)	$	8,359,564
Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position	'
		December 31, 2012
		Less than 12 months						12 months or longer						Total
	(dollars in thousands)	Estimated			Gross			Estimated			Gross			Estimated		Gross
		Fair			Unrealized			Fair			Unrealized			Fair		Unrealized
		Value			Holding and			Value			Holding and			Value		Holding and
					Other-Than-						Other-Than-					Other-Than-
					Temporary						Temporary					Temporary
					Impairment						Impairment					Impairment
					Losses						Losses					Losses
	Fixed maturities:
	Obligations of states, municipalities and political subdivisions	$	2,833		$	(46	)	$	3,616		$	(316	)	$	6,449	$	(362	)
	Residential mortgage-backed securities	364			(2,260		)	201			(3		)	565		(2,263		)
	Corporate bonds	—			(6,822		)	3,860			(121		)	3,860		(6,943		)
	Total fixed maturities	3,197			(9,128		)	7,677			(440		)	10,874		(9,568		)
	Equity securities:
	Insurance, banks and other financial institutions	2,431			(138		)	—			—			2,431		(138		)
	Industrial, consumer and all other	82,109			(7,310		)	1,983			(123		)	84,092		(7,433		)
	Total equity securities	84,540			(7,448		)	1,983			(123		)	86,523		(7,571		)
	Short-term investments	228,820			(14		)	—			—			228,820		(14		)
	Total	$	316,557		$	(16,590	)	$	9,660		$	(563	)	$	326,217	$	(17,153	)
	The following tables summarize gross unrealized investment losses by the length of time that securities have continuously been in an unrealized loss position.
		September 30, 2013
		Less than 12 months						12 months or longer						Total
	(dollars in thousands)	Estimated			Gross			Estimated			Gross			Estimated		Gross
		Fair			Unrealized			Fair			Unrealized			Fair		Unrealized
		Value			Holding and			Value			Holding and			Value		Holding and
					Other-Than-						Other-Than-					Other-Than-
					Temporary						Temporary					Temporary
					Impairment						Impairment					Impairment
					Losses						Losses					Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	652,752		$	(24,660	)	$	—		$	—		$	652,752	$	(24,660	)
	Obligations of states, municipalities and political subdivisions	307,119			(23,439		)	3,196			(122		)	310,315		(23,561		)
	Foreign governments	907,456			(45,888		)	—			—			907,456		(45,888		)
	Commercial mortgage-backed securities	378,001			(10,443		)	—			—			378,001		(10,443		)
	Residential mortgage-backed securities	455,942			(14,269		)	166			(1		)	456,108		(14,270		)
	Asset-backed securities	189,070			(1,779		)	—			—			189,070		(1,779		)
	Corporate bonds	2,127,583			(69,391		)	—			—			2,127,583		(69,391		)
	Total fixed maturities	5,017,923			(189,869		)	3,362			(123		)	5,021,285		(189,992		)
	Equity securities:
	Insurance, banks and other financial institutions	11,518			(241		)	—			—			11,518		(241		)
	Industrial, consumer and all other	51,261			(2,957		)	125			(12		)	51,386		(2,969		)
	Total equity securities	62,779			(3,198		)	125			(12		)	62,904		(3,210		)
	Short-term investments	525,986			(6		)	—			—			525,986		(6		)
	Total	$	5,606,688		$	(193,073	)	$	3,487		$	(135	)	$	5,610,175	$	(193,208	)
Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturities By Contractual Maturity Date	'
	The amortized cost and estimated fair value of fixed maturities at September 30, 2013 are shown below by contractual maturity.
	(dollars in thousands)	Amortized			Estimated
		Cost			Fair Value
	Due in one year or less	$	900,263		$	905,564
	Due after one year through five years	3,151,362			3,220,106
	Due after five years through ten years	2,227,954			2,277,491
	Due after ten years	2,416,521			2,377,219
		8,696,100			8,780,380
	Commercial mortgage-backed securities	401,036			390,709
	Residential mortgage-backed securities	927,880			925,974
	Asset-backed securities	217,394			215,876
	Total fixed maturities	$	10,242,410		$	10,312,939
Components Of Net Investment Income	'
	The following table presents the components of net investment income.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Interest:
	Municipal bonds (tax-exempt)	$	20,347		$	22,508		$	62,307		$	68,287
	Municipal bonds (taxable)	7,452			5,670			19,467			17,055
	Other taxable bonds	38,211			25,335			93,565			82,930
	Short-term investments, including overnight deposits	1,042			721			2,316			1,995
	Dividends on equity securities	12,059			10,128			35,600			30,586
	Change in fair value of credit default swap	1,660			1,480			8,860			13,681
	Other	9,004			875			16,036			(356		)
		89,775			66,717			238,151			214,178
	Investment expenses	(3,583		)	(2,279		)	(9,363		)	(6,344		)
	Net investment income	$	86,192		$	64,438		$	228,788		$	207,834
Summary Of Net Realized Investment Gains And The Change In Net Unrealized Gains On Investments	'
	The following table presents net realized investment gains and the change in net unrealized gains on investments.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Realized gains:
	Sales of fixed maturities	$	4,933		$	2,576		$	6,031		$	7,863
	Sales of equity securities	24,648			10,403			58,563			25,865
	Other	398			—			451			655
	Total realized gains	29,979			12,979			65,045			34,383
	Realized losses:
	Sales of fixed maturities	(18,741		)	(118		)	(19,585		)	(457		)
	Sales of equity securities	—			(383		)	(170		)	(331		)
	Other-than-temporary impairments	—			(3,159		)	(4,589		)	(4,151		)
	Other	—			(4,088		)	—			(4,088		)
	Total realized losses	(18,741		)	(7,748		)	(24,344		)	(9,027		)
	Net realized investment gains	$	11,238		$	5,231		$	40,701		$	25,356
	Change in net unrealized gains on investments:
	Fixed maturities	$	(8,679	)	$	37,488		$	(346,476	)	$	82,078
	Equity securities	117,899			98,964			457,505			248,214
	Short-term investments	(13		)	18			(15		)	13
	Net increase	$	109,207		$	136,470		$	111,014		$	330,305
Summary Of Other-Than-Temporary Impairment Losses Recognized In Net Income And In Net Realized Investment Gains By Investment Type	'
	The following table presents other-than-temporary impairment losses recognized in net income and included in net realized investment gains by investment type.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Fixed maturities:
	Obligations of states, municipalities and political subdivisions	$	—		$	—		$	(1,242	)	$	—
	Residential mortgage-backed securities	—			—			(523		)	—
	Total fixed maturities	—			—			(1,765		)	—
	Equity securities:
	Insurance, banks and other financial institutions	—			(1,615		)	—			(2,441		)
	Industrial, consumer and all other	—			(1,544		)	(2,824		)	(1,710		)
	Total equity securities	—			(3,159		)	(2,824		)	(4,151		)
	Total	$	—		$	(3,159	)	$	(4,589	)	$	(4,151	)
Schedule Of Restricted Assets	'
	Total restricted assets are included on the Company's consolidated balance sheets as follows.
	(dollars in thousands)	September 30, 2013			December 31, 2012
	Investments, available-for-sale	$	5,121,181		$	1,262,755
	Restricted cash and cash equivalents	600,171			109,415
	Other assets	38,435			—
	Total	$	5,759,787		$	1,372,170

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]	'
Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis	'
	The following tables present the balances of assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy.
		September 30, 2013
	(dollars in thousands)	Level 1			Level 2			Level 3			Total
	Assets:
	Investments available-for-sale:
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	—		$	1,244,128		$	—		$	1,244,128
	Obligations of states, municipalities and political subdivisions	—			2,851,061			—			2,851,061
	Foreign governments	—			1,423,684			—			1,423,684
	Commercial mortgage-backed securities	—			390,709			—			390,709
	Residential mortgage-backed securities	—			925,974			—			925,974
	Asset-backed securities	—			215,876			—			215,876
	Corporate bonds	—			3,261,507			—			3,261,507
	Total fixed maturities	—			10,312,939			—			10,312,939
	Equity securities:
	Insurance, banks and other financial institutions	945,431			—			—			945,431
	Industrial, consumer and all other	2,043,183			—			—			2,043,183
	Total equity securities	2,988,614			—			—			2,988,614
	Short-term investments	1,016,185			143,272			—			1,159,457
	Total investments available-for-sale	$	4,004,799		$	10,456,211		$	—		$	14,461,010
	Liabilities:
	Derivative contracts	$	—		$	—		$	3,830		$	3,830
		December 31, 2012
	(dollars in thousands)	Level 1			Level 2			Level 3			Total
	Assets:
	Investments available-for-sale:
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	—		$	317,507		$	—		$	317,507
	Obligations of states, municipalities and political subdivisions	—			2,831,562			—			2,831,562
	Foreign governments	—			556,608			—			556,608
	Residential mortgage-backed securities	—			215,377			—			215,377
	Asset-backed securities	—			14,345			—			14,345
	Corporate bonds	—			1,043,884			—			1,043,884
	Total fixed maturities	—			4,979,283			—			4,979,283
	Equity securities:
	Insurance, banks and other financial institutions	898,067			—			—			898,067
	Industrial, consumer and all other	1,508,884			—			—			1,508,884
	Total equity securities	2,406,951			—			—			2,406,951
	Short-term investments	888,758			84,572			—			973,330
	Total investments available-for-sale	$	3,295,709		$	5,063,855		$	—		$	8,359,564
	Liabilities:
	Derivative contracts	$	—		$	—		$	12,690		$	12,690
Summary Of Changes In Level 3 Liabilities Measured At Fair Value On A Recurring Basis	'
	The following table summarizes changes in Level 3 liabilities measured at fair value on a recurring basis.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Derivatives, beginning of period	$	5,490		$	17,130		$	12,690		$	29,331
	Total gains included in:
	Net income	(1,660		)	(1,480		)	(8,860		)	(13,681		)
	Other comprehensive income	—			—			—			—
	Transfers into Level 3	—			—			—			—
	Transfers out of Level 3	—			—			—			—
	Derivatives, end of period	$	3,830		$	15,650		$	3,830		$	15,650
	Net unrealized gains included in net income relating to liabilities held at September 30, 2013 and 2012 (1)	$	1,660		$	1,480		$	8,860		$	13,681
	(1)	Included in net investment income in the consolidated statements of income and comprehensive income.

