Markel (MKL) 10-K 27 Feb 15 2014 FY Annual report Financial data

Document_And_Entity_Informatio

Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 09, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Entity Registrant Name	MARKEL CORP
Entity Central Index Key	1096343
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		13,962,386
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$8,808,000,000

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
ASSETS
Fixed maturities (amortized cost of $9,929,137 in 2014 and $10,129,141 in 2013)	$10,422,882		$10,142,536
Equity securities (cost of $1,951,658 in 2014 and $1,566,553 in 2013)	4,137,576		3,251,798
Short-term investments (estimated fair value approximates cost)	1,594,849		1,452,288
Total Investments	16,155,307		14,846,622
Cash and cash equivalents	1,960,169		1,978,526
Restricted cash and cash equivalents	522,225		786,926
Receivables	1,135,217		1,141,773
Reinsurance recoverable on unpaid losses	1,868,669		1,854,414
Reinsurance recoverable on paid losses	102,206		102,002
Deferred policy acquisition costs	353,410		260,967
Prepaid reinsurance premiums	365,458		383,559
Goodwill	1,049,115	[1]	967,717
Intangible assets	702,747		565,083
Other assets	985,834		1,067,922
Total Assets	25,200,357		23,955,511
LIABILITIES AND EQUITY
Unpaid losses and loss adjustment expenses	10,404,152		10,262,056
Life and annuity benefits	1,305,818		1,486,574
Unearned premiums	2,245,690		2,127,115
Payables to insurance and reinsurance companies	276,122		295,496
Senior long-term debt and other debt (estimated fair value of $2,493,000 in 2014 and $2,372,000 in 2013)	2,253,594		2,256,227
Other liabilities	1,051,931		777,850
Total Liabilities	17,537,307		17,205,318
Redeemable noncontrolling interests	61,048		72,183
Commitments and contingencies
Shareholders' equity:
Common stock	3,308,395		3,288,863
Retained earnings	2,581,866		2,294,909
Accumulated other comprehensive income	1,704,557		1,089,805
Total Shareholders' Equity	7,594,818		6,673,577
Noncontrolling interests	7,184		4,433
Total Equity	7,602,002		6,678,010
Total Liabilities and Equity	$25,200,357		$23,955,511
[1]	Goodwill is net of accumulated impairment losses of $13.7 million included in Other.

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$9,929,137	$10,129,141
Equity securities, cost	1,951,658	1,566,553
Senior long-term debt and other debt, estimated fair value	$2,493,000	$2,372,000

Consolidated_Statements_Of_Inc

Consolidated Statements Of Income And Comprehensive Income (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
OPERATING REVENUES
Earned premiums	$3,840,912	$3,231,616	$2,147,128
Net investment income	363,230	317,373	282,107
Net realized investment gains:
Other-than-temporary impairment losses	-4,784	-4,706	-12,078
Net realized investment gains, excluding other-than-temporary impairment losses	50,784	67,858	43,671
Net realized investment gains	46,000	63,152	31,593
Other revenues	883,525	710,942	539,284
Total Operating Revenues	5,133,667	4,323,083	3,000,112
OPERATING EXPENSES
Losses and loss adjustment expenses	2,202,467	1,816,273	1,154,068
Underwriting, acquisition and insurance expenses	1,460,882	1,312,312	929,472
Amortization of intangible assets	57,627	55,223	33,512
Other expenses	854,871	663,528	478,248
Total Operating Expenses	4,575,847	3,847,336	2,595,300
Operating Income	557,820	475,747	404,812
Interest expense	117,442	114,004	92,762
Income Before Income Taxes	440,378	361,743	312,050
Income tax expense	116,690	77,898	53,802
Net Income	323,688	283,845	258,248
Net income attributable to noncontrolling interests	2,506	2,824	4,863
Net Income to Shareholders	321,182	281,021	253,385
OTHER COMPREHENSIVE INCOME
Net holding gains arising during the period	687,735	225,545	266,425
Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	173	-141	-160
Reclassification adjustments for net gains included in net income	-26,161	-40,830	-24,051
Change in net unrealized gains on investments, net of taxes	661,747	184,574	242,214
Change in foreign currency translation adjustments, net of taxes	-32,241	-10,143	1,534
Change in net actuarial pension loss, net of taxes	-14,750	4,065	6,664
Total Other Comprehensive Income	614,756	178,496	250,412
Comprehensive Income	938,444	462,341	508,660
Comprehensive income attributable to noncontrolling interests	2,510	2,852	4,858
Comprehensive Income to Shareholders	$935,934	$459,489	$503,802
NET INCOME PER SHARE
Basic (dollars per share)	$22.38	$22.57	$25.96
Diluted (dollars per share)	$22.27	$22.48	$25.89

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, redeemable noncontrolling interests at Dec. 31, 2011								$74,231
Balance at Dec. 31, 2011		3,388,115	3,387,513	891,507	1,835,086	660,920	602
Balance, shares at Dec. 31, 2011				9,621
Net income (loss)	258,248	253,123	253,385		253,385	0	-262	5,125
Other comprehensive income (loss)	250,412	250,417	250,417		0	250,417	0	-5
Comprehensive Income	508,660	503,540	503,802				-262	5,120
Issuance of common stock		9,145	9,145	9,145	0	0	0	0
Issuance of common stock, shares				47
Repurchase of common stock		-16,873	-16,873	0	-16,873	0	0	0
Repurchase of common stock, shares				-39
Restricted stock awards expensed		6,462	6,462	6,462	0	0	0	0
Restricted stock awards expensed, shares				0
Acquisitions		0	0	0	0	0	0	15,055
Acquisitions, shares				0
Adjustment of redeemable noncontrolling interests	3,100	-3,101	-3,101	0	-3,101	0	0	3,101
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		1,430	1,430	1,430	0	0	0	-3,573
Purchase of noncontrolling interest, shares				0
Other		299	279	436	-157	0	20	-7,709
Other, shares				0
Balance, redeemable noncontrolling interests at Dec. 31, 2012								86,225
Balance at Dec. 31, 2012		3,889,017	3,888,657	908,980	2,068,340	911,337	360
Balance, shares at Dec. 31, 2012				9,629
Net income (loss)	283,845	280,063	281,021		281,021	0	-958	3,782
Other comprehensive income (loss)	178,496	178,468	178,468		0	178,468	0	28
Comprehensive Income	462,341	458,531	459,489				-958	3,810
Issuance of common stock		24,518	24,518	24,518	0	0	0	0
Issuance of common stock, shares				71
Repurchase of common stock		-57,388	-57,388	0	-57,388	0	0	0
Repurchase of common stock, shares				-109
Restricted stock awards expensed		25,239	25,239	25,239	0	0	0	0
Restricted stock awards expensed, shares				-3
Acquisitions, shares				4,398
Acquisition of Alterra		2,330,199	2,330,199	2,330,199	0	0	0	0
Adjustment of redeemable noncontrolling interests	-2,000	1,963	1,963	0	1,963	0	0	-1,963
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		-136	-136	-136	0	0	0	-11,716
Purchase of noncontrolling interest, shares				0
Other		6,067	1,036	63	973	0	5,031	-4,173
Other, shares				0
Balance, redeemable noncontrolling interests at Dec. 31, 2013	72,183							72,183
Balance at Dec. 31, 2013	6,678,010	6,678,010	6,673,577	3,288,863	2,294,909	1,089,805	4,433
Balance, shares at Dec. 31, 2013				13,986
Net income (loss)	323,688	319,201	321,182		321,182	0	-1,981	4,487
Other comprehensive income (loss)	614,756	614,752	614,752		0	614,752	0	4
Comprehensive Income	938,444	933,953	935,934				-1,981	4,491
Issuance of common stock		5,691	5,691	5,691	0	0	0	0
Issuance of common stock, shares				19
Repurchase of common stock		-26,053	-26,053	0	-26,053	0	0	0
Repurchase of common stock, shares				-43
Restricted stock awards expensed		22,935	22,935	22,935	0	0	0	0
Restricted stock awards expensed, shares				0
Adjustment of redeemable noncontrolling interests	8,200	-8,186	-8,186	0	-8,186	0	0	8,186
Adjustment of redeemable noncontrolling interests, shares				0
Purchase of noncontrolling interest		-9,352	-10,257	-10,257	0	0	905	-18,566
Purchase of noncontrolling interest, shares				0
Other		5,004	1,177	1,163	14	0	3,827	-5,246
Other, shares				0
Balance, redeemable noncontrolling interests at Dec. 31, 2014	61,048							61,048
Balance at Dec. 31, 2014	$7,602,002	$7,602,002	$7,594,818	$3,308,395	$2,581,866	$1,704,557	$7,184
Balance, shares at Dec. 31, 2014				13,962

Consolidated_Statements_Of_Cas

Consolidated Statements Of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
OPERATING ACTIVITIES
Net income	$323,688	$283,845	$258,248
Adjustments to reconcile net income to net cash provided by operating activities:
Deferred income tax expense	84,543	4,050	37,648
Depreciation and amortization	203,580	190,066	87,326
Net realized investment gains	-46,000	-63,152	-31,593
Decrease (increase) in receivables	21,148	142,065	-36,590
Decrease (increase) in deferred policy acquisition costs	-99,387	-103,704	37,209
Increase (decrease) in unpaid losses and loss adjustment expenses, net	249,873	290,130	-28,052
Decrease in life and annuity benefits	-62,883	-40,235	0
Increase in unearned premiums, net	147,840	97,249	71,073
Increase (decrease) in payables to insurance and reinsurance companies	-45,204	-150,764	19,190
Increase (decrease) in income taxes payable	-46,576	81,995	-9,909
Other	-13,830	13,976	-12,017
Net Cash Provided By Operating Activities	716,792	745,521	392,533
INVESTING ACTIVITIES
Proceeds from sales of fixed maturities and equity securities	1,286,871	879,564	336,548
Proceeds from maturities, calls and prepayments of fixed maturities	1,420,817	1,475,938	510,697
Cost of fixed maturities and equity securities purchased	-3,153,055	-1,651,397	-426,439
Net change in short-term investments	-129,164	-470,423	-428,292
Proceeds from sales of equity method investments	107,292	313,557	0
Cost of equity method investments	-16,081	-38,018	-40,650
Change in restricted cash and cash equivalents	264,701	-263,014	-37,642
Additions to property and equipment	-82,132	-47,725	-45,519
Acquisitions, net of cash acquired	-319,086	-12,198	-243,675
Other	-2,368	1,103	-2,158
Net Cash Provided (Used) By Investing Activities	-622,205	187,387	-377,130
FINANCING ACTIVITIES
Additions to senior long-term debt and other debt	89,480	547,214	492,792
Repayment and retirement of senior long-term debt and other debt	-83,722	-321,978	-313,790
Repurchases of common stock	-26,053	-57,388	-16,873
Issuance of common stock	5,691	24,518	9,145
Purchase of redeemable noncontrolling interests	-25,918	-11,852	-2,143
Distributions to noncontrolling interests	-5,245	-5,124	-7,684
Other	-21,357	-23	-19,485
Net Cash Provided (Used) By Financing Activities	-67,124	175,367	141,962
Effect of foreign currency rate changes on cash and cash equivalents	-45,820	6,485	3,142
Increase (decrease) in cash and cash equivalents	-18,357	1,114,760	160,507
Cash and cash equivalents at beginning of year	1,978,526	863,766	703,259
CASH AND CASH EQUIVALENTS AT END OF YEAR	$1,960,169	$1,978,526	$863,766