Segment_Reporting_Disclosures_

Segment Reporting Disclosures (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract]	'
Company's Segment Disclosures	'
	The following tables summarize the Company's segment disclosures.
		Quarter Ended September 30, 2013
	(dollars in thousands)	Excess and			Specialty			London			Alterra				Other				Investing			Consolidated
		Surplus			Admitted			Insurance							Insurance
		Lines						Market							(Discontinued
															Lines)
	Gross premium volume	$	273,757		$	230,450		$	199,575		$	372,516			$	—			$	—		$	1,076,298
	Net written premiums	232,396			219,955			174,203			216,378				(1		)		—			842,931
	Earned premiums	218,538			197,372			199,633			304,181				(1		)		—			919,723
	Losses and loss adjustment expenses:
	Current accident year	(147,204		)	(126,649		)	(135,098		)	(201,732		)		—				—			(610,683		)
	Prior accident years	40,799			28,410			40,562			—				(32,460		)		—			77,311
	Underwriting, acquisition and insurance expenses:
	Transaction costs and other acquisition-related expenses (1)	—			—			—			(8,508		)		—				—			(8,508		)
	All other expenses	(85,570		)	(80,023		)	(73,335		)	(104,608		)		(82		)		—			(343,618		)
	Underwriting profit (loss)	26,563			19,110			31,762			(10,667		)		(32,543		)		—			34,225
	Net investment income	—			—			—			—				—				86,192			86,192
	Net realized investment gains	—			—			—			—				—				11,238			11,238
	Other revenues (insurance)	—			1,402			311			59				160				—			1,932
	Other expenses (insurance)	—			(6,292		)	(1,167		)	—				(8,789		)		—			(16,248		)
	Segment profit (loss)	$	26,563		$	14,220		$	30,906		$	(10,608	)		$	(41,172	)		$	97,430		$	117,339
	Other revenues (non-insurance)																					172,580
	Other expenses (non-insurance)																					(150,318		)
	Amortization of intangible assets																					(16,848		)
	Interest expense																					(30,619		)
	Income before income taxes																					$	92,134
	U.S. GAAP combined ratio (2)	88		%	90		%	84		%	104		%		NM			(3)				96		%
	(1)	In connection with the acquisition of Alterra, the Company incurred acquisition-related costs totaling $8.5 million for the quarter ended September 30, 2013, which included severance costs of $3.5 million, stay bonuses of $3.9 million and other compensation costs of $1.1 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
	(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
	(3)	NM – Ratio is not meaningful.
		Quarter Ended September 30, 2012
	(dollars in thousands)	Excess and			Specialty			London			Other				Investing				Consolidated
		Surplus			Admitted			Insurance			Insurance
		Lines						Market			(Discontinued
											Lines)
	Gross premium volume	$	253,014		$	167,747		$	190,071		$	1			$	—			$	610,833
	Net written premiums	211,538			157,894			170,193			(1		)		—				539,624
	Earned premiums	195,478			153,009			182,052			(2		)		—				530,537
	Losses and loss adjustment expenses:
	Current accident year	(134,504		)	(116,044		)	(122,192		)	—				—				(372,740		)
	Prior accident years	51,310			11,504			32,744			(26,277		)		—				69,281
	Underwriting, acquisition and insurance expenses:
	Prospective adoption of ASU 2010-26 (1)	(2,125		)	(2,600		)	(1,809		)	—				—				(6,534		)
	All other expenses	(88,093		)	(60,008		)	(76,820		)	(386		)		—				(225,307		)
	Underwriting profit (loss)	22,066			(14,139		)	13,975			(26,665		)		—				(4,763		)
	Net investment income	—			—			—			—				64,438				64,438
	Net realized investment gains	—			—			—			—				5,231				5,231
	Other revenues (insurance)	—			11,536			223			—				—				11,759
	Other expenses (insurance)	—			(12,181		)	(970		)	—				—				(13,151		)
	Segment profit (loss)	$	22,066		$	(14,784	)	$	13,228		$	(26,665	)		$	69,669			$	63,514
	Other revenues (non-insurance)																		153,810
	Other expenses (non-insurance)																		(132,188		)
	Amortization of intangible assets																		(7,959		)
	Interest expense																		(24,692		)
	Income before income taxes																		$	52,485
	U.S. GAAP combined ratio (2)	89		%	109		%	92		%	NM			(3)					101		%
	(1)	Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included $43.1 million of costs that no longer met the criteria for deferral as of January 1, 2012 and were recognized into income during 2012, consistent with policy terms. The quarter ended September 30, 2012 included $6.5 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.
	(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
	(3)	NM – Ratio is not meaningful.
		Nine Months Ended September 30, 2013
	(dollars in thousands)	Excess and			Specialty			London			Alterra				Other				Investing			Consolidated
		Surplus			Admitted			Insurance							Insurance
		Lines						Market							(Discontinued
									Lines)
Gross premium volume	$	792,877		$	687,669		$	725,153		$	714,844			$	35			$	—		$	2,920,578
Net written premiums	673,741			655,350			634,180			469,894				35				—			2,433,200
Earned premiums	629,288			533,037			577,101			529,668				35				—			2,269,129
Losses and loss adjustment expenses:
Current accident year	(424,398		)	(353,581		)	(386,617		)	(380,513		)		—				—			(1,545,109		)
Prior accident years	165,587			45,863			101,589			—				(31,604		)		—			281,435
Underwriting, acquisition and insurance expenses:
Transaction costs and other acquisition-related expenses (1)	—			—			—			(70,317		)		—				—			(70,317		)
All other expenses	(250,101		)	(229,462		)	(222,469		)	(171,762		)		217				—			(873,577		)
Underwriting profit (loss)	120,376			(4,143		)	69,604			(92,924		)		(31,352		)		—			61,561
Net investment income	—			—			—			—				—				228,788			228,788
Net realized investment gains	—			—			—			—				—				40,701			40,701
Other revenues (insurance)	—			11,945			4,778			1,256				479				—			18,458
Other expenses (insurance)	—			(14,447		)	(3,762		)	—				(13,039		)		—			(31,248		)
Segment profit (loss)	$	120,376		$	(6,645	)	$	70,620		$	(91,668	)		$	(43,912	)		$	269,489		$	318,260
Other revenues (non-insurance)																					486,222
Other expenses (non-insurance)																					(428,394		)
Amortization of intangible assets																					(37,755		)
Interest expense																					(82,754		)
Income before income taxes																					$	255,579
U.S. GAAP combined ratio (2)	81		%	101		%	88		%	118		%		NM			(3)				97		%
(1)	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the nine months ended September 30, 2013, which primarily consist of due diligence, legal and investment banking costs. Additionally, the Company incurred severance costs of $31.7 million, stay bonuses of $10.0 million and other compensation costs totaling $12.6 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
(3)	NM – Ratio is not meaningful.
	Nine Months Ended September 30, 2012
(dollars in thousands)	Excess and			Specialty			London			Other				Investing				Consolidated
	Surplus			Admitted			Insurance			Insurance
	Lines						Market			(Discontinued
										Lines)
Gross premium volume	$	705,849		$	496,019		$	704,511		$	(6	)		$	—			$	1,906,373
Net written premiums	597,742			467,722			621,947			(7		)		—				1,687,404
Earned premiums	584,524			431,179			557,493			(7		)		—				1,573,189
Losses and loss adjustment expenses:
Current accident year	(390,254		)	(310,115		)	(372,869		)	—				—				(1,073,238		)
Prior accident years	132,583			27,747			118,994			(19,160		)		—				260,164
Underwriting, acquisition and insurance expenses:
Prospective adoption of ASU 2010-26 (1)	(16,652		)	(12,863		)	(11,578		)	—				—				(41,093		)
All other expenses	(255,295		)	(171,111		)	(226,770		)	(1,053		)		—				(654,229		)
Underwriting profit (loss)	54,906			(35,163		)	65,270			(20,220		)		—				64,793
Net investment income	—			—			—			—				207,834				207,834
Net realized investment gains	—			—			—			—				25,356				25,356
Other revenues (insurance)	—			36,065			4,753			—				—				40,818
Other expenses (insurance)	—			(35,184		)	(2,722		)	—				—				(37,906		)
Segment profit (loss)	$	54,906		$	(34,282	)	$	67,301		$	(20,220	)		$	233,190			$	300,895
Other revenues (non-insurance)																		344,960
Other expenses (non-insurance)																		(305,556		)
Amortization of intangible assets																		(25,078		)
Interest expense																		(69,068		)
Income before income taxes																		$	246,153
U.S. GAAP combined ratio (2)	91		%	108		%	88		%	NM			(3)					96		%
(1)	Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included $43.1 million of costs that no longer met the criteria for deferral as of January 1, 2012 and were recognized into income during 2012, consistent with policy terms. The nine months ended September 30, 2012 included $41.1 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.
(2)	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
(3)	NM – Ratio is not meaningful.
Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets	'
	The following table reconciles segment assets to the Company's consolidated balance sheets.
	(dollars in thousands)	September 30, 2013			December 31, 2012
	Segment assets:
	Investing	$	17,125,848		$	9,277,697
	Underwriting	5,778,934			2,387,305
	Total segment assets	22,904,782			11,665,002
	Non-insurance operations	903,795			891,586
	Total assets	$	23,808,577		$	12,556,588

Reinsurance_Tables

Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2013
Reinsurance Disclosures [Abstract]	'
Effect Of Reinsurance On Premiums Written And Earned	'
	The following tables summarize the effect of reinsurance on premiums written and earned.
		Quarter Ended September 30,
		2013						2012
	(dollars in thousands)	Written			Earned			Written			Earned
	Direct	$	836,117		$	818,854		$	519,408		$	505,514
	Assumed	240,181			331,495			91,425			94,822
	Ceded	(233,367		)	(230,626		)	(71,209		)	(69,799		)
	Net premiums	$	842,931		$	919,723		$	539,624		$	530,537
		Nine Months Ended September 30,
		2013						2012
	(dollars in thousands)	Written			Earned			Written			Earned
	Direct	$	2,310,966		$	2,076,682		$	1,568,092		$	1,505,710
	Assumed	609,612			675,286			338,281			269,771
	Ceded	(487,378		)	(482,839		)	(218,969		)	(202,292		)
	Net premiums	$	2,433,200		$	2,269,129		$	1,687,404		$	1,573,189

Life_And_Annuity_Benefits_Tabl

Life And Annuity Benefits (Tables)	9 Months Ended
	Sep. 30, 2013
Liability for Future Policy Benefits [Abstract]	'
Schedule Of Life And Annuity Benefits	'
	The following table presents life and annuity benefits as of September 30, 2013:
		(dollars in thousands)
	Life	$	189,792
	Annuities	1,188,914
	Accident and health	104,292
		$	1,482,998

Senior_LongTerm_Debt_And_Other1

Senior Long-Term Debt And Other Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Summary Of Senior Long-Term Debt And Other Debt	'
	The following table summarizes the Company's senior long-term debt and other debt.
	(dollars in thousands)	September 30, 2013		December 31, 2012
	6.80% unsecured senior notes, due February 15, 2013, interest payable semi-annually, net of unamortized discount of $45 in 2012	$	—	$	246,619
	7.20% unsecured senior notes, due April 14, 2017, interest payable semi-annually, net of unamortized premium of $5,160 in 2013	95,789		—
	7.125% unsecured senior notes, due September 30, 2019, interest payable semi-annually, net of unamortized discount of $1,697 in 2013 and $1,909 in 2012	348,303		348,091
	6.25% unsecured senior notes, due September 30, 2020, interest payable semi-annually, net of unamortized premium of $64,049 in 2013	414,049		—
	5.35% unsecured senior notes, due June 1, 2021, interest payable semi-annually, net of unamortized discount of $1,583 in 2013 and $1,738 in 2012	248,417		248,262
	4.90% unsecured senior notes, due July 1, 2022, interest payable semi-annually, net of unamortized discount of $2,444 in 2013 and $2,653 in 2012	347,556		347,347
	3.625% unsecured senior notes, due March 30, 2023, interest payable semi-annually, net of unamortized discount of $1,910 in 2013	248,090		—
	7.35% unsecured senior notes, due August 15, 2034, interest payable semi-annually, net of unamortized discount of $2,211 in 2013 and $2,291 in 2012	197,789		197,710
	5.0% unsecured senior notes, due March 30, 2043, interest payable semi-annually, net of unamortized discount of $6,607 in 2013	243,393		—
	Other debt, at various interest rates ranging from 1.6% to 6.5%	107,093		104,521
	Senior long-term debt and other debt	$	2,250,479	$	1,492,550
Summary Of Future Principal Payments Due At Maturity On Senior Long-Term Debt And Other Debt	'
	The following table summarizes the future principal payments due at maturity on senior long-term debt and other debt as of September 30, 2013.
	Years Ending December 31,	(dollars in
		thousands)
	2013	$	4,498
	2014	17,857
	2015	5,090
	2016	12,092
	2017	104,366
	2018 and thereafter	2,053,819
	Total principal payments	$	2,197,722
	Net unamortized premium	52,757
	Senior long-term debt and other debt	$	2,250,479