Summary_Of_Significant_Account

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary Of Significant Accounting Policies	Summary of Significant Accounting Policies
	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. Through its wholly-owned subsidiary, Markel Ventures, Inc. (Markel Ventures), 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).
	a)Basis of Presentation. 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 December 31, 2014 and not in any prior periods, except with respect to the Supplemental Pro Forma Information included in note 2. The Company consolidates the results of its Markel Ventures subsidiaries on a one-month lag. Certain prior year amounts have been reclassified to conform to the current presentation.
	b)Use of Estimates. 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. Management periodically reviews its estimates and assumptions. Quarterly reviews include evaluating the adequacy of reserves for unpaid losses and loss adjustment expenses, life and annuity reinsurance benefit reserves, litigation contingencies, the reinsurance allowance for doubtful accounts and income tax liabilities, as well as analyzing the recoverability of deferred tax assets, estimating reinsurance premiums written and earned and evaluating the investment portfolio for other-than-temporary declines in estimated fair value. Estimates and assumptions for goodwill and intangible assets are reviewed in conjunction with an acquisition, and goodwill and indefinite-lived intangible assets are reassessed at least annually for impairment. Actual results may differ materially from the estimates and assumptions used in preparing the consolidated financial statements.
	c)Investments. Available-for-sale investments are recorded at estimated fair value. Unrealized gains and losses on investments, net of deferred income taxes, are included in accumulated other comprehensive income in shareholders' equity. 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.
	Premiums and discounts are amortized or accreted over the lives of the related fixed maturities as an adjustment to the yield using the effective interest method. Dividend and interest income are recognized when earned. Realized investment gains or losses are included in earnings. Realized gains or losses from sales of investments are derived using the first-in, first-out method.
	Investments accounted for under the equity method of accounting are recorded at cost within other assets on the consolidated balance sheets and subsequently increased or decreased by the Company's proportionate share of the net income or loss of the investee. The Company records its proportionate share of net income or loss of the investee in net investment income. The Company records its proportionate share of other comprehensive income or loss of the investee as a component of other comprehensive income. Dividends or other equity distributions are recorded as a reduction of the investment. The Company reviews equity method investments for impairment when events or circumstances indicate that a decline in the fair value of the investment below its carrying value is other-than-temporary.
	d)Cash and Cash Equivalents. The Company considers all investments with original maturities of 90 days or less to be cash equivalents. The carrying value of the Company's cash and cash equivalents and restricted cash and cash equivalents approximates fair value.
	e)Receivables. Receivables include amounts receivable from agents, brokers and insureds, which represent premiums that are both currently due and amounts not yet due on insurance and reinsurance policies. Premiums for insurance policies are generally due at inception. Premiums for reinsurance policies generally become due over the period of coverage based on the policy terms. The Company monitors the credit risk associated with premiums receivable, taking into consideration the fact that in certain instances credit risk may be reduced by the Company's right to offset loss obligations or unearned premiums against premiums receivable. Amounts deemed uncollectible are charged to net income in the period they are determined. Changes in the estimate of reinsurance premiums written will result in an adjustment to premiums receivable in the period they are determined.
	f)Reinsurance Recoverables. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured business. Allowances are established for amounts deemed uncollectible and reinsurance recoverables are recorded net of these allowances. The Company evaluates the financial condition of its reinsurers and monitors concentration risk to minimize its exposure to significant losses from individual reinsurers.
	g)Deferred Policy Acquisition Costs. Costs directly related to the acquisition of insurance premiums are deferred and amortized over the related policy period, generally one year. Concurrent with the adoption of Financial Accounting Standards Board (FASB) Accounting Standards Update (ASU) No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts, effective January 1, 2012, the Company only defers acquisition costs incurred that are related directly to the successful acquisition of new or renewal insurance contracts, including commissions to agents and brokers and premium taxes. Commissions received related to reinsurance premiums ceded are netted against broker commissions in determining acquisition costs eligible for deferral. To the extent that future policy revenues on existing policies are not adequate to cover related costs and expenses, deferred policy acquisition costs are charged to earnings. The Company does not consider anticipated investment income in determining whether a premium deficiency exists.
	h)Goodwill and Intangible Assets. Goodwill and intangible assets are recorded as a result of business acquisitions. Goodwill represents the excess of the amount paid to acquire a business over the net fair value of assets acquired and liabilities assumed at the date of acquisition. Indefinite-lived and other intangible assets are recorded at fair value as of the acquisition date. The determination of the fair value of certain assets acquired and liabilities assumed involves significant judgment and the use of valuation models and other estimates, which require assumptions that are inherently subjective. Goodwill and indefinite-lived intangible assets are tested for impairment at least annually. The Company completes an annual test during the fourth quarter of each year based upon the results of operations through September 30. Intangible assets with definite lives are amortized using the straight-line method over their estimated useful lives, generally five to 20 years, and are reviewed for impairment when events or circumstances indicate that their carrying value may not be recoverable.
	i)Property and Equipment. Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization of property and equipment are calculated using the straight-line method over the estimated useful lives (generally, the life of the lease for leasehold improvements, ten to 40 years for buildings, seven to 40 years for land improvements, three to ten years for furniture and equipment and three to 25 years for other property and equipment).
	j)Redeemable Noncontrolling Interests. The Company owns controlling interests in various companies through its Markel Ventures operations. In some cases, the Company has the option to acquire the remaining equity interests, and the remaining equity interests have the option to sell their interests to the Company, in the future. The redemption value of the remaining equity interests is generally based on the respective company's earnings in specified periods preceding the redemption date. The redeemable noncontrolling interests generally become redeemable through 2018.
	The Company recognizes changes in the redemption value that exceed the carrying value of redeemable noncontrolling interests to retained earnings as if the balance sheet date were also the redemption date. Changes in the redemption value also result in an adjustment to net income to shareholders in the calculation of basic and diluted net income per share. The adjustment recorded to retained earnings during 2014, 2013 and 2012 was an increase of $8.2 million, a decrease of $2.0 million, and an increase of $3.1 million, respectively.
	k)Income Taxes. The Company records deferred income taxes to reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and their tax bases. Deferred tax assets are reduced by a valuation allowance when management believes it is more likely than not that some, or all, of the deferred tax assets will not be realized. The Company recognizes the tax benefit from an uncertain tax position taken or expected to be taken in income tax returns only if it is more likely than not that the tax position will be sustained upon examination by tax authorities, based on the technical merits of the position. Tax positions that meet the more likely than not threshold are then measured using a probability weighted approach, whereby the largest amount of tax benefit that is greater than 50% likely of being realized upon ultimate settlement is recognized. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense.
	l)Unpaid Losses and Loss Adjustment Expenses. Unpaid losses and loss adjustment expenses on our property and casualty insurance business are based on evaluations of reported claims and estimates for losses and loss adjustment expenses incurred but not reported. Estimates for losses and loss adjustment expenses incurred but not reported are based on reserve development studies, among other things. The Company does not discount reserves for losses and loss adjustment expenses to reflect estimated present value, except for reserves assumed in connection with an acquisition, which are recorded at fair value at the acquisition date. Recorded reserves are estimates, and the ultimate liability may be greater or less than the estimates.
	m)Life and Annuity Benefits. Prior to its acquisition by the Company, Alterra entered into long duration reinsurance contracts for life and annuity benefits which subject the Company to mortality, longevity and morbidity risks. 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 an unlocking event occurs. To the extent existing policy reserves, together with the present value of future gross premiums and expected investment income earned thereon, are not adequate to cover the present value of future benefits, settlement and maintenance costs, the locked-in assumptions are revised to current best estimate assumptions and a charge to earnings for life and annuity benefits is recognized at that time. Because of the assumptions and estimates used in establishing reserves for life and annuity benefit obligations and the long-term nature of these reinsurance contracts, the ultimate liability may be greater or less than the estimates.
	Results attributable to the run-off of 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.
	n)Revenue Recognition.
	Property and Casualty Premiums
	Insurance premiums are generally earned on a pro rata basis over the policy period, typically one year. The cost of reinsurance ceded is initially recorded as prepaid reinsurance premiums and is amortized over the reinsurance contract period in proportion to the amount of insurance protection provided. Premiums ceded are netted against premiums written.
	Assumed reinsurance premiums are recorded at the inception of each contract based upon contract terms and information received from cedents and brokers and are earned on a pro rata basis over the coverage period, or for multi-year contracts, in proportion with the underlying risk exposure to the extent there is variability in the exposure through the coverage period. Changes in reinsurance premium estimates are expected and may result in significant adjustments in any period. These estimates change over time as additional information regarding changes in underlying exposures is obtained. Any subsequent differences arising on such estimates are recorded as premiums written in the period they are determined. The Company uses the periodic method to account for assumed reinsurance from foreign reinsurers. The Company's foreign reinsurers provide sufficient information to record foreign assumed business in the same manner as the Company records assumed business from United States reinsurers.
	Certain contracts that the Company writes provide for reinstatement of coverage. Reinstatement premiums are the premiums for the restoration of the insurance or reinsurance limit of a contract to its full amount after a loss occurrence by the insured or reinsured. The Company accrues for reinstatement premiums resulting from losses recorded. Such accruals are based upon contractual terms and management judgment is involved with respect to the amount of losses recorded. Changes in estimates of losses recorded on contracts with reinstatement premium features will result in changes in reinstatement premiums based on contractual terms. Reinstatement premiums are recognized at the time losses are recorded and are earned on a pro-rata basis over the coverage period.
	Other Revenues
	Other revenues primarily relate to the Company's Markel Ventures operations and consist of revenues from the sale of manufactured products and service revenues. Revenues from manufactured products are generally recognized at the time title transfers to the customer, which typically occurs at the point of shipment or delivery to the customer, depending on the terms of the sales arrangement. Revenues from services are generally recognized as the services are performed. Services provided pursuant to a contract are recognized either over the contract period or upon completion of the elements specified in the contract, depending on the terms of the contract.
	o)Stock-based Compensation. Stock-based compensation expense is recognized as part of underwriting, acquisition and insurance expenses over the requisite service period. Stock-based compensation expense, net of taxes, was $18.7 million in 2014, $18.4 million in 2013 and $4.4 million in 2012. See note 12.
	p)Foreign Currency Translation. The functional currencies of the Company's foreign operations are the currencies in which the majority of their business is transacted. Assets and liabilities of foreign operations are translated into the United States Dollar using the exchange rates in effect at the balance sheet date. Revenues and expenses of foreign operations are translated using the average exchange rate for the period. Gains or losses from translating the financial statements of foreign operations are included, net of taxes, in shareholders' equity as a component of accumulated other comprehensive income. Gains and losses arising from transactions denominated in a foreign currency, other than a functional currency, are included in net income.
	The Company manages its exposure to foreign currency risk primarily by matching assets, other than goodwill and intangible assets, and liabilities denominated in the same currency. To the extent that assets and liabilities in foreign currencies are not matched, the Company is exposed to foreign currency risk. For functional currencies, the related exchange rate fluctuations are reflected in other comprehensive income. The cumulative foreign currency translation adjustment, net of taxes, was a loss of $43.5 million and $11.2 million at December 31, 2014 and 2013, respectively.
	q)Derivative Financial Instruments. Derivative instruments, including derivative instruments resulting from hedging activities, are measured at fair value and recognized as either assets or liabilities on the consolidated balance sheets. The changes in fair value of derivatives are recognized in earnings unless the derivative is designated as a hedge and qualifies for hedge accounting.
	The Company's foreign currency forward contracts are generally designated and qualify as hedges of a net investment in a foreign operation. The effective portion of the change in fair value resulting from these hedges is reported in currency translation adjustments as part of other comprehensive income. The ineffective portion of the change in fair value is recognized in earnings.
	r)Comprehensive Income. Comprehensive income represents all changes in equity that result from recognized transactions and other economic events during the period. Other comprehensive income refers to revenues, expenses, gains and losses that under U.S. GAAP are included in comprehensive income but excluded from net income, such as unrealized gains or losses on investments, foreign currency translation adjustments and changes in net actuarial pension loss.
	s)Net Income Per Share. Basic net income per share is computed by dividing adjusted net income to shareholders by the weighted average number of common shares outstanding during the year. 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. See note 12(b).
	t)Recent Accounting Pronouncements. Effective January 1, 2014, the Company adopted FASB ASU No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. ASU No. 2013-11 requires that a liability related to an unrecognized tax benefit be offset against a deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward if such settlement is required or expected in the event the uncertain tax position is disallowed. In that case, the liability associated with the unrecognized tax benefit is presented in the financial statements as a reduction to the related deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward. Otherwise, the unrecognized tax benefit should be presented in the financial statements as a liability and should not be combined with deferred tax assets. The adoption of this guidance did not have an impact on the Company's financial position, results of operations or cash flows.
	In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which creates a new comprehensive revenue recognition standard that will serve as a single source of revenue guidance for all companies in all industries. The guidance applies to all companies that either enter into contracts with customers to transfer goods or services or enter into contracts for the transfer of nonfinancial assets, unless those contracts are within the scope of other standards, such as insurance contracts. ASU No. 2014-09's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under the current guidance. These may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU No. 2014-09 becomes effective for the Company during the first quarter of 2017 and may be applied retrospectively or under a modified retrospective method where the cumulative effect is recognized at the date of initial application. Early application is not permitted. The Company is currently evaluating ASU No. 2014-09 to determine the potential impact that adopting this standard will have on its consolidated financial statements.

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2014
Acquisitions [Abstract]
Acquisitions	Acquisitions
	Acquisition of Alterra
	a)Overview. On May 1, 2013, the Company completed the acquisition of 100% of the 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. Results attributable to Alterra's property and casualty insurance and reinsurance business are included in each of the Company's underwriting segments, which were redefined during the first quarter of 2014. 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 19 for further discussion of the Company's reportable 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. 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.
	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 and restricted stock units issued was net of income taxes of $1.9 million, $10.1 million and $0.7 million, respectively. See note 12 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 $295.7 million, which 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. See note 7 for goodwill recorded by reportable segment. None of the goodwill that was recorded is deductible for income tax purposes. The Company also recognized indefinite lived intangible assets of $37.5 million and other intangible assets of $170.0 million, which will be amortized over a weighted average period of 17 years.
	The following table summarizes the 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	859,884
	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	223,108
	Net assets	2,801,103
	Goodwill	295,690
	Intangible assets	207,500
	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 - Fixed maturity 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 historical amortized cost of the fixed maturity investments acquired and their estimated fair value as of the Acquisition Date, $495.5 million, represents incremental premium that will be amortized to net investment income over the term of the underlying securities.  The amount of the unamortized incremental premium as of December 31, 2014 and 2013 was $281.1 million and $398.1 million, respectively. The decrease in the unamortized incremental premium is due to amortization expense of $59.3 million and $58.3 million for the years ended December 31, 2014 and 2013, respectively, and sales of securities.
	•	Intangible assets - Establish the 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 was 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 expired. As of December 31, 2014, this adjustment was fully amortized.
	•	Unpaid losses and loss adjustment expenses - Unpaid losses and loss adjustment expenses acquired include an 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. The amount of the unamortized fair value adjustment included in unpaid losses and loss adjustment expenses as of December 31, 2014 and 2013 was $114.6 million and $136.5 million, respectively.
	•	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 10 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 11. The amount of the unamortized premium on the acquired senior long-term debt as of December 31, 2014 and 2013 was $56.7 million and $66.1 million, respectively.
	The following table summarizes the intangible assets recorded in connection with the acquisition.
	(dollars in thousands)	Amount			Economic
					Useful Life
	Customer relationships	$	132,000		18 years
	Broker relationships	19,000			18 years
	Technology	18,000			Ten years
	Trade names	1,000			One year
	Lloyd's syndicate capacity	12,000			Indefinite
	Insurance licenses	25,500			Indefinite
	Intangible assets as of the Acquisition Date	$	207,500
Customer relationships represent policyholder relationships and the network of insurance companies through which Alterra conducted 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 of London (Lloyd's) insurance market. The Lloyd's capacity is renewed annually at no cost to the Company or 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, is required to record deferred taxes at the 35% statutory U.S. rate.
As part of the allocation of the purchase price, the Company recorded net deferred tax assets of $310.1 million. Of this amount, $343.9 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 $64.6 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. As of the Acquisition Date, 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 was recorded.
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)	Year Ended
	31-Dec-13
Transaction costs	$	15,981
Acquisition-related costs:
Severance costs	31,734
Stay bonuses	14,804
Acceleration of Alterra long-term incentive compensation awards and restricted stock awards	12,621
Total transaction and acquisition-related costs	$	75,140
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 have been 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 $12.6 million related to the acceleration of certain of these awards during the year ended December 31, 2013.
f)Financial Results. The following table summarizes the results of Alterra from the Acquisition Date through December 31, 2013 that have been included within the Company's consolidated statements of income and comprehensive income.
(dollars in thousands)	Year Ended
	December 31, 2013
Operating revenues	$	912,387
Net loss to shareholders	$	(93,074	)
g)Supplemental Pro Forma Information (unaudited). Alterra's results have been included in the Company's Consolidated Financial Statements from the Acquisition Date to December 31, 2014. The following table presents unaudited pro forma consolidated information for the years ended December 31, 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 year ended December 31, 2013, severance costs attributable to the acquisition totaling $31.7 million for the year ended December 31, 2013, and stay bonuses of $14.8 million for the year ended December 31, 2013. The acceleration of compensation expense during the year ended December 31, 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.
	Unaudited
	Consolidated Pro Forma
	Years Ended December 31,
(in thousands, except per share amounts)	2013			2012
Earned premiums	$	3,680,220		$	3,509,834
Operating revenues	4,899,628			4,561,107
Net income to shareholders	422,829			308,496
U.S. GAAP combined ratio (1)	95		%	99		%
Basic net income per share	$	30.33		$	21.79
Diluted net income per share	$	30.19		$	21.71
Weighted average common shares outstanding:
Basic	14,007			14,014
Diluted	14,069			14,065
(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 Abbey Protection
On January 17, 2014, the Company completed its acquisition of 100% of the share capital of Abbey Protection plc (Abbey), an integrated specialty insurance and consultancy group headquartered in London. Abbey's business is focused on the underwriting and sale of insurance products to small and medium-sized 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 and professional consulting services. Results attributable to Abbey's insurance operations are included in the International Insurance segment. Results attributable to Abbey's consultancy operations are reported with the Company's non-insurance operations, which are not included in a reportable segment.
Total consideration for this acquisition was $190.7 million, all of which was cash consideration. The purchase price was allocated to the acquired assets and liabilities based on estimated fair values on January 17, 2014. The Company recognized goodwill of $65.8 million, of which $43.0 million was allocated to the International Insurance segment and $22.8 million was allocated to the Company's non-insurance operations. None of the goodwill recognized is expected to be deductible for income tax purposes. The goodwill is primarily attributable to Abbey's assembled workforce and synergies that are expected to result upon integration of Abbey into the Company's insurance operations. The Company also recognized other intangible assets of $113.4 million, including $103.5 million of customer relationships and $9.9 million of trade names. These intangible assets are expected to be amortized over 20 years and 14 years, respectively.
Acquisition of Essentia
Effective 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 U.S. Insurance segment.
Effective January 1, 2014, Hagerty exercised its option to purchase 9.9% of the outstanding shares of Essentia, which reduced the Company's ownership interest in Essentia to 90.1%.
Markel Ventures Acquisitions
In July 2014, the Company acquired 100% of the outstanding shares of Cottrell, Inc. (Cottrell), a privately held company headquartered in Gainesville, Georgia. Cottrell is a leading manufacturer of over-the-road car hauler equipment and related car hauler parts. In June and August 2014, ParkLand Ventures also completed the acquisition of several manufactured housing communities. Total consideration for these non-insurance acquisitions was $187.0 million, which primarily consisted of cash consideration. Total consideration includes the estimated fair value of contingent consideration we expect to pay based on Cottrell's earnings, as defined in the stock purchase agreement, in 2014 and 2015. The Company recognized goodwill of $38.7 million related to these acquisitions, the majority of which we expect to amortize for income tax purposes. The Company also recognized other intangible assets of $78.7 million, including $53.7 million of customer relationships and $13.0 million of trade names, which are expected to be amortized over a weighted average period of 17 years and 10 years, respectively. Results attributable to these acquisitions are included with the Company's non-insurance operations, which are not included in a reportable segment.