Employee_Benefit_Plans_Tables

Employee Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2013
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'
Components Of Net Periodic Benefit Income For The Terra Nova Pension Plan	'
	The following table presents the components of net periodic benefit income for the Terra Nova Pension Plan, a defined benefit plan.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Service cost	$	—		$	90		$	—		$	269
	Interest cost	1,611			1,693			4,837			5,079
	Expected return on plan assets	(2,671		)	(2,432		)	(8,015		)	(7,295		)
	Amortization of net actuarial pension loss	477			643			1,432			1,930
	Net periodic benefit income	$	(583	)	$	(6	)	$	(1,746	)	$	(17	)

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2013
Stockholders' Equity Note [Abstract]	'
Schedule Of Net Income Per Share	'
	Net income per share was determined by dividing adjusted net income to shareholders by the applicable weighted average shares outstanding. Unvested share-based compensation awards that contain non-forfeitable rights to dividends or dividend equivalents (whether paid or unpaid) are considered participating awards and are included in the computation of net income per share. Non-participating unvested share-based compensation awards are excluded from the computation of net income per share. Diluted net income per share is computed by dividing adjusted net income to shareholders by the weighted average number of common shares and dilutive potential common shares outstanding during the year.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(in thousands, except per share amounts)	2013			2012			2013			2012
	Net income to shareholders	$	65,599		$	49,653		$	182,257		$	196,593
	Adjustment of redeemable noncontrolling interests	—			1,702			3,101			(6,484		)
	Adjusted net income to shareholders	$	65,599		$	51,355		$	185,358		$	190,109
	Basic common shares outstanding	14,001			9,632			12,050			9,641
	Dilutive potential common shares from conversion of options	16			4			11			7
	Dilutive potential common shares from conversion of non participating restricted stock	39			23			32			19
	Diluted shares outstanding	14,056			9,659			12,093			9,667
	Basic net income per share	$	4.69		$	5.33		$	15.38		$	19.72
	Diluted net income per share	$	4.67		$	5.32		$	15.33		$	19.67
Summary Of Options For The Alterra Equity Award Plans	'
	The following table summarizes additional information with respect to these options.
		Number of Shares		Weighted Average Exercise Price			Weighted Average Remaining Contractual Term (years)		Intrinsic Value (in millions)
	Outstanding and exercisable, May 1, 2013	101,875		$	398.96
	Exercised	49,763		$	401.83
	Outstanding and exercisable, September 30, 2013	52,112		$	396.22		2.6		$	6.7
Summary Of Nonvested Share-Based Awards For The Alterra Equity Award Plans	'
	The following table summarizes activity related to these nonvested restricted stock awards.
		Number		Weighted Average
		of Awards		Grant-Date
				Fair Value
	Nonvested awards issued on May 1, 2013	154,103		$	529.59
	Vested	(76,287	)	529.59
	Forfeited	(7,476	)	529.59
	Nonvested awards at September 30, 2013	70,340		$	529.59

Other_Comprehensive_Income_Tab

Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2013
Other Comprehensive Income (Loss), Tax [Abstract]	'
Change In Accumulated Other Comprehensive Income By Component, Net Of Taxes And Noncontrolling Interest	'
	The following table presents the change in accumulated other comprehensive income by component, net of taxes and noncontrolling interests, for the nine months ended September 30, 2013 and 2012.
	(dollars in thousands)	Unrealized Holding Gains on Available-for-Sale Securities			Foreign Currency			Net Actuarial Pension Loss			Total
	31-Dec-11	$	704,719		$	(2,614	)	$	(41,185	)	$	660,920
	Other comprehensive income before reclassifications	245,920			2,969			—			248,889
	Amounts reclassified from accumulated other comprehensive income	(20,154		)	—			1,487			(18,667		)
	Total other comprehensive income	225,766			2,969			1,487			230,222
	30-Sep-12	$	930,485		$	355		$	(39,698	)	$	891,142
	31-Dec-12	$	946,933		$	(1,075	)	$	(34,521	)	$	911,337
	Other comprehensive income before reclassifications	107,433			(9,931		)	—			97,502
	Amounts reclassified from accumulated other comprehensive income	(27,866		)	—			1,146			(26,720		)
	Total other comprehensive income	79,567			(9,931		)	1,146			70,782
	30-Sep-13	$	1,026,500		$	(11,006	)	$	(33,375	)	$	982,119
Summary Of Deferred Tax Expense (Benefit) Of Other Comprehensive Income	'
	The following table summarizes the deferred tax expense (benefit) associated with each component of other comprehensive income.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Change in net unrealized gains on investments:
	Net holding gains arising during the period	$	35,765		$	45,325		$	43,840		$	113,215
	Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	(63		)	(38		)	(9		)	(41		)
	Reclassification adjustments for net gains included in net income	(3,186		)	(2,835		)	(12,384		)	(8,635		)
	Change in net unrealized gains on investments	32,516			42,452			31,447			104,539
	Change in foreign currency translation adjustments	—			(409		)	38			(457		)
	Change in net actuarial pension loss	67			121			286			443
	Total	$	32,583		$	42,164		$	31,771		$	104,525
Reclassifications From Accumulated Other Comprehensive Income Into Income, By Component	'
	The following table presents the details of amounts reclassified from accumulated other comprehensive income into income, by component.
		Quarter Ended September 30,						Nine Months Ended September 30,
	(dollars in thousands)	2013			2012			2013			2012
	Unrealized holding gains on available-for-sale securities:
	Net realized investment gains	$	10,840		$	9,319		$	40,250		$	28,789
	Income taxes	(3,186		)	(2,835		)	(12,384		)	(8,635		)
	Reclassification of unrealized holding gains, net of taxes	$	7,654		$	6,484		$	27,866		$	20,154
	Net actuarial pension loss:
	Underwriting, acquisition and insurance expenses	$	(477	)	$	(643	)	$	(1,432	)	$	(1,930	)
	Income taxes	67			121			286			443
	Reclassification of net actuarial pension loss, net of taxes	$	(410	)	$	(522	)	$	(1,146	)	$	(1,487	)

Basis_Of_Presentation_Narrativ

Basis Of Presentation (Narrative) (Details) (May 1, 2013 Alterra Capital Holdings [Member], USD $)	6 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]	'
Business Acquisition [Line Items]	'
Percentage of issued and outstanding common stock acquired	100.00%
Total purchase consideration	$3,304,293

Acquisitions_Narrative_Details

Acquisitions (Narrative) (Details)

3 Months Ended

9 Months Ended

9 Months Ended

3 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2012

Mar. 31, 2013

Sep. 30, 2013

Mar. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Oct. 09, 2013

Oct. 09, 2013

USD ($)

USD ($)

January 1, 2013 Essentia Insurance Company [Member]

January 1, 2013 Essentia Insurance Company [Member]

January 1, 2013 Essentia Insurance Company [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

Pre-Acquisition Board Of Directors [Member]

Additional Alterra Board Of Directors [Member]

Abbey Protection plc [Member]

Abbey Protection plc [Member]

USD ($)

Customer Relationships [Member]

Customer Relationships [Member]

USD ($)

USD ($)

USD ($)

Customer Relationships [Member]

Customer Relationships [Member]

Lloyds Syndicate Fourteen Hundred [Member]

Lloyds Syndicate Two Thousand Five Hundred Twenty Six [Member]

directors

directors

Subsequent Event [Member]

Subsequent Event [Member]

USD ($)

USD ($)

USD ($)

GBP (£)

Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of issued and outstanding common stock acquired

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

Number of Markel Corporation shares issued for each outstanding share of Alterra common stock

'

'

'

'

'

'

'

0.04315

'

'

'

'

'

'

'

'

'

Cash paid for each outstanding share of Alterra common stock

'

'

'

'

'

'

'

$10

'

'

'

'

'

'

'

'

'

Total purchase consideration

'

'

'

'

'

'

'

$3,304,293,000

'

'

'

'

'

'

'

'

'

Cash consideration

'

'

'

'

'

'

'

964,330,000

'

'

'

'

'

'

'

'

'

Stock consideration, shares

'

'

'

'

'

'

'

4,300,000

'

'

'

'

'

'

'

'

'

Number of board of directors

'

'

'

'

'

'

'

'

'

'

'

'

'

10

2

'

'

Options issued, taxes

'

'

'

'

'

'

'

1,900,000

'

'

'

'

'

'

'

'

'

Restricted stock awards issued, taxes

'

'

'

'

'

'

'

10,100,000

'

'

'

'

'

'

'

'

'

Partially vested restricted stock units issued, taxes

'

'

'

'

'

'

'

700,000

'

'

'

'

'

'

'

'

'

Goodwill recognized

'

'

'

'

'

'

'

336,965,000

'

'

'

'

'

'

'

'

'

Goodwill, expected tax deductible

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

Indefinite lived intangible assets

'

'

'

'

'

'

'

37,500,000

'

'

'

'

'

'

'

'

'

Other intangible assets

'

'

'

'

'

'

'

150,000,000

'

113,000,000

'

'

'

'

'

'

'

Other intangible assets, weighted average amortization period, years

'

'

'

'

'6 years

'

'

'

'17 years

'

'18 years

'

'

'

'

'

'

Investments acquired, adjustment to fair value

'

'

'

'

'

'

'

223,100,000

'

'

'

'

'

'

'

'

'

Fixed maturity securities, investments acquired, amount to be amortized to net investment income

'

'

'

'

'

'

'

552,400,000

'

'

'

'

'

'

'

'

'

Unearned premiums acquired, adjustment to fair value

'

'

'

'

'

'

'

-176,300,000

'

'

'

'

'

'

'

'

'

Unearned premiums, weighted average amortization period, years

'

'

'

'

'

'

'

'

'1 year

'

'

'

'

'

'

'

'

Unpaid losses and loss adjustment expenses acquired, adjustment to fair value

'

'

'

'

'

'

'

120,800,000

'

'

'

'

'

'

'

'

'

Unpaid losses and loss adjustment expenses acquired pre acquisition adjustment, unamortized balance

'

'

'

'

'

'

'

26,500,000

'

'

'

'

'

'

'

'

'

Unpaid losses and loss adjustment expenses, weighted average amortization period, years

'

'

'

'

'

'

'

'

'5 years

'

'

'

'

'

'

'

'

Life and annuity benefits acquired, adjustment to fair value

'

'

'

'

'

'

'

329,600,000

'

'

'

'

'

'

'

'

'

Senior long-term debt acquired, adjustment to fair value

'

'

'

'

'

'

'

71,900,000

'

'

'

'

'

'

'

'

'

Percentage of proportionate share of syndicates

'

'

'

'

'

'

'

'

'

'

'

100.00%

20.00%

'

'

'

'

United States corporate tax rate

'

35.00%

35.00%

'

'

'

'

'

35.00%

'

'

'

'

'

'

'

'

Net deferred tax assets recognized

'

'

'

'

'

'

'

299,000,000

'

'

'

'

'

'

'

'

'

Deferred tax assets recognized, accrued losses and loss adjustment expenses and life and annuity benefits

'

'

'

'

'

'

'

320,000,000

'

'

'

'

'

'

'

'

'

Deferred tax liabilities recognized, intangible assets

'

'

'

'

'

'

'

56,800,000

'

'

'

'

'

'

'

'

'

Provision for United States income taxes on earnings of the Company's foreign subsidiaries

'

'

'

'

'

'

0

'

0

'

'

'

'

'

'

'

'

Deferred tax assets, increase in provisional allocation of purchase price

'

'

'

'

'

'

43,200,000

'

'

'

'

'

'

'

'

'

'

Pro forma transaction costs, Alterra

'

'

'

'

'

'

'

'

23,000,000

'

'

'

'

'

'

'

'

Acceleration of Alterra long-term incentive compensation awards and restricted stock awards

1,083,000

[1]

12,621,000

[2]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Transaction costs

0

15,981,000

[2]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Pro forma transaction costs, total

'

39,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Other acquisition related costs, severance costs

3,519,000

[1]

31,734,000

[2]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Other acquisition related costs, stay bonuses

3,906,000

[1]

9,981,000

[2]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of outstanding shares acquired

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

Other intangible assets recognized, asset acquisition

'

'

'

35,400,000

'

25,000,000

'

'

'

'

'

'

'

'

'

'

'

Approximate total purchase price

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$187,400,000

£ 116,500,000

[1]

In connection with the acquisition of Alterra, the Company incurred acquisition-related costs totaling $8.5 million for the quarter ended SeptemberB 30, 2013, which included severance costs of $3.5 million, stay bonuses of $3.9 million and other compensation costs of $1.1 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.

[2]

In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the nine months ended SeptemberB 30, 2013, which primarily consist of due diligence, legal and investment banking costs. Additionally, the Company incurred severance costs of $31.7 million, stay bonuses of $10.0 million and other compensation costs totaling $12.6 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.

Acquisitions_Calculation_Of_To

Acquisitions (Calculation Of Total Purchase Price) (Details) (May 1, 2013 Alterra Capital Holdings [Member], USD $)	6 Months Ended
In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013
Business Acquisition [Line Items]	'
Exchange ratio per the Merger Agreement	0.04315
Markel share issuance to Alterra shareholders	4,300,000
Incentive award ratio per the Merger Agreement	0.06252
Multiplied by Markel's weighted average stock price on April 30, 2013	$529.59	[1]
Markel share and restricted stock issuance consideration, net of taxes	$2,267,648
Alterra common shares outstanding as of the Acquisition Date that received cash consideration	96,433,000
Multiplied by cash price per share component per the Merger Agreement	$10
Markel cash consideration	964,330
Fair value of Markel warrant issuance to Alterra warrant holders as of the Acquisition Date	73,685
Fair value of Markel stock option issuance to Alterra stock option holders as of the Acquisition Date, net of taxes	12,335
Fair value of partially vested Markel restricted stock unit issuance as of the Acquisition Date, net of taxes	6,867
Unrecognized compensation on unvested restricted stock and restricted stock units	-20,572
Total acquisition consideration	$3,304,293
Common Stock [Member]	'
Business Acquisition [Line Items]	'
Shares of Alterra stock outstanding as of the Acquisition Date	96,433,000
Markel share issuance to Alterra shareholders	4,161,000
Restricted Stock [Member]	'
Business Acquisition [Line Items]	'
Shares of Alterra stock outstanding as of the Acquisition Date	2,239,000
Markel share issuance to Alterra shareholders	140,000
[1]	The fair value of the shares issued by the Company was calculated as the weighted average price of the Company's stock on April 30, 2013, the day preceding the Acquisition Date.

Acquisitions_Summary_Of_Provis

Acquisitions (Summary Of Provisional Fair Value Of Assets Acquired And Liabilities Assumed) (Details) (May 1, 2013 Alterra Capital Holdings [Member], USD $)	Jun. 30, 2013
In Thousands, unless otherwise specified
May 1, 2013 Alterra Capital Holdings [Member]	'
ASSETS	'
Investments	$6,407,841
Cash and cash equivalents	1,036,274
Restricted cash and cash equivalents	414,497
Receivables	866,388
Reinsurance recoverable on unpaid losses	1,169,084
Reinsurance recoverable on paid losses	80,672
Prepaid reinsurance premiums	317,445
Other assets	834,024
LIABILITIES	'
Unpaid losses and loss adjustment expenses	4,719,461
Life and annuity benefits	1,477,482
Unearned premiums	1,075,610
Payables to insurance and reinsurance companies	342,858
Senior long-term debt	512,463
Other liabilities	218,523
Net assets	2,779,828
Goodwill	336,965
Intangible assets	187,500
Acquisition date fair value	$3,304,293

Acquisitions_Summary_Of_Provis1

Acquisitions (Summary Of Provisional Net Intangible Assets Acquired) (Details) (USD $)

3 Months Ended

9 Months Ended

5 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

6 Months Ended

9 Months Ended

In Thousands, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Sep. 30, 2013

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

Lloyd's Syndicate Capacity [Member]

Lloyd's Syndicate Capacity [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Customer Relationships [Member]

Customer Relationships [Member]

Broker Relationships [Member]

Broker Relationships [Member]

Technology [Member]

Technology [Member]

Trade Names [Member]

Trade Names [Member]

Schedule of Intangible Assets [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Finite lived intangible assets

'

'

'

'

'

'

$150,000

'

'

'

'

'

$113,000

'

$18,000

'

$18,000

'

$1,000

'

Indefinite lived intangible assets

'

'

'

'

'

'

37,500

'

12,000

'

25,500

'

'

'

'

'

'

'

'

'

Economic useful lives of finite lived intangible assets, years

'

'

'

'

'

'

'

'17 years

'

'

'

'

'

'18 years

'

'18 years

'

'10 years

'

'1 year

Economic useful lives of indefinite lived intangible assets, years

'

'

'

'

'

'

'

'

'

'Indefinite

'

'Indefinite

'

'

'

'

'

'

'

'

Intangible assets, before amortization, as of the Acquisition Date

'

'

'

'

'

'

187,500

'

'

'

'

'

'

'

'

'

'

'

'

'

Amortization (from the Acquisition Date through September 30, 2013)

16,848

7,959

37,755

25,078

'

4,246

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net intangible assets as of September 30, 2013

$560,836

'

$560,836

'

$374,295

$183,254

'

$183,254

'

'

'

'

'

'

'

'

'

'

'

'

Acquisitions_Summary_Of_Transa

Acquisitions (Summary Of Transaction And Acquisition-Related Costs) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2013
Acquisitions [Abstract]	'		'
Transaction costs	$0		$15,981	[1]
Severance costs	3,519	[2]	31,734	[1]
Stay bonuses	3,906	[2]	9,981	[1]
Acceleration of Alterra long-term incentive compensation awards and restricted stock awards	1,083	[2]	12,621	[1]
Total transaction and acquisition-related costs	$8,508	[2]	$70,317	[1]
[1]	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the nine months ended SeptemberB 30, 2013, which primarily consist of due diligence, legal and investment banking costs. Additionally, the Company incurred severance costs of $31.7 million, stay bonuses of $10.0 million and other compensation costs totaling $12.6 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
[2]	In connection with the acquisition of Alterra, the Company incurred acquisition-related costs totaling $8.5 million for the quarter ended SeptemberB 30, 2013, which included severance costs of $3.5 million, stay bonuses of $3.9 million and other compensation costs of $1.1 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.

Acquisitions_Summary_Of_Result

Acquisitions (Summary Of Results Of Operations) (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	5 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2013
					May 1, 2013 Alterra Capital Holdings [Member]	May 1, 2013 Alterra Capital Holdings [Member]
Business Acquisition [Line Items]	'	'	'	'	'	'
Operating revenues	$1,191,665	$765,775	$3,043,298	$2,192,157	$314,200	$557,446
Net loss to shareholders	$65,599	$49,653	$182,257	$196,593	($11,312)	($67,055)

Acquisitions_Unaudited_Pro_For

Acquisitions (Unaudited Pro Forma Consolidated Financial Information) (Details) (USD $)	3 Months Ended				9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Acquisitions [Abstract]	'		'		'		'
Earned premiums	$919,723		$862,534		$2,717,732		$2,592,818
Operating revenues	1,234,380		1,146,994		3,629,659		3,365,605
Net income to shareholders	$90,910		$60,090		$298,970		$265,664
U.S. GAAP combined ratio	95.00%	[1]	102.00%	[1]	95.00%	[1]	96.00%	[1]
Basic net income per share	$6.49		$4.29		$21.33		$18.96
Diluted net income per share	$6.47		$4.27		$21.24		$18.90
Weighted average common shares outstanding, basic	14,001		14,015		14,014		14,012
Weighted average common shares outstanding, diluted	14,057		14,067		14,075		14,060
[1]	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

Investments_Narrative_Details

Investments (Narrative) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified	securities	securities
Schedule of Investments [Line Items]	'	'
Available-for-sale securities in unrealized loss position, number of positions	1,569	35
Available-for-sale securities, estimated fair value	$5,610,175	$326,217
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-193,208	-17,153
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	12	10
Available-for-sale securities, estimated fair value, 12 months or longer	3,487	9,660
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-135	-563
Restricted assets	5,759,787	1,372,170
Held In Trust Or On Deposit [Member]	'	'
Schedule of Investments [Line Items]	'	'
Restricted assets	5,100,000	1,300,000
Held In Trust Or On Deposit [Member] | Alterra Capital Holdings [Member]	'	'
Schedule of Investments [Line Items]	'	'
Restricted assets	3,600,000	'
Cash And Cash Equivalents And Securities Pledged As Collateral [Member]	'	'
Schedule of Investments [Line Items]	'	'
Restricted assets	706,800	23,200
Cash And Cash Equivalents And Securities Pledged As Collateral [Member] | Alterra Capital Holdings [Member]	'	'
Schedule of Investments [Line Items]	'	'
Restricted assets	671,000	'
Fixed Maturities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Available-for-sale securities, estimated fair value	5,021,285	10,874
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-189,992	-9,568
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	10	8
Available-for-sale securities, estimated fair value, 12 months or longer	3,362	7,677
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-123	-440
Equity Securities [Member]	'	'
Schedule of Investments [Line Items]	'	'
Available-for-sale securities, estimated fair value	62,904	86,523
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-3,210	-7,571
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	2	2
Available-for-sale securities, estimated fair value, 12 months or longer	125	1,983
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	($12)	($123)