Investments

Investments	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Investments	Investments
	a)The following tables summarize the Company's available-for-sale investments.
		December 31, 2014
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized Other-			Estimated
		Cost			Unrealized			Unrealized			Than-Temporary			Fair
					Holding			Holding			Impairment Losses			Value
					Gains			Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	662,462		$	12,963		$	(2,163	)	$	—		$	673,262
	Obligations of states, municipalities and political subdivisions	4,075,748			245,158			(3,359		)	—			4,317,547
	Foreign governments	1,458,255			154,707			(1,041		)	—			1,611,921
	Commercial mortgage-backed securities	427,904			5,325			(2,602		)	—			430,627
	Residential mortgage-backed securities	954,263			34,324			(3,482		)	(2,258		)	982,847
	Asset-backed securities	100,073			99			(682		)	—			99,490
	Corporate bonds	2,250,432			69,016			(10,441		)	(1,819		)	2,307,188
	Total fixed maturities	9,929,137			521,592			(23,770		)	(4,077		)	10,422,882
	Equity securities:
	Insurance, banks and other financial institutions	523,739			789,717			(1,531		)	—			1,311,925
	Industrial, consumer and all other	1,427,919			1,403,566			(5,834		)	—			2,825,651
	Total equity securities	1,951,658			2,193,283			(7,365		)	—			4,137,576
	Short-term investments	1,594,819			36			(6		)	—			1,594,849
	Investments, available-for-sale	$	13,475,614		$	2,714,911		$	(31,141	)	$	(4,077	)	$	16,155,307
		December 31, 2013
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized Other-			Estimated
		Cost			Unrealized			Unrealized			Than-Temporary			Fair
					Holding			Holding			Impairment Losses			Value
					Gains			Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	1,215,522		$	9,051		$	(30,342	)	$	—		$	1,194,231
	Obligations of states, municipalities and political subdivisions	2,986,758			116,341			(27,384		)	—			3,075,715
	Foreign governments	1,484,818			30,647			(54,411		)	—			1,461,054
	Commercial mortgage-backed securities	379,555			62			(11,796		)	—			367,821
	Residential mortgage-backed securities	875,902			13,046			(16,442		)	(2,258		)	870,248
	Asset-backed securities	189,646			257			(1,614		)	—			188,289
	Corporate bonds	2,996,940			54,777			(61,650		)	(4,889		)	2,985,178
	Total fixed maturities	10,129,141			224,181			(203,639		)	(7,147		)	10,142,536
	Equity securities:
	Insurance, banks and other financial institutions	422,975			592,112			(4		)	—			1,015,083
	Industrial, consumer and all other	1,143,578			1,094,251			(1,114		)	—			2,236,715
	Total equity securities	1,566,553			1,686,363			(1,118		)	—			3,251,798
	Short-term investments	1,452,270			18			—			—			1,452,288
	Investments, available-for-sale	$	13,147,964		$	1,910,562		$	(204,757	)	$	(7,147	)	$	14,846,622
	b)The following tables summarize gross unrealized investment losses by the length of time that securities have continuously been in an unrealized loss position.
		December 31, 2014
		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	$	108,250		$	(62	)	$	163,359		$	(2,101	)	$	271,609	$	(2,163	)
	Obligations of states, municipalities and political subdivisions	58,583			(542		)	92,441			(2,817		)	151,024		(3,359		)
	Foreign governments	18,856			(386		)	56,217			(655		)	75,073		(1,041		)
	Commercial mortgage-backed securities	45,931			(210		)	147,558			(2,392		)	193,489		(2,602		)
	Residential mortgage-backed securities	9,613			(2,285		)	207,374			(3,455		)	216,987		(5,740		)
	Asset-backed securities	30,448			(20		)	45,160			(662		)	75,608		(682		)
	Corporate bonds	141,176			(2,263		)	621,821			(9,997		)	762,997		(12,260		)
	Total fixed maturities	412,857			(5,768		)	1,333,930			(22,079		)	1,746,787		(27,847		)
	Equity securities:
	Insurance, banks and other financial institutions	16,219			(1,531		)	—			—			16,219		(1,531		)
	Industrial, consumer and all other	86,062			(5,834		)	—			—			86,062		(5,834		)
Total equity securities	102,281			(7,365		)	—			—			102,281		(7,365		)
Short-term investments	181,964			(6		)	—			—			181,964		(6		)
Total	$	697,102		$	(13,139	)	$	1,333,930		$	(22,079	)	$	2,031,032	$	(35,218	)
At December 31, 2014, the Company held 552 securities with a total estimated fair value of $2.0 billion and gross unrealized losses of $35.2 million. Of these 552 securities, 396 securities had been in a continuous unrealized loss position for one year or longer and had a total estimated fair value of $1.3 billion and gross unrealized losses of $22.1 million. All 396 securities were fixed maturities, of which 391 represent securities acquired in the Alterra transaction, for which a new amortized cost was established at fair value as of the Acquisition Date. The Company does not intend to sell or believe it will be required to sell these fixed maturities before recovery of their amortized cost.
	December 31, 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	$	587,929		$	(30,342	)	$	—		$	—		$	587,929	$	(30,342	)
Obligations of states, municipalities and political subdivisions	513,608			(27,238		)	3,512			(146		)	517,120		(27,384		)
Foreign governments	950,040			(54,411		)	—			—			950,040		(54,411		)
Commercial mortgage-backed securities	357,737			(11,796		)	—			—			357,737		(11,796		)
Residential mortgage-backed securities	437,675			(18,700		)	—			—			437,675		(18,700		)
Asset-backed securities	142,011			(1,614		)	—			—			142,011		(1,614		)
Corporate bonds	1,817,737			(66,539		)	—			—			1,817,737		(66,539		)
Total fixed maturities	4,806,737			(210,640		)	3,512			(146		)	4,810,249		(210,786		)
Equity securities:
Insurance, banks and other financial institutions	144			(4		)	—			—			144		(4		)
Industrial, consumer and all other	20,943			(714		)	27,735			(400		)	48,678		(1,114		)
Total equity securities	21,087			(718		)	27,735			(400		)	48,822		(1,118		)
Total	$	4,827,824		$	(211,358	)	$	31,247		$	(546	)	$	4,859,071	$	(211,904	)
At December 31, 2013, the Company held 1,364 securities with a total estimated fair value of $4.9 billion and gross unrealized losses of $211.9 million. Of these 1,364 securities, nine securities had been in a continuous unrealized loss position for one year or longer and had a total estimated fair value of $31.2 million and gross unrealized losses of $0.5 million. Of these securities, eight securities were fixed maturities and one was an equity security.
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.
c)The amortized cost and estimated fair value of fixed maturities at December 31, 2014 are shown below by contractual maturity.
(dollars in thousands)	Amortized			Estimated
	Cost			Fair Value
Due in one year or less	$	747,997		$	752,715
Due after one year through five years	2,007,798			2,059,192
Due after five years through ten years	2,026,628			2,142,582
Due after ten years	3,664,474			3,955,429
	8,446,897			8,909,918
Commercial mortgage-backed securities	427,904			430,627
Residential mortgage-backed securities	954,263			982,847
Asset-backed securities	100,073			99,490
Total fixed maturities	$	9,929,137		$	10,422,882
Expected maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, and the lenders may have the right to put the securities back to the borrower. Based on expected maturities, the estimated average duration of fixed maturities was 5.8 years.
d)The following table presents the components of net investment income.
	Years Ended December 31,
(dollars in thousands)	2014			2013			2012
Interest:
Municipal bonds (tax-exempt)	$	98,262		$	82,308		$	90,316
Municipal bonds (taxable)	49,345			28,041			22,663
Other taxable bonds	152,789			134,377			107,270
Short-term investments, including overnight deposits	5,959			3,573			2,755
Dividends on equity securities	65,031			48,641			50,416
Change in fair value of credit default swap	2,230			10,460			16,641
Income from equity method investments	4,766			21,898			1,961
Other	108			355			(41		)
	378,490			329,653			291,981
Investment expenses	(15,260		)	(12,280		)	(9,874		)
Net investment income	$	363,230		$	317,373		$	282,107
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 $12.7 million at December 31, 2014 and 2013 and $21.4 million at December 31, 2012.
f)The following table presents net realized investment gains and the change in net unrealized gains on investments.
	Years Ended December 31,
(dollars in thousands)	2014			2013			2012
Realized gains:
Sales of fixed maturities	$	8,417		$	13,772		$	18,337
Sales of equity securities	51,356			73,592			29,578
Other	15,205			5,940			749
Total realized gains	74,978			93,304			48,664
Realized losses:
Sales of fixed maturities	(18,136		)	(25,168		)	(563		)
Sales of equity securities	(802		)	(278		)	(342		)
Other-than-temporary impairments	(4,784		)	(4,706		)	(12,078		)
Other	(5,256		)	—			(4,088		)
Total realized losses	(28,978		)	(30,152		)	(17,071		)
Net realized investment gains	$	46,000		$	63,152		$	31,593
Change in net unrealized gains on investments:
Fixed maturities	$	480,350		$	(403,610	)	$	51,783
Equity securities	500,673			665,599			302,013
Short-term investments	12			6			12
Net increase	$	981,035		$	261,995		$	353,808
Other-than-temporary impairment losses recognized in net income and included in net realized investment gains included losses attributable to equity securities totaling $4.5 million, $2.8 million and $12.1 million for the years ended December 31, 2014, 2013 and 2012, respectively. Other-than-temporary impairment losses recognized in net income and included in net realized investment gains included losses attributable to fixed maturities totaling $0.3 million and $1.9 million for the years ended December 31, 2014 and 2013, respectively.
g)The following table presents the components of restricted assets.
	December 31,
(dollars in thousands)	2014			2013
Restricted assets held in trust or on deposit to support underwriting activities	$	4,961,061		$	5,161,871
Investments and cash and cash equivalents pledged as security for letters of credit	635,340			695,072
Investments and cash pledged as collateral for credit default swap	—			33,430
Cash held in escrow for Abbey acquisition	—			199,006
Total	$	5,596,401		$	6,089,379
Total restricted assets are included on the Company's consolidated balance sheets as follows.
	December 31,
(dollars in thousands)	2014			2013
Investments, available-for-sale	$	5,040,413		$	5,225,701
Restricted cash and cash equivalents	522,225			786,926
Other assets	33,763			76,752
Total	$	5,596,401		$	6,089,379
h)At December 31, 2014 and December 31, 2013, investments in U.S. Treasury securities and obligations of U.S. government agencies were the only investments in any one issuer that exceeded 10% of shareholders' equity.
At December 31, 2014, the Company's ten largest equity holdings represented $1.9 billion, or 45%, of the equity portfolio. Investments in the property and casualty insurance industry represented $756.3 million, or 18%, of the equity portfolio at December 31, 2014. Investments in the property and casualty insurance industry included a $470.7 million investment in the common stock of Berkshire Hathaway Inc.

Receivables

Receivables	12 Months Ended
	Dec. 31, 2014
Receivables [Abstract]
Receivables	Receivables
	The following table presents the components of receivables.
		December 31,
	(dollars in thousands)	2014			2013
	Amounts receivable from agents, brokers and insureds	$	1,031,519		$	1,058,021
	Trade accounts receivable	97,225			85,203
	Employee stock loans receivable (see note 12(c))	15,044			12,822
	Other	8,601			5,420
		1,152,389			1,161,466
	Allowance for doubtful receivables	(17,172		)	(19,693		)
	Receivables	$	1,135,217		$	1,141,773

Deferred_Policy_Acquisition_Co

Deferred Policy Acquisition Costs	12 Months Ended
	Dec. 31, 2014
Deferred Policy Acquisition Costs Disclosures [Abstract]
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs
	The following table presents the amounts of policy acquisition costs deferred and amortized.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Balance, beginning of year	$	260,967		$	157,465		$	194,674
	Policy acquisition costs deferred	754,303			577,620			390,360
	Amortization of policy acquisition costs	(654,916		)	(471,915		)	(428,109		)
	Foreign currency movements	(6,944		)	(2,203		)	540
	Deferred policy acquisition costs	$	353,410		$	260,967		$	157,465
	The following table presents the components of underwriting, acquisition and insurance expenses.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Transaction costs and other acquisition-related expenses (1)	$	—		$	75,140		$	—
	Prospective adoption of FASB ASU No. 2010-26 (2)	—			—			43,093
	Other amortization of policy acquisition costs	654,916			471,915			385,016
	Other operating expenses	805,966			765,257			501,363
	Underwriting, acquisition and insurance expenses	$	1,460,882		$	1,312,312		$	929,472
	(1)	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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)	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.

Property_And_Equipment

Property And Equipment	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property And Equipment	Property and Equipment
	The following table presents the components of property and equipment, which are included in other assets on the consolidated balance sheets.
		December 31,
	(dollars in thousands)	2014			2013
	Land	$	56,848		$	48,036
	Buildings	78,786			59,307
	Leasehold improvements	98,098			68,363
	Land improvements	70,596			57,673
	Furniture and equipment	255,566			217,528
	Other	116,884			104,690
		676,778			555,597
	Accumulated depreciation and amortization	(255,388		)	(207,688		)
	Property and equipment	$	421,390		$	347,909
	Depreciation and amortization expense of property and equipment was $51.2 million, $51.5 million and $36.0 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	The Company does not own any material properties. The Company leases substantially all of the facilities used by its insurance operations and certain furniture and equipment under operating leases. The Company leases offices for the U.S. Insurance segment in Glen Allen, Virginia and in 25 other locations; the Company leases offices for the International Insurance segment in London, England, Hamilton, Bermuda and 35 other locations; and the Company leases offices for the Reinsurance segment primarily in Summit, New Jersey and Hamilton, Bermuda. The Company's Markel Ventures operations own certain of their office, clinic, manufacturing, warehouse and distribution facilities and lease others. The Company believes these facilities are suitable and adequate for the Company’s insurance and non-insurance operations.

Goodwill_And_Intangible_Assets

Goodwill And Intangible Assets	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill And Intangible Assets	Goodwill and Intangible Assets
	The following table presents the components of goodwill. As described in note 19, effective January 1, 2014, the Company redefined its segments. As a result, goodwill was reallocated as of December 31, 2012 using a relative fair value allocation approach. The following table presents the components of goodwill by reportable segment.
	(dollars in thousands)	U.S. Insurance		International Insurance			Reinsurance		Other(1)			Total
	January 1, 2013	$	172,824	$	309,662		$	—	$	192,444		$	674,930
	Acquisitions (see note 2)	107,755		65,190			122,745		1,185			296,875
	Foreign currency movements and other adjustments	—		(2,088		)	—		(2,000		)	(4,088		)
	December 31, 2013	$	280,579	$	372,764		$	122,745	$	191,629		$	967,717
	Acquisitions (see note 2)	—		42,989			—		61,539			104,528
	Impairment loss	—		—			—		(13,737		)	(13,737		)
	Foreign currency movements and other adjustments	—		(7,570		)	—		(1,823		)	(9,393		)
	December 31, 2014 (2)	$	280,579	$	408,183		$	122,745	$	237,608		$	1,049,115
	(1)	Amounts included in Other above are related to the Company's non-insurance operations, which are not included in a reportable segment.
	(2)	Goodwill is net of accumulated impairment losses of $13.7 million included in Other.
	Goodwill and indefinite-lived intangible assets are tested for impairment at least annually. The Company completes an annual test during the fourth quarter of each year based upon the results of operations through September 30. As part of our annual impairment test, during the fourth quarter of 2014, the Company recorded a non-cash goodwill impairment charge of $13.7 million to other expenses, to reduce the carrying value of the Diamond Healthcare reporting unit's goodwill to its implied fair value. Diamond Healthcare's operations consist of the planning, development and operation of behavioral health services in partnership with healthcare organizations, and are reported in our non-insurance operations. The Company determined the goodwill for the reporting unit was impaired as a result of lower than expected earnings and lower estimated future earnings. To determine the value of the impairment loss, the Company estimated the fair value of the reporting unit primarily using an income approach based on a discounted cash flow model.
	There were no impairment losses recognized during 2013 or 2012.
	The following table presents the components of intangible assets with a net carrying amount.
		December 31,
		2014					2013
	(dollars in thousands)	Gross		Accumulated			Gross		Accumulated
		Carrying		Amortization			Carrying		Amortization
		Amount					Amount
	Customer relationships	$	452,157	$	(69,483	)	$	298,886	$	(43,286	)
	Broker relationships	175,681		(34,827		)	178,693		(23,255		)
	Trade names	94,795		(17,673		)	65,880		(12,666		)
	Technology	62,288		(22,671		)	56,429		(16,222		)
	Insurance licenses	39,985		—			40,185		(133		)
	Lloyd's syndicate capacity	12,000		—			12,000		—
	Other	18,903		(8,408		)	14,197		(5,625		)
	Total	$	855,809	$	(153,062	)	$	666,270	$	(101,187	)
	Amortization of intangible assets was $57.6 million, $55.2 million and $33.5 million for the years ended December 31, 2014, 2013 and 2012, respectively. Amortization of intangible assets is estimated to be $58.3 million for 2015, $55.2 million for 2016, $52.8 million for 2017, $52.5 million for 2018 and $46.7 million for 2019. Indefinite-lived intangible assets were $58.0 million at December 31, 2014 and December 31, 2013.
	In 2014, the Company acquired $204.4 million of intangible assets. The definite-lived intangible assets acquired are expected to be amortized over a weighted average period of 16.8 years. The definite-lived intangible assets acquired during 2014 include customer relationships, trade names and technology, which are expected to be amortized over a weighted average period of 18.6, 12.5 and 7.0 years, respectively.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	Income before income taxes includes the following components.
		Years Ended December 31,
	(dollars in thousands)	2014				2013				2012
	Domestic operations	$	240,279			$	325,133			$	126,466
	Foreign operations	200,099				36,610				185,584
	Income before income taxes	$	440,378			$	361,743			$	312,050
	Income tax expense includes the following components.
		Years Ended December 31,
	(dollars in thousands)	2014				2013				2012
	Current:
	Domestic	$	7,573			$	50,683			$	14,340
	Foreign	24,574				23,165				1,814
	Total current tax expense	32,147				73,848				16,154
	Deferred:
	Domestic	43,673				23,906				(3,734		)
	Foreign	40,870				(19,856		)		41,382
	Total deferred tax expense	84,543				4,050				37,648
	Income tax expense	$	116,690			$	77,898			$	53,802
	Foreign income tax expense (benefit) includes United States tax expense (benefit) on foreign operations.
	State income tax expense is not material to the consolidated financial statements.
	The Company made income tax payments of $89.5 million, $35.7 million and $30.0 million in 2014, 2013 and 2012, respectively. Current income taxes payable were $37.6 million and $84.2 million at December 31, 2014 and 2013, respectively, and were included in other liabilities on the consolidated balance sheets.
	Reconciliations of the United States corporate income tax rate to the effective tax rate on income before income taxes are presented in the following table.
		Years Ended December 31,
		2014			2013			2012
	United States corporate tax rate	35	%		35	%		35	%
	Tax-exempt investment income	(9	)		(9	)		(12	)
	Tax credits	(1	)		—			—
	Foreign operations	—			(4	)		(5	)
	Other	1			—			(1	)
	Effective tax rate	26	%		22	%		17	%
	The following table presents the components of domestic and foreign deferred tax assets and liabilities.
		December 31,
	(dollars in thousands)	2014				2013
	Assets:
	Unpaid losses and loss adjustment expenses	$	239,588			$	283,365
	Life and annuity benefits	143,102				161,209
	Unearned premiums recognized for income tax purposes	108,960				76,862
	Net operating loss carryforwards	36,359				43,010
	Tax credit carryforwards	32,525				33,773
	Other-than-temporary impairments not yet deductible for income tax purposes	28,106				34,978
	Differences between financial reporting and tax bases	132,878				139,357
	Total gross deferred tax assets	721,518				772,554
	Less valuation allowance	(4,801		)		—
	Total gross deferred tax assets, net of allowance	716,717				772,554
	Liabilities:
	Net unrealized gains on investments	759,212				459,015
	Amortization of goodwill and other intangible assets	106,927				97,580
	Deferred policy acquisition costs	101,766				65,543
	Differences between financial reporting and tax bases	59,359				46,699
	Total gross deferred tax liabilities	1,027,264				668,837
	Net deferred tax asset (liability)	$	(310,547	)		$	103,717
	The net deferred tax liability at December 31, 2014 was included in other liabilities on the consolidated balance sheet. The net deferred tax asset at December 31, 2013 was included in other assets on the consolidated balance sheet.
	At December 31, 2014, the Company had tax credit carryforwards of $32.5 million. The earliest any of these credits will expire is 2019.
	At December 31, 2014, the Company had net operating losses of $61.9 million that can be used to offset future income that is taxable in the United States from Markel Capital Limited, a wholly owned United Kingdom subsidiary. The Company's ability to use these losses in the United States expires between the years 2021 and 2032. At December 31, 2014, the Company had net operating losses of $61.2 million that can be used to offset future income that is taxable in the United States. The Company's ability to use these losses in the United States expires between the years 2027 and 2033.
	The Company believes that it is more likely than not that it will realize $716.7 million of gross deferred tax assets, including net operating losses, recorded at December 31, 2014, through generating taxable income or the reversal of existing temporary differences attributable to the gross deferred tax liabilities. The Company recorded a valuation allowance to offset the deferred tax asset on losses incurred in our Brazilian subsidiary.
	At December 31, 2014, the Company had unrecognized tax benefits of $17.7 million. If recognized, $15.6 million of these tax benefits would decrease the annual effective tax rate. The Company does not currently anticipate any significant changes in unrecognized tax benefits during 2015.
	The following table presents a reconciliation of unrecognized tax benefits.
		Years Ended December 31,
	(dollars in thousands)	2014				2013
	Unrecognized tax benefits, beginning of year	$	18,219			$	18,870
	Increases for tax positions taken in prior years	3				—
	Lapse of statute of limitations	(522		)		(651		)
	Unrecognized tax benefits, end of year	$	17,700			$	18,219
At December 31, 2014, earnings of the Company's foreign subsidiaries, with the exception of certain of our Bermuda subsidiaries, are considered reinvested indefinitely and no provision for deferred United States income taxes has been recorded. It is not practicable to determine the amount of unrecorded deferred tax liabilities associated with such earnings due to the complexity of this calculation.
The Company is subject to income tax in the United States and in foreign jurisdictions. With few exceptions, the Company is no longer subject to income tax examination by tax authorities for years ended before January 1, 2011.