Investments_AvailableForSale_I

Investments (Available-For-Sale Investments) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	$12,913,333	$6,922,901
Available-for-sale investments, gross unrealized holding gains	1,740,885	1,453,816
Available-for-sale investments, gross unrealized holding losses	-184,078	-8,073
Available-for-sale investments, unrealized other-than-temporary impairment losses	-9,130	-9,080
Total investments, available-for-sale	14,461,010	8,359,564
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	1,257,855	297,663
Available-for-sale investments, gross unrealized holding gains	10,933	19,844
Available-for-sale investments, gross unrealized holding losses	-24,660	0
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	1,244,128	317,507
Obligations Of States, Municipalities And Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	2,739,483	2,586,867
Available-for-sale investments, gross unrealized holding gains	135,139	245,057
Available-for-sale investments, gross unrealized holding losses	-23,561	-362
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	2,851,061	2,831,562
Foreign Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	1,435,249	503,844
Available-for-sale investments, gross unrealized holding gains	34,323	52,764
Available-for-sale investments, gross unrealized holding losses	-45,888	0
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	1,423,684	556,608
Commercial Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	401,036	'
Available-for-sale investments, gross unrealized holding gains	116	'
Available-for-sale investments, gross unrealized holding losses	-10,443	'
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	'
Total investments, available-for-sale	390,709	'
Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	927,880	202,644
Available-for-sale investments, gross unrealized holding gains	12,364	14,996
Available-for-sale investments, gross unrealized holding losses	-12,012	-5
Available-for-sale investments, unrealized other-than-temporary impairment losses	-2,258	-2,258
Total investments, available-for-sale	925,974	215,377
Asset-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	217,394	13,828
Available-for-sale investments, gross unrealized holding gains	261	517
Available-for-sale investments, gross unrealized holding losses	-1,779	0
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	215,876	14,345
Corporate Bonds [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	3,263,513	957,432
Available-for-sale investments, gross unrealized holding gains	67,385	93,395
Available-for-sale investments, gross unrealized holding losses	-62,519	-121
Available-for-sale investments, unrealized other-than-temporary impairment losses	-6,872	-6,822
Total investments, available-for-sale	3,261,507	1,043,884
Total Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	10,242,410	4,562,278
Available-for-sale investments, gross unrealized holding gains	260,521	426,573
Available-for-sale investments, gross unrealized holding losses	-180,862	-488
Available-for-sale investments, unrealized other-than-temporary impairment losses	-9,130	-9,080
Total investments, available-for-sale	10,312,939	4,979,283
Insurance, Banks And Other Financial Institutions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	404,923	508,771
Available-for-sale investments, gross unrealized holding gains	540,749	389,434
Available-for-sale investments, gross unrealized holding losses	-241	-138
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	945,431	898,067
Industrial, Consumer And All Other [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	1,106,540	878,534
Available-for-sale investments, gross unrealized holding gains	939,612	637,783
Available-for-sale investments, gross unrealized holding losses	-2,969	-7,433
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	2,043,183	1,508,884
Total Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	1,511,463	1,387,305
Available-for-sale investments, gross unrealized holding gains	1,480,361	1,027,217
Available-for-sale investments, gross unrealized holding losses	-3,210	-7,571
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	2,988,614	2,406,951
Short-Term Investments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale investments, amortized cost	1,159,460	973,318
Available-for-sale investments, gross unrealized holding gains	3	26
Available-for-sale investments, gross unrealized holding losses	-6	-14
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Total investments, available-for-sale	$1,159,457	$973,330

Investments_Summary_Of_Gross_U

Investments (Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	$5,606,688	$316,557
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-193,073	-16,590
Available-for-sale securities, estimated fair value, 12 months or longer	3,487	9,660
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-135	-563
Available-for-sale securities, estimated fair value	5,610,175	326,217
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-193,208	-17,153
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	652,752	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-24,660	'
Available-for-sale securities, estimated fair value, 12 months or longer	0	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	'
Available-for-sale securities, estimated fair value	652,752	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-24,660	'
Obligations Of States, Municipalities And Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	307,119	2,833
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-23,439	-46
Available-for-sale securities, estimated fair value, 12 months or longer	3,196	3,616
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-122	-316
Available-for-sale securities, estimated fair value	310,315	6,449
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-23,561	-362
Foreign Governments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	907,456	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-45,888	'
Available-for-sale securities, estimated fair value, 12 months or longer	0	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	'
Available-for-sale securities, estimated fair value	907,456	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-45,888	'
Commercial Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	378,001	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-10,443	'
Available-for-sale securities, estimated fair value, 12 months or longer	0	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	'
Available-for-sale securities, estimated fair value	378,001	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-10,443	'
Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	455,942	364
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-14,269	-2,260
Available-for-sale securities, estimated fair value, 12 months or longer	166	201
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-1	-3
Available-for-sale securities, estimated fair value	456,108	565
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-14,270	-2,263
Asset-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	189,070	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-1,779	'
Available-for-sale securities, estimated fair value, 12 months or longer	0	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	'
Available-for-sale securities, estimated fair value	189,070	'
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-1,779	'
Corporate Bonds [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	2,127,583	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-69,391	-6,822
Available-for-sale securities, estimated fair value, 12 months or longer	0	3,860
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	-121
Available-for-sale securities, estimated fair value	2,127,583	3,860
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-69,391	-6,943
Total Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	5,017,923	3,197
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-189,869	-9,128
Available-for-sale securities, estimated fair value, 12 months or longer	3,362	7,677
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-123	-440
Available-for-sale securities, estimated fair value	5,021,285	10,874
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-189,992	-9,568
Insurance, Banks And Other Financial Institutions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	11,518	2,431
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-241	-138
Available-for-sale securities, estimated fair value, 12 months or longer	0	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	0
Available-for-sale securities, estimated fair value	11,518	2,431
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-241	-138
Industrial, Consumer And All Other [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	51,261	82,109
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-2,957	-7,310
Available-for-sale securities, estimated fair value, 12 months or longer	125	1,983
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-12	-123
Available-for-sale securities, estimated fair value	51,386	84,092
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-2,969	-7,433
Total Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	62,779	84,540
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-3,198	-7,448
Available-for-sale securities, estimated fair value, 12 months or longer	125	1,983
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-12	-123
Available-for-sale securities, estimated fair value	62,904	86,523
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-3,210	-7,571
Short-Term Investments [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Available-for-sale securities, estimated fair value, less than 12 months	525,986	228,820
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-6	-14
Available-for-sale securities, estimated fair value, 12 months or longer	0	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	0
Available-for-sale securities, estimated fair value	525,986	228,820
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	($6)	($14)

Investments_Schedule_Of_Amorti

Investments (Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturities By Contractual Maturity Date) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Due in one year or less, amortized cost	$900,263	'
Due after one year through five years, amortized cost	3,151,362	'
Due after five years through ten years, amortized cost	2,227,954	'
Due after ten years, amortized cost	2,416,521	'
Amortized cost, sub-total	8,696,100	'
Total fixed maturities, amortized cost	10,242,410	4,562,278
Due in one year or less, estimated fair value	905,564	'
Due after one year through five years, estimated fair value	3,220,106	'
Due after five years through ten years, estimated fair value	2,277,491	'
Due after ten years, estimated fair value	2,377,219	'
Estimated fair value, sub-total	8,780,380	'
Total fixed maturities, estimated fair value	10,312,939	4,979,283
Commercial Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, amortized cost	401,036	'
Fixed maturities, estimated fair value	390,709	'
Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, amortized cost	927,880	'
Fixed maturities, estimated fair value	925,974	'
Asset-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, amortized cost	217,394	'
Fixed maturities, estimated fair value	$215,876	'

Investments_Components_Of_Net_

Investments (Components Of Net Investment Income) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Municipal bonds (tax-exempt)	$20,347	$22,508	$62,307	$68,287
Short-term investments, including overnight deposits	1,042	721	2,316	1,995
Dividends on equity securities	12,059	10,128	35,600	30,586
Change in fair value of credit default swap	1,660	1,480	8,860	13,681
Other	9,004	875	16,036	-356
Total investment income	89,775	66,717	238,151	214,178
Investment expenses	-3,583	-2,279	-9,363	-6,344
Net investment income	86,192	64,438	228,788	207,834
Taxable Municipal Bonds [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Taxable bonds	7,452	5,670	19,467	17,055
Other Taxable Bonds [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Taxable bonds	$38,211	$25,335	$93,565	$82,930

Investments_Summary_Of_Activit

Investments (Summary Of Activity For Credit Losses) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Investments, Debt and Equity Securities [Abstract]	'	'	'	'
Cumulative credit loss recognized in income	$21,370	$21,370	$21,370	$21,370
Changes in cumulative credit losses	$0	$0	$0	$0

Investments_Summary_Of_Net_Rea

Investments (Summary Of Net Realized Investment Gains And The Change In Net Unrealized Gains On Investments) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Gain (Loss) on Investments [Line Items]	'	'	'	'
Realized gains	$29,979	$12,979	$65,045	$34,383
Realized losses	-18,741	-7,748	-24,344	-9,027
Other-than-temporary impairment losses recognized in net income	0	-3,159	-4,589	-4,151
Net realized investment gains	11,238	5,231	40,701	25,356
Change in net unrealized gains on investments	109,207	136,470	111,014	330,305
Sales Of Fixed Maturities [Member]	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Realized gains	4,933	2,576	6,031	7,863
Realized losses	-18,741	-118	-19,585	-457
Sales Of Equity Securities [Member]	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Realized gains	24,648	10,403	58,563	25,865
Realized losses	0	-383	-170	-331
Other [Member]	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Realized gains	398	0	451	655
Realized losses	0	-4,088	0	-4,088
Fixed Maturities [Member]	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Change in net unrealized gains on investments	-8,679	37,488	-346,476	82,078
Equity Securities [Member]	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Change in net unrealized gains on investments	117,899	98,964	457,505	248,214
Short-Term Investments [Member]	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Change in net unrealized gains on investments	($13)	$18	($15)	$13

Investments_Summary_Of_OtherTh

Investments (Summary Of Other-Than-Temporary Impairment Losses Recognized In Net Income And In Net Realized Investment Gains (Losses) By Investment Type) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	$0	($3,159)	($4,589)	($4,151)
Fixed Maturities [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	0	0	-1,765	0
Fixed Maturities [Member] | Obligations Of States, Municipalities And Political Subdivisions [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	0	0	-1,242	0
Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	0	0	-523	0
Equity Securities [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	0	-3,159	-2,824	-4,151
Equity Securities [Member] | Insurance, Banks And Other Financial Institutions [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	0	-1,615	0	-2,441
Equity Securities [Member] | Industrial, Consumer And All Other [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairment losses recognized in net income	$0	($1,544)	($2,824)	($1,710)

Investments_Schedule_Of_Restri

Investments (Schedule Of Restricted Assets) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]	'	'
Investments, available-for-sale	$5,121,181	$1,262,755
Restricted cash and cash equivalents	600,171	109,415
Other assets	38,435	0
Total	$5,759,787	$1,372,170

Fair_Value_Measurements_Narrat

Fair Value Measurements (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value Disclosures [Abstract]	'	'	'	'
Fair value, level 1 to level 2 transfers, amount	$0	$0	$0	$0
Fair value, level 2 to level 1 transfers, amount	0	0	0	0
Assets measured at fair value on a nonrecurring basis	0	0	0	0
Liabilities measured at fair value on a nonrecurring basis	$0	$0	$0	$0