Unpaid_Losses_And_Loss_Adjustm

Unpaid Losses And Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Unpaid Losses And Loss Adjustment Expenses	Unpaid Losses and Loss Adjustment Expenses
	a)The following table presents a reconciliation of consolidated beginning and ending reserves for losses and loss adjustment expenses.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Net reserves for losses and loss adjustment expenses, beginning of year	$	8,407,642		$	4,592,652		$	4,607,767
	Foreign currency movements, commutations and other	(137,385		)	(780		)	30,359
	Adjusted net reserves for losses and loss adjustment expenses, beginning of year	8,270,257			4,591,872			4,638,126
	Incurred losses and loss adjustment expenses:
	Current year	2,638,012			2,227,402			1,553,070
	Prior years	(435,545		)	(411,129		)	(399,002		)
	Total incurred losses and loss adjustment expenses	2,202,467			1,816,273			1,154,068
	Payments:
	Current year	502,107			670,928			268,745
	Prior years	1,436,851			906,302			931,955
	Total payments	1,938,958			1,577,230			1,200,700
	Effect of foreign currency rate changes	(19,476		)	(7,915		)	1,158
	Net reserves for losses and loss adjustment expenses of acquired insurance companies	21,193			3,584,642			—
	Net reserves for losses and loss adjustment expenses, end of year	8,535,483			8,407,642			4,592,652
	Reinsurance recoverable on unpaid losses	1,868,669			1,854,414			778,774
	Gross reserves for losses and loss adjustment expenses, end of year	$	10,404,152		$	10,262,056		$	5,371,426
	Beginning of year net reserves for losses and loss adjustment expenses are adjusted, when applicable, for the impact of changes in foreign currency rates, commutations and other items. In 2014, beginning of year net reserves for losses and loss adjustment expenses were decreased by a movement of $127.7 million in foreign currency rates of exchange. In 2013, beginning of year net reserves for losses and loss adjustment expenses were increased by a movement of $0.7 million in foreign currency rates of exchange, which was more than offset by commutations. In 2012, beginning of year net reserves for losses and loss adjustment expenses were increased by a movement of $23.4 million in foreign currency rates of exchange.
	In 2014, the Company recorded net reserves for losses and loss adjustment expenses of $21.2 million as a result of the acquisition of Abbey. These reserves were recorded at fair value as part of the Company's purchase accounting.
	In 2014, incurred losses and loss adjustment expenses included $435.5 million of favorable development on prior years' loss reserves, which was due in part to $250.4 million of loss reserve redundancies on our long-tail casualty and professional liability lines within the U.S. Insurance segment and on our professional liability and marine and energy product lines within the International Insurance segment, as actual claims reporting patterns on prior accident years have been more favorable than the Company's actuarial analyses initially anticipated. The favorable development on prior years' loss reserves in 2014 was partially offset by $32.8 million of adverse development in prior years' loss reserves on asbestos and environmental (A&E) exposures.
	Once a year, generally during the third quarter, the Company completes an in-depth, actuarial review of its A&E exposures. Over the past few years, the number of A&E claims reported each year across the property and casualty industry has been on the decline. However, at the same time, the likelihood of making an indemnity payment has risen, thus increasing the average cost per reported claim. During the 2012 annual review, the Company reduced its estimate of the ultimate claims count, while increasing its estimate of the number of claims that would ultimately be closed with an indemnity payment. During the annual review for both 2014 and 2013, the Company increased its expectation of the severity of the outcome of certain claims subject to litigation. As the ultimate outcome of known claims increases, the Company's expected ultimate closure value on unreported claims also increases. As a result, prior years' loss reserves for A&E exposures were increased by $27.2 million in 2014, $28.4 million in 2013 and $31.1 million in 2012.
	In 2013, the Company recorded net reserves for losses and loss adjustment expenses of $3.6 billion as a result of the acquisition of Alterra. These reserves were recorded at fair value as part of the Company's purchase accounting. See note 2 for a discussion of the Company's acquisition of Alterra.
	In 2013, incurred losses and loss adjustment expenses included $411.1 million of favorable development on prior years' loss reserves, which was due in part to $255.2 million of loss reserve redundancies on our long-tail casualty and professional liability product lines within the U.S. Insurance segment and on our professional liability and marine and energy product lines within the International Insurance segment, as actual claims reporting patterns on prior accident years have been more favorable than the Company's actuarial analyses initially anticipated. The favorable development on prior years' loss reserves in 2013 was partially offset by $30.1 million of adverse development in prior years' loss reserves on A&E exposures.
	Current year incurred losses and loss adjustment expenses for 2012 included $99.6 million of estimated net losses related to Hurricane Sandy. The estimated net losses on Hurricane Sandy were net of estimated reinsurance recoverables of $77.6 million.
	In 2012, incurred losses and loss adjustment expenses included $399.0 million of favorable development on prior years' loss reserves, which was due in part to $292.3 million of loss reserve redundancies on our long-tail casualty and professional liability lines within our U.S. Insurance segment and on our professional liability and marine and energy product lines within the International Insurance segment, as actual claims reporting patterns on prior accident years have been more favorable than the Company's actuarial analyses initially anticipated. The favorable development on prior years' loss reserves in 2012 was partially offset by $38.2 million of adverse development in prior years' loss reserves on A&E exposures.
	Inherent in the Company's reserving practices is the desire to establish loss reserves that are more likely redundant than deficient. As such, the Company seeks to establish loss reserves that will ultimately prove to be adequate. As part of the Company's acquisition of insurance operations, to the extent the reserving philosophy of the acquired business differs from the Company's reserving philosophy, the post-acquisition loss reserves will be built until total loss reserves are consistent with the Company's target level of confidence. Furthermore, the Company's philosophy is to price its insurance products to make an underwriting profit. Management continually attempts to improve its loss estimation process by refining its ability to analyze loss development patterns, claim payments and other information, but uncertainty remains regarding the potential for adverse development of estimated ultimate liabilities.
	The Company uses a variety of techniques to establish the liabilities for unpaid losses and loss adjustment expenses, all of which involve significant judgments and assumptions. These techniques include detailed statistical analysis of past claim reporting, settlement activity, claim frequency and severity, policyholder loss experience, industry loss experience and changes in market conditions, policy forms and exposures. Greater judgment may be required when new product lines are introduced or when there have been changes in claims handling practices, as the statistical data available may be insufficient. The Company's estimates reflect implicit and explicit assumptions regarding the potential effects of external factors, including economic and social inflation, judicial decisions, changes in law, general economic conditions and recent trends in these factors. In some of the Company's markets, and where the Company acts as a reinsurer, the timing and amount of information reported about underlying claims are in the control of third parties. There is often a time lag between cedents establishing case reserves and re-estimating their reserves, and notifying the Company of the new or revised case reserves. This can also affect estimates and require re-estimation as new information becomes available.
	The Company believes the process of evaluating past experience, adjusted for the effects of current developments and anticipated trends, is an appropriate basis for predicting future events. Management currently believes the Company's gross and net reserves, including the reserves for A&E exposures, are adequate. However, there is no precise method for evaluating the impact of any significant factor on the adequacy of reserves, and actual results will differ from original estimates.
	b)The Company's exposure to A&E claims results from policies written by acquired insurance operations before their acquisitions by the Company. The Company's exposure to A&E claims originated from umbrella, excess and commercial general liability (CGL) insurance policies and assumed reinsurance contracts that were written on an occurrence basis from the 1970s to mid-1980s. Exposure also originated from claims-made policies that were designed to cover environmental risks provided that all other terms and conditions of the policy were met.
	A&E claims include property damage and clean-up costs related to pollution, as well as personal injury allegedly arising from exposure to hazardous materials. After 1986, the Company began underwriting CGL coverage with pollution exclusions, and in some lines of business the Company began using a claims-made form. These changes significantly reduced the Company's exposure to future A&E claims on post-1986 business.
	The following table provides a reconciliation of beginning and ending A&E reserves for losses and loss adjustment expenses, which are a component of consolidated unpaid losses and loss adjustment expenses. Amounts included in the following table are presented before consideration of reinsurance allowances.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Net reserves for A&E losses and loss adjustment expenses, beginning of year	$	272,194		$	260,791		$	244,772
	Commutations and other	115			(5,067		)	(897		)
	Adjusted net reserves for A&E losses and loss adjustment expenses, beginning of year	272,309			255,724			243,875
	Incurred losses and loss adjustment expenses	32,840			30,128			38,179
	Payments	(17,426		)	(13,658		)	(21,263		)
	Net reserves for A&E losses and loss adjustment expenses, end of year	287,723			272,194			260,791
	Reinsurance recoverable on unpaid losses	102,719			100,784			100,063
	Gross reserves for A&E losses and loss adjustment expenses, end of year	$	390,442		$	372,978		$	360,854
	At December 31, 2014, asbestos-related reserves were $290.1 million and $218.9 million on a gross and net basis, respectively. Net reserves for reported claims and net incurred but not reported reserves for A&E exposures were $173.8 million and $113.9 million, respectively, at December 31, 2014. Inception-to-date net paid losses and loss adjustment expenses for A&E related exposures totaled $415.7 million at December 31, 2014, which includes $85.7 million of litigation-related expense.
	The Company's reserves for losses and loss adjustment expenses related to A&E exposures represent management's best estimate of ultimate settlement values. A&E reserves are monitored by management, and the Company's statistical analysis of these reserves is reviewed by the Company's independent actuaries. A&E exposures are subject to significant uncertainty due to potential loss severity and frequency resulting from the uncertain and unfavorable legal climate. A&E reserves could be subject to increases in the future; however, management believes the Company's gross and net A&E reserves at December 31, 2014 are adequate.