Fair_Value_Measurements_Balanc

Fair Value Measurements (Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	$10,312,939	$4,979,283
Total equity securities	2,988,614	2,406,951
Short-term investments	1,159,457	973,330
Derivative contracts	3,800	12,700
Fair Value, Measurements, Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	10,312,939	4,979,283
Total equity securities	2,988,614	2,406,951
Short-term investments	1,159,457	973,330
Available-for-sale investments, estimated fair value	14,461,010	8,359,564
Derivative contracts	3,830	12,690
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Total equity securities	2,988,614	2,406,951
Short-term investments	1,016,185	888,758
Available-for-sale investments, estimated fair value	4,004,799	3,295,709
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	10,312,939	4,979,283
Total equity securities	0	0
Short-term investments	143,272	84,572
Available-for-sale investments, estimated fair value	10,456,211	5,063,855
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Total equity securities	0	0
Short-term investments	0	0
Available-for-sale investments, estimated fair value	0	0
Derivative contracts	3,830	12,690
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	1,244,128	317,507
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	1,244,128	317,507
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Obligations Of States, Municipalities And Political Subdivisions [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	2,851,061	2,831,562
Fair Value, Measurements, Recurring [Member] | Obligations Of States, Municipalities And Political Subdivisions [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Obligations Of States, Municipalities And Political Subdivisions [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	2,851,061	2,831,562
Fair Value, Measurements, Recurring [Member] | Obligations Of States, Municipalities And Political Subdivisions [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	1,423,684	556,608
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	1,423,684	556,608
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	390,709	'
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	'
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	390,709	'
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	'
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	925,974	215,377
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	925,974	215,377
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	215,876	14,345
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	215,876	14,345
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	3,261,507	1,043,884
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	3,261,507	1,043,884
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Insurance, Banks And Other Financial Institutions [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	945,431	898,067
Fair Value, Measurements, Recurring [Member] | Insurance, Banks And Other Financial Institutions [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	945,431	898,067
Fair Value, Measurements, Recurring [Member] | Insurance, Banks And Other Financial Institutions [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	0	0
Fair Value, Measurements, Recurring [Member] | Insurance, Banks And Other Financial Institutions [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	0	0
Fair Value, Measurements, Recurring [Member] | Industrial, Consumer And All Other [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	2,043,183	1,508,884
Fair Value, Measurements, Recurring [Member] | Industrial, Consumer And All Other [Member] | Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	2,043,183	1,508,884
Fair Value, Measurements, Recurring [Member] | Industrial, Consumer And All Other [Member] | Level 2 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	0	0
Fair Value, Measurements, Recurring [Member] | Industrial, Consumer And All Other [Member] | Level 3 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total equity securities	$0	$0

Fair_Value_Measurements_Summar

Fair Value Measurements (Summary Of Changes In Level 3 Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $)	3 Months Ended				9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Fair Value Disclosures [Abstract]	'		'		'		'
Derivatives, beginning of period	$5,490		$17,130		$12,690		$29,331
Total gains included in net income	-1,660		-1,480		-8,860		-13,681
Total gains included in other comprehensive income	0		0		0		0
Transfers into Level 3	0		0		0		0
Transfers out of Level 3	0		0		0		0
Derivatives, end of period	3,830		15,650		3,830		15,650
Net unrealized gains included in net income relating to liabilities held at September 30, 2013 and 2012	$1,660	[1]	$1,480	[1]	$8,860	[1]	$13,681	[1]
[1]	Included in net investment income in the consolidated statements of income and comprehensive income.

Segment_Reporting_Disclosures_1

Segment Reporting Disclosures (Company's Segment Disclosures) (Details) (USD $)	3 Months Ended				9 Months Ended				12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
					segment
Segment Reporting Information [Line Items]	'		'		'		'		'
Number of segments of the specialty insurance marketplace in which the Company historically operated	'		'		3		'		'
Gross premium volume	$1,076,298		$610,833		$2,920,578		$1,906,373		'
Net written premiums	842,931		539,624		2,433,200		1,687,404		'
Earned premiums	919,723		530,537		2,269,129		1,573,189		'
Losses and loss adjustment expenses, current accident year	-610,683		-372,740		-1,545,109		-1,073,238		'
Losses and loss adjustment expenses, prior accident years	77,311		69,281		281,435		260,164		'
Transaction costs and other acquisition-related expenses	-8,508	[1]	'		-70,317	[2]	'		'
Transaction costs	0		'		-15,981	[2]	'		'
Other acquisition related costs, severance costs	-3,519	[1]	'		-31,734	[2]	'		'
Other acquisition related costs, stay bonuses	-3,906	[1]	'		-9,981	[2]	'		'
Other acquisition related costs, other compensation costs	-1,083	[1]	'		-12,621	[2]	'		'
Prospective adoption of ASU 2010-26	'		-6,534	[3]	'		-41,093	[4]	-43,100	[3],[4]
All other expenses, excluding transaction costs and other acquisition-related expenses	-343,618		'		-873,577		'		'
All other expenses, excluding prospective adoption of ASU 2010-26	'		-225,307		'		-654,229		'
Underwriting profit (loss)	34,225		-4,763		61,561		64,793		'
Net investment income	86,192		64,438		228,788		207,834		'
Net realized investment gains	11,238		5,231		40,701		25,356		'
Other revenues (insurance)	1,932		11,759		18,458		40,818		'
Other expenses (insurance)	-16,248		-13,151		-31,248		-37,906		'
Segment profit (loss)	117,339		63,514		318,260		300,895		'
Other revenues (non-insurance)	172,580		153,810		486,222		344,960		'
Other expenses (non-insurance)	-150,318		-132,188		-428,394		-305,556		'
Amortization of intangible assets	-16,848		-7,959		-37,755		-25,078		'
Interest expense	-30,619		-24,692		-82,754		-69,068		'
Income Before Income Taxes	92,134		52,485		255,579		246,153		'
U.S. GAAP combined ratio	96.00%	[5]	101.00%	[5]	97.00%	[5]	96.00%	[5]	'
Excess And Surplus Lines [Member]	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premium volume	273,757		253,014		792,877		705,849		'
Net written premiums	232,396		211,538		673,741		597,742		'
Earned premiums	218,538		195,478		629,288		584,524		'
Losses and loss adjustment expenses, current accident year	-147,204		-134,504		-424,398		-390,254		'
Losses and loss adjustment expenses, prior accident years	40,799		51,310		165,587		132,583		'
Transaction costs and other acquisition-related expenses	0	[1]	'		0	[2]	'		'
Prospective adoption of ASU 2010-26	'		-2,125	[3]	'		-16,652	[4]	'
All other expenses, excluding transaction costs and other acquisition-related expenses	-85,570		'		-250,101		'		'
All other expenses, excluding prospective adoption of ASU 2010-26	'		-88,093		'		-255,295		'
Underwriting profit (loss)	26,563		22,066		120,376		54,906		'
Net investment income	0		0		0		0		'
Net realized investment gains	0		0		0		0		'
Other revenues (insurance)	0		0		0		0		'
Other expenses (insurance)	0		0		0		0		'
Segment profit (loss)	26,563		22,066		120,376		54,906		'
U.S. GAAP combined ratio	88.00%	[5]	89.00%	[5]	81.00%	[5]	91.00%	[5]	'
Specialty Admitted [Member]	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premium volume	230,450		167,747		687,669		496,019		'
Net written premiums	219,955		157,894		655,350		467,722		'
Earned premiums	197,372		153,009		533,037		431,179		'
Losses and loss adjustment expenses, current accident year	-126,649		-116,044		-353,581		-310,115		'
Losses and loss adjustment expenses, prior accident years	28,410		11,504		45,863		27,747		'
Transaction costs and other acquisition-related expenses	0	[1]	'		0	[2]	'		'
Prospective adoption of ASU 2010-26	'		-2,600	[3]	'		-12,863	[4]	'
All other expenses, excluding transaction costs and other acquisition-related expenses	-80,023		'		-229,462		'		'
All other expenses, excluding prospective adoption of ASU 2010-26	'		-60,008		'		-171,111		'
Underwriting profit (loss)	19,110		-14,139		-4,143		-35,163		'
Net investment income	0		0		0		0		'
Net realized investment gains	0		0		0		0		'
Other revenues (insurance)	1,402		11,536		11,945		36,065		'
Other expenses (insurance)	-6,292		-12,181		-14,447		-35,184		'
Segment profit (loss)	14,220		-14,784		-6,645		-34,282		'
U.S. GAAP combined ratio	90.00%	[5]	109.00%	[5]	101.00%	[5]	108.00%	[5]	'
London Insurance Market [Member]	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premium volume	199,575		190,071		725,153		704,511		'
Net written premiums	174,203		170,193		634,180		621,947		'
Earned premiums	199,633		182,052		577,101		557,493		'
Losses and loss adjustment expenses, current accident year	-135,098		-122,192		-386,617		-372,869		'
Losses and loss adjustment expenses, prior accident years	40,562		32,744		101,589		118,994		'
Transaction costs and other acquisition-related expenses	0	[1]	'		0	[2]	'		'
Prospective adoption of ASU 2010-26	'		-1,809	[3]	'		-11,578	[4]	'
All other expenses, excluding transaction costs and other acquisition-related expenses	-73,335		'		-222,469		'		'
All other expenses, excluding prospective adoption of ASU 2010-26	'		-76,820		'		-226,770		'
Underwriting profit (loss)	31,762		13,975		69,604		65,270		'
Net investment income	0		0		0		0		'
Net realized investment gains	0		0		0		0		'
Other revenues (insurance)	311		223		4,778		4,753		'
Other expenses (insurance)	-1,167		-970		-3,762		-2,722		'
Segment profit (loss)	30,906		13,228		70,620		67,301		'
U.S. GAAP combined ratio	84.00%	[5]	92.00%	[5]	88.00%	[5]	88.00%	[5]	'
Alterra [Member]	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premium volume	372,516		'		714,844		'		'
Net written premiums	216,378		'		469,894		'		'
Earned premiums	304,181		'		529,668		'		'
Losses and loss adjustment expenses, current accident year	-201,732		'		-380,513		'		'
Losses and loss adjustment expenses, prior accident years	0		'		0		'		'
Transaction costs and other acquisition-related expenses	-8,508	[1]	'		-70,317	[2]	'		'
All other expenses, excluding transaction costs and other acquisition-related expenses	-104,608		'		-171,762		'		'
Underwriting profit (loss)	-10,667		'		-92,924		'		'
Net investment income	0		'		0		'		'
Net realized investment gains	0		'		0		'		'
Other revenues (insurance)	59		'		1,256		'		'
Other expenses (insurance)	0		'		0		'		'
Segment profit (loss)	-10,608		'		-91,668		'		'
U.S. GAAP combined ratio	104.00%	[5]	'		118.00%	[5]	'		'
Other Insurance (Discontinued Lines) [Member]	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premium volume	0		1		35		-6		'
Net written premiums	-1		-1		35		-7		'
Earned premiums	-1		-2		35		-7		'
Losses and loss adjustment expenses, current accident year	0		0		0		0		'
Losses and loss adjustment expenses, prior accident years	-32,460		-26,277		-31,604		-19,160		'
Transaction costs and other acquisition-related expenses	0	[1]	'		0	[2]	'		'
Prospective adoption of ASU 2010-26	'		0	[3]	'		0	[4]	'
All other expenses, excluding transaction costs and other acquisition-related expenses	-82		'		217		'		'
All other expenses, excluding prospective adoption of ASU 2010-26	'		-386		'		-1,053		'
Underwriting profit (loss)	-32,543		-26,665		-31,352		-20,220		'
Net investment income	0		0		0		0		'
Net realized investment gains	0		0		0		0		'
Other revenues (insurance)	160		0		479		0		'
Other expenses (insurance)	-8,789		0		-13,039		0		'
Segment profit (loss)	-41,172		-26,665		-43,912		-20,220		'
U.S. GAAP combined ratio - not meaniningful	'NM	[5],[6]	'NM	[5],[6]	'NM	[5],[6]	'NM	[5],[6]	'
Investing [Member]	'		'		'		'		'
Segment Reporting Information [Line Items]	'		'		'		'		'
Gross premium volume	0		0		0		0		'
Net written premiums	0		0		0		0		'
Earned premiums	0		0		0		0		'
Losses and loss adjustment expenses, current accident year	0		0		0		0		'
Losses and loss adjustment expenses, prior accident years	0		0		0		0		'
Transaction costs and other acquisition-related expenses	0	[1]	'		0	[2]	'		'
Prospective adoption of ASU 2010-26	'		0	[3]	'		0	[4]	'
All other expenses, excluding transaction costs and other acquisition-related expenses	0		'		0		'		'
All other expenses, excluding prospective adoption of ASU 2010-26	'		0		'		0		'
Underwriting profit (loss)	0		0		0		0		'
Net investment income	86,192		64,438		228,788		207,834		'
Net realized investment gains	11,238		5,231		40,701		25,356		'
Other revenues (insurance)	0		0		0		0		'
Other expenses (insurance)	0		0		0		0		'
Segment profit (loss)	$97,430		$69,669		$269,489		$233,190		'
[1]	In connection with the acquisition of Alterra, the Company incurred acquisition-related costs totaling $8.5 million for the quarter ended SeptemberB 30, 2013, which included severance costs of $3.5 million, stay bonuses of $3.9 million and other compensation costs of $1.1 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
[2]	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the nine months ended SeptemberB 30, 2013, which primarily consist of due diligence, legal and investment banking costs. Additionally, the Company incurred severance costs of $31.7 million, stay bonuses of $10.0 million and other compensation costs totaling $12.6 million related to the acceleration of certain long-term incentive compensation awards and restricted stock awards that were granted by Alterra prior to the acquisition.
[3]	Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included $43.1 million of costs that no longer met the criteria for deferral as of January 1, 2012 and were recognized into income during 2012, consistent with policy terms. The quarter ended September 30, 2012 included $6.5 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.
[4]	Effective January 1, 2012, the Company prospectively adopted FASB ASU No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts. At December 31, 2011, deferred policy acquisition costs included $43.1 million of costs that no longer met the criteria for deferral as of January 1, 2012 and were recognized into income during 2012, consistent with policy terms. The nine months ended September 30, 2012 included $41.1 million of underwriting, acquisition and insurance expenses that were deferred as of December 31, 2011 and no longer met the criteria for deferral as of January 1, 2012.
[5]	The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.
[6]	NM b Ratio is not meaningful.

Segment_Reporting_Disclosures_2

Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Segment Reporting Information [Line Items]	'	'
Assets	$23,808,577	$12,556,588
Non-Insurance Operations [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Assets	903,795	891,586
Segment Assets [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Assets	22,904,782	11,665,002
Segment Assets [Member] | Investing [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Assets	17,125,848	9,277,697
Segment Assets [Member] | Underwriting [Member]	'	'
Segment Reporting Information [Line Items]	'	'
Assets	$5,778,934	$2,387,305

Reinsurance_Narrative_Details

Reinsurance (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
	reinsurer		reinsurer		reinsurer
Reinsurance Disclosures [Line Items]	'	'	'	'	'
Percentage of ceded earned premiums to gross earned premiums	20.00%	12.00%	18.00%	11.00%	'
Number of largest company reinsurers	10	'	10	'	10
Reinsurance recoverables	$98	$15.30	$179.90	$79.60	'
Ten Largest Reinsurers [Member]	'	'	'	'	'
Reinsurance Disclosures [Line Items]	'	'	'	'	'
Recoverable balances as a percentage of the total balance	61.00%	'	61.00%	'	74.00%
Fairfax Financial Group [Member]	'	'	'	'	'
Reinsurance Disclosures [Line Items]	'	'	'	'	'
Recoverable balances as a percentage of the total balance	10.00%	'	10.00%	'	'

Reinsurance_Effect_Of_Reinsura

Reinsurance (Effect Of Reinsurance On Premiums Written And Earned) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reinsurance Disclosures [Abstract]	'	'	'	'
Direct premiums written	$836,117	$519,408	$2,310,966	$1,568,092
Direct premiums earned	818,854	505,514	2,076,682	1,505,710
Assumed premiums written	240,181	91,425	609,612	338,281
Assumed premiums earned	331,495	94,822	675,286	269,771
Ceded premiums written	-233,367	-71,209	-487,378	-218,969
Ceded premiums earned	-230,626	-69,799	-482,839	-202,292
Net premiums written	842,931	539,624	2,433,200	1,687,404
Net premiums earned	$919,723	$530,537	$2,269,129	$1,573,189

Life_And_Annuity_Benefits_Narr

Life And Annuity Benefits (Narrative) (Details)	Sep. 30, 2013
Liability for Future Policy Benefits [Abstract]	'
Average reserve valuation rate	2.30%

Life_And_Annuity_Benefits_Sche

Life And Annuity Benefits (Schedule Of Life And Annuity Benefits) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Liability for Future Policy Benefits [Abstract]	'	'
Life	$189,792	'
Annuities	1,188,914	'
Accident and health	104,292	'
Total life and annuity benefits	$1,482,998	$0

Senior_LongTerm_Debt_And_Other2

Senior Long-Term Debt And Other Debt (Narrative) (Details) (USD $)

9 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

9 Months Ended

Sep. 30, 2013

Dec. 31, 2012

Jul. 02, 2012

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Mar. 08, 2013

Sep. 30, 2013

Mar. 08, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Jun. 30, 2013

Sep. 30, 2013

Jun. 30, 2013

Apr. 16, 2007

Sep. 30, 2013

Jun. 30, 2013

Sep. 27, 2010

Sep. 30, 2013

4.90% Unsecured Senior Notes [Member]

4.90% Unsecured Senior Notes [Member]

4.90% Unsecured Senior Notes [Member]

7.50% Unsecured Senior Debentures [Member]

6.80% Unsecured Senior Notes [Member]

6.80% Unsecured Senior Notes [Member]

3.625% Unsecured Senior Notes [Member]

3.625% Unsecured Senior Notes [Member]

5.0% Unsecured Senior Notes [Member]

5.0% Unsecured Senior Notes [Member]

3.625% And 5.0% Unsecured Senior Notes [Member]

7.20% Unsecured Senior Notes [Member]

6.25% Unsecured Senior Notes [Member]

7.35% Unsecured Senior Notes [Member]

7.35% Unsecured Senior Notes [Member]

Revolving Credit Facility [Member]

Revolving Credit Facility [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

May 1, 2013 Alterra Capital Holdings [Member]

option

7.20% Unsecured Senior Notes [Member]

7.20% Unsecured Senior Notes [Member]

7.20% Unsecured Senior Notes [Member]

6.25% Unsecured Senior Notes [Member]

6.25% Unsecured Senior Notes [Member]

6.25% Unsecured Senior Notes [Member]

Senior Credit Facility [Member]

Debt Instrument [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt instrument, face amount

'

'

$350,000,000

$150,000,000

'

'

'

$250,000,000

'

$250,000,000

'

'

'

'

'

'

'

'

'

'

$100,000,000

'

'

$350,000,000

'

Debt instrument, interest rate

4.90%

4.90%

'

7.50%

6.80%

'

3.63%

'

5.00%

'

'

7.20%

6.25%

7.35%

7.35%

'

'

'

7.20%

'

'

6.25%

'

'

'

Net proceeds from issuance of unsecured debt

347,200,000

'

'

'

'

'

'

'

'

'

491,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt instrument, redemption price, percentage of face amount

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Loss on extinguishment of debt

'

'

'

-4,100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt instrument, principal amount outstanding

'

'

'

'

'

246,700,000

'

'

'

'

'

'

'

'

'

'

'

'

'

90,600,000

'

'

'

'

'

Debt instrument, estimated fair value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

512,463,000

'

95,800,000

'

'

416,600,000

'

'

Line of credit facility, previous capacity available

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

150,000,000

'

'

'

'

'

'

'

'

'

Line of credit facility, capacity available

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

300,000,000

'

'

'

'

'

'

'

'

250,000,000

Number of interest rate options on revolving credit facility

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2

'

'

'

'

'

'

'

'

'

Line of credit facility, unused capacity, commitment fee, percent

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.25%

'

'

'

'

'

'

'

'

'

Borrowings outstanding under the facility

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0

0

'

'

'

'

'

'

'

0

Credit facility, maximum capacity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

900,000,000

Credit facility, potential increased maximum capacity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,400,000,000

Credit facility, letters of credit issued and outstanding

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$439,600,000

Senior_LongTerm_Debt_And_Other3

Senior Long-Term Debt And Other Debt (Summary Of Senior Long-Term Debt And Other Debt) (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]	'	'
Senior long-term debt and other debt	$2,250,479	$1,492,550
6.80% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	0	246,619
Debt instrument, interest rate	6.80%	6.80%
Debt Instrument, unamortized discount	0	45
7.20% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	95,789	0
Debt instrument, interest rate	7.20%	'
Debt instrument, unamortized premium	5,160	'
7.125% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	348,303	348,091
Debt instrument, interest rate	7.13%	7.13%
Debt Instrument, unamortized discount	1,697	1,909
6.25% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	414,049	0
Debt instrument, interest rate	6.25%	'
Debt instrument, unamortized premium	64,049	'
5.35% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	248,417	248,262
Debt instrument, interest rate	5.35%	5.35%
Debt Instrument, unamortized discount	1,583	1,738
4.90% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	347,556	347,347
Debt instrument, interest rate	4.90%	4.90%
Debt Instrument, unamortized discount	2,444	2,653
3.625% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	248,090	0
Debt instrument, interest rate	3.63%	'
Debt Instrument, unamortized discount	1,910	'
7.35% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	197,789	197,710
Debt instrument, interest rate	7.35%	7.35%
Debt Instrument, unamortized discount	2,211	2,291
5.0% Unsecured Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured senior notes	243,393	0
Debt instrument, interest rate	5.00%	'
Debt Instrument, unamortized discount	6,607	'
Subsidiary And Other Debt [Member]	'	'
Debt Instrument [Line Items]	'	'
Senior long-term debt and other debt	$107,093	$104,521
Debt Instrument, interest rate, percentage rate range, minimum	1.60%	'
Debt Instrument, interest rate, percentage rate range, maximum	6.50%	'

Senior_LongTerm_Debt_And_Other4

Senior Long-Term Debt And Other Debt (Summary Of Future Principal Payments Due At Maturity On Senior Long-Term Debt And Other Debt) (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Debt Disclosure [Abstract]	'	'
2013	$4,498	'
2014	17,857	'
2015	5,090	'
2016	12,092	'
2017	104,366	'
2018 and thereafter	2,053,819	'
Total principal payments	2,197,722	'
Net unamortized premium	52,757	'
Senior long-term debt and other debt	$2,250,479	$1,492,550

Derivatives_Narrative_Details

Derivatives (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Credit Derivatives [Line Items]	'	'	'	'	'
Credit default swap, notional value	$33,100,000	'	$33,100,000	'	$33,100,000
Credit default swap, exposure to loss, maximum per reference entity	20,000,000	'	20,000,000	'	'
Credit default swap, fair value	3,800,000	'	3,800,000	'	12,700,000
Gain on derivative instrument recognized in earnings	$1,660,000	$1,480,000	$8,860,000	$13,681,000	'
Weighted-average expected default rate	1.00%	'	1.00%	'	3.00%
Weighted-average future expected recovery rate	39.00%	'	39.00%	'	39.00%
Minimum [Member]	'	'	'	'	'
Credit Derivatives [Line Items]	'	'	'	'	'
Expected default rate	0.25%	'	0.25%	'	1.00%
Future expected recovery rate	20.00%	'	20.00%	'	20.00%
Maximum [Member]	'	'	'	'	'
Credit Derivatives [Line Items]	'	'	'	'	'
Expected default rate	10.00%	'	10.00%	'	43.00%
Future expected recovery rate	40.00%	'	40.00%	'	40.00%

Income_Taxes_Narrative_Details

Income Taxes (Narrative) (Details)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Income Tax Disclosure [Abstract]	'	'
Effective tax rate	28.00%	19.00%
United States corporate tax rate	35.00%	35.00%

Employee_Benefit_Plans_Narrati

Employee Benefit Plans (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'	'	'	'
Expenses relating to defined contribution plans	$6,200,000	$4,600,000	$16,700,000	$13,900,000
Contributions to pension plan	'	'	5,300,000	'
Estimated future employer contributions, current fiscal year	$0	'	$0	'

Employee_Benefit_Plans_Compone

Employee Benefit Plans (Components Of Net Periodic Benefit Income Pension Plan) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'	'	'	'
Service cost	$0	$90	$0	$269
Interest cost	1,611	1,693	4,837	5,079
Expected return on plan assets	-2,671	-2,432	-8,015	-7,295
Amortization of net actuarial pension loss	477	643	1,432	1,930
Net periodic benefit income	($583)	($6)	($1,746)	($17)

Shareholders_Equity_Narrative_

Shareholders' Equity (Narrative) (Details) (USD $)	Jun. 30, 2013	Sep. 30, 2013	31-May-13	31-May-13	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	31-May-13	Sep. 30, 2013
	May 1, 2013 Alterra Capital Holdings [Member]	Warrant [Member]	2012 Compensation Plan [Member]	Alterra Equity Award Plans [Member]	Alterra Equity Award Plans [Member]	Alterra Equity Award Plans [Member]	Alterra Equity Award Plans [Member]	Alterra Equity Award Plans [Member]	Alterra Equity Award Plans [Member]
		May 1, 2013 Alterra Capital Holdings [Member]	Restricted Stock Units (RSUs) [Member]	Restricted Stock Units (RSUs) [Member]	Restricted Stock Units (RSUs) [Member]	Restricted Stock Units (RSUs) [Member]	Restricted Stock Units (RSUs) [Member]	Stock Options [Member]	Stock Options [Member]
		warrant_holders	Retention [Member]
			Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'	'	'	'	'	'
Number of restricted stock units awarded	'	'	31,669	'	'	'	'	'	'
Restricted stock awards, grant date fair value	'	'	$16,600,000	'	'	$81,600,000	'	'	'
Restricted stock awards, vesting period	'	'	'3 years	'	'	'	'	'	'
Number of shares of common stock issuable for each vested restricted stock unit	'	'	1	'	'	'	'	'	'
Number of awards issued	'	'	'	154,103	'	'	'	101,875	'
Weighted average exercise price of replacement options issued	'	'	'	'	'	'	'	'	$398.96
Weighted average grant-date fair value of replacement options issued	'	'	'	'	'	'	'	'	$140.08
Expected annual volatility, assumption used in pricing model	'	'	'	'	'	'	'	'	19.04%
Risk free rate, assumption used in pricing model	'	'	'	'	'	'	'	'	0.20%
Expected term, assumption used in pricing model	'	'	'	'	'	'	'	'	'2 years
Number of options exercised under option plans	'	'	'	'	'	'	'	'	49,763
Cash proceeds resulting from exercise of stock options	'	'	'	'	'	'	'	'	18,500,000
Current tax benefits resulting from exercise of stock options	'	'	'	'	'	'	'	'	700,000
Intrinsic value of options exercised	'	'	'	'	'	'	'	'	6,400,000
Weighted average grant-date fair value of restricted stock awards	'	'	'	'	'	$529.59	'	'	'
Weighted average remaining service period of restricted stock awards	'	'	'	'	'	'1 year	'1 year	'	'
Recognized compensation expense related to vested restricted stock awards as of the acquisition date	'	'	'	'	'	-61,000,000	'	'	'
Unrecognized compensation on unvested restricted stock and restricted stock units	-20,572,000	'	'	'	-20,600,000	-20,600,000	-20,600,000	'	'
Recognized compensation expense related to restricted stock awards	'	'	'	'	-1,500,000	'	-10,900,000	'	'
Unrecognized compensation cost related to nonvested restricted stock awards	'	'	'	'	-6,500,000	-6,500,000	-6,500,000	'	'
Fair value of the vested restricted stock awards	'	'	'	'	'	'	40,400,000	'	'
Number of Company Converted Warrants issued	'	225,529	'	'	'	'	'	'	'
Fair value of Company Converted Warrants	'	$73,700,000	'	'	'	'	'	'	'
Number of warrant holders who elected to have their warrants remain outstanding	'	3	'	'	'	'	'	'	'
Number of Company Converted Warrants exercised	'	225,499	'	'	'	'	'	'	'
Number of shares of the Company's common stock stock warrant holders received under the cashless exercise provision of warrants	'	100,115	'	'	'	'	'	'	'
Number of shares of the Company's common stock available for purchase related to outstanding warrants	'	30	'	'	'	'	'	'	'

Shareholders_Equity_Schedule_O

Shareholders' Equity (Schedule Of Net Income Per Share) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Line Items]	'	'	'	'
Net income to shareholders	$65,599	$49,653	$182,257	$196,593
Adjustment of redeemable noncontrolling interests	0	1,702	3,101	-6,484
Adjusted net income to shareholders	$65,599	$51,355	$185,358	$190,109
Basic common shares outstanding	14,001	9,632	12,050	9,641
Diluted shares outstanding	14,056	9,659	12,093	9,667
Basic net income per share	$4.69	$5.33	$15.38	$19.72
Diluted net income per share	$4.67	$5.32	$15.33	$19.67
Stock Options [Member]	'	'	'	'
Earnings Per Share [Line Items]	'	'	'	'
Dilutive potential common shares	16	4	11	7
Restricted Stock Units (RSUs) [Member]	'	'	'	'
Earnings Per Share [Line Items]	'	'	'	'
Dilutive potential common shares	39	23	32	19

Shareholders_Equity_Summary_Of

Shareholders' Equity (Summary Of Options For The Alterra Equity Award Plans) (Details) (Stock Options [Member], Alterra Equity Award Plans [Member], USD $)	5 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013
Stock Options [Member] | Alterra Equity Award Plans [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]	'
Outstanding and exercisable, May 1, 2013, number of shares	101,875
Exercised, number of shares	49,763
Outstanding and exercisable, September 30, 2013, number of shares	52,112
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]	'
Outstanding and exercisable, May 1, 2013, weighted average exercise price (dollars per share)	$398.96
Exercised, weighted average exercise price (dollars per share)	$401.83
Outstanding and exercisable, September 30, 2013, weighted average exercise price (dollars per share)	$396.22
Outstanding and exercisable, September 30, 2013, weighted average remaining contractual term	'2 years 7 months
Outstanding and exercisable, September 30, 2013, intrinsic value	$6.70

Shareholders_Equity_Summary_Of1

Shareholders' Equity (Summary Of Nonvested Share-Based Awards For The Alterra Equity Award Plans) (Details) (Restricted Stock Units (RSUs) [Member], Alterra Equity Award Plans [Member], USD $)	5 Months Ended
	Sep. 30, 2013
Restricted Stock Units (RSUs) [Member] | Alterra Equity Award Plans [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]	'
Number of awards, nonvested awards issued on May 1, 2013	154,103
Number of awards, vested	-76,287
Number of awards, forfeited	-7,476
Number of awards, nonvested awards at September 30, 2013	70,340
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]	'
Weighted average grant-date fair value, nonvested awards issued on May 1, 2013	$529.59
Weighted average grant-date fair value, vested	$529.59
Weighted average grant-date fair value, forfeited	$529.59
Weighted average grant-date fair value, nonvested awards at September 30, 2013	$529.59

Other_Comprehensive_Income_Cha

Other Comprehensive Income (Change In Accumulated Other Comprehensive Income By Component, Net Of Taxes And Noncontrolling Interest) (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income [Line Items]	'	'
Accumulated other comprehensive income, beginning balance	$911,337	$660,920
Other comprehensive income before reclassifications	97,502	248,889
Amounts reclassified from accumulated other comprehensive income	-26,720	-18,667
Total other comprehensive income	70,782	230,222
Accumulated other comprehensive income, ending balance	982,119	891,142
Unrealized Holding Gains On Available-For-Sale Securities [Member]	'	'
Accumulated Other Comprehensive Income [Line Items]	'	'
Accumulated other comprehensive income, beginning balance	946,933	704,719
Other comprehensive income before reclassifications	107,433	245,920
Amounts reclassified from accumulated other comprehensive income	-27,866	-20,154
Total other comprehensive income	79,567	225,766
Accumulated other comprehensive income, ending balance	1,026,500	930,485
Foreign Currency [Member]	'	'
Accumulated Other Comprehensive Income [Line Items]	'	'
Accumulated other comprehensive income, beginning balance	-1,075	-2,614
Other comprehensive income before reclassifications	-9,931	2,969
Amounts reclassified from accumulated other comprehensive income	0	0
Total other comprehensive income	-9,931	2,969
Accumulated other comprehensive income, ending balance	-11,006	355
Net Actuarial Pension Loss [Member]	'	'
Accumulated Other Comprehensive Income [Line Items]	'	'
Accumulated other comprehensive income, beginning balance	-34,521	-41,185
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	1,146	1,487
Total other comprehensive income	1,146	1,487
Accumulated other comprehensive income, ending balance	($33,375)	($39,698)

Other_Comprehensive_Income_Sch

Other Comprehensive Income (Schedule Of Deferred Tax Expense (Benefit) Of Other Comprehensive Income) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Comprehensive Income (Loss), Tax [Abstract]	'	'	'	'
Net holding gains arising during the period	$35,765	$45,325	$43,840	$113,215
Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	-63	-38	-9	-41
Reclassification adjustments for net gains included in net income	-3,186	-2,835	-12,384	-8,635
Change in net unrealized gains on investments	32,516	42,452	31,447	104,539
Change in foreign currency translation adjustments	0	-409	38	-457
Change in net actuarial pension loss	67	121	286	443
Total	$32,583	$42,164	$31,771	$104,525

Other_Comprehensive_Income_Amo

Other Comprehensive Income (Amounts Reclassified From Accumulated Other Comprehensive Income Into Income, By Component) (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Net realized investment gains	$11,238	$5,231	$40,701	$25,356
Underwriting, acquisition and insurance expenses	-352,126	-231,841	-943,894	-695,322
Income taxes	-25,167	-811	-70,673	-45,998
Net Income	66,967	51,674	184,906	200,155
Reclassification Out Of Accumulated Other Comprehensive Income [Member] | Unrealized Holding Gains On Available-For-Sale Securities [Member]	'	'	'	'
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Net realized investment gains	10,840	9,319	40,250	28,789
Income taxes	-3,186	-2,835	-12,384	-8,635
Net Income	7,654	6,484	27,866	20,154
Reclassification Out Of Accumulated Other Comprehensive Income [Member] | Net Actuarial Pension Loss [Member]	'	'	'	'
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'
Underwriting, acquisition and insurance expenses	-477	-643	-1,432	-1,930
Income taxes	67	121	286	443
Net Income	($410)	($522)	($1,146)	($1,487)

Commitments_And_Contingencies_

Commitments And Contingencies (Narrative) (Details) (New Point VI Limited [Member], USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
New Point VI Limited [Member]	'
Other Commitments [Line Items]	'
Commitment under subscription agreement	$75