Life_And_Annuity_Benefits

Life And Annuity Benefits	12 Months Ended
	Dec. 31, 2014
Liability for Future Policy Benefits [Abstract]
Life And Annuity Benefits	Life and Annuity Benefits
	The following table presents life and annuity benefits.
		December 31,
	(dollars in thousands)	2014		2013
	Life	$	182,604	$	190,765
	Annuities	1,031,946		1,194,558
	Accident and health	91,268		101,251
	Total	$	1,305,818	$	1,486,574
	Life and annuity benefits 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 cedent experience, industry mortality tables, and 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 an unlocking event occurs. To the extent existing policy reserves, together with the present value of future gross premiums and expected investment income earned thereon, are not adequate to cover the present value of future benefits, settlement and maintenance costs, the locked-in assumptions are revised to current best estimate assumptions and a charge to earnings for life and annuity benefits is recognized at that time.
	Because of the assumptions and estimates used in establishing the Company's reserves for life and annuity benefit obligations and the long-term nature of these reinsurance contracts, the ultimate liability may be greater or less than the estimates. The average discount rate for the life and annuity benefit reserves was 2.3% as of December 31, 2014.
	As of December 31, 2014, the largest life and annuity benefits reserve for a single contract was 33.5% of the total.
	No annuities included in life and annuity benefits in the consolidated balance sheet are subject to discretionary withdrawal.

Senior_LongTerm_Debt_And_Other

Senior Long-Term Debt And Other Debt	12 Months Ended
	Dec. 31, 2014
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.
		December 31,
	(dollars in thousands)	2014		2013
	7.20% unsecured senior notes, due April 14, 2017, interest payable semi-annually, net of unamortized premium of $3,526 in 2014 and $4,822 in 2013	$	94,155	$	95,451
	7.125% unsecured senior notes, due September 30, 2019, interest payable semi-annually, net of unamortized discount of $1,343 in 2014 and $1,626 in 2013	348,657		348,374
	6.25% unsecured senior notes, due September 30, 2020, interest payable semi-annually, net of unamortized premium of $53,172 in 2014 and $61,273 in 2013	403,172		411,273
	5.35% unsecured senior notes, due June 1, 2021, interest payable semi-annually, net of unamortized discount of $1,325 in 2014 and $1,531 in 2013	248,675		248,469
	4.90% unsecured senior notes, due July 1, 2022, interest payable semi-annually, net of unamortized discount of $2,095 in 2014 and $2,374 in 2013	347,905		347,626
	3.625% unsecured senior notes, due March 30, 2023, interest payable semi-annually, net of unamortized discount of $1,659 in 2014 and $1,860 in 2013	248,341		248,140
	7.35% unsecured senior notes, due August 15, 2034, interest payable semi-annually, net of unamortized discount of $2,078 in 2014 and $2,185 in 2013	197,922		197,815
	5.0% unsecured senior notes, due March 30, 2043, interest payable semi-annually, net of unamortized discount of $6,327 in 2014 and $6,551 in 2013	243,673		243,449
	Other debt, at various interest rates ranging from 1.6% to 6.5%	121,094		115,630
	Senior long-term debt and other debt	$	2,253,594	$	2,256,227
	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.
	On June 30, 2014, Markel Corporation entered into agreements guaranteeing the 7.20% unsecured senior notes and 6.25% unsecured senior notes. These guarantee agreements were issued pursuant to supplemental indentures entered into by the Company on June 30, 2014 and are in addition to the existing guarantees provided by Alterra.
	Effective August 1, 2014, both Alterra Finance and Alterra USA provided guarantees for the Company's revolving credit facility. As a result, the Company's revolving credit facility ranks equally with the 6.25% unsecured senior notes and the 7.20% 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 Markel Ventures operations and is non-recourse to the holding company. The debt of the Company's Markel Ventures subsidiaries generally is secured by the assets of those subsidiaries. ParkLand Ventures, Inc. (ParkLand), 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 estimated fair value based on quoted market prices of the Company's senior long-term debt and other debt was $2.5 billion and $2.4 billion at December 31, 2014 and 2013, respectively.
	The following table summarizes the future principal payments due at maturity on senior long-term debt and other debt as of December 31, 2014.
	Years Ending December 31,	(dollars in
		thousands)
	2015	$	23,340
	2016	21,850
	2017	112,942
	2018	2,348
	2019	352,199
	2020 and thereafter	1,699,044
	Total principal payments	$	2,211,723
	Net unamortized premium	41,871
	Senior long-term debt and other debt	$	2,253,594
	On August 1, 2014, the Company entered into a credit agreement for a revolving credit facility, which provides $300 million of capacity for future acquisitions, investments, repurchases of capital stock of the Company and for general corporate purposes. At the Company's discretion, $200 million of the total capacity may be used for secured letters of credit. The Company may increase the capacity of the facility to $500 million subject to certain terms and conditions. The Company pays interest on balances outstanding under the facility and a utilization fee for letters of credit issued under the facility. The Company also pays a commitment fee (0.225% at December 31, 2014) on the unused portion of the facility based on the Company's debt to equity leverage ratio as calculated under the credit agreement. Markel Corporation, along with Alterra Finance and Alterra USA, guaranteed the Company's obligations under the facility. At December 31, 2014, the Company had no borrowings outstanding under this revolving credit facility. This facility replaced the Company's previous $300 million revolving credit facility and expires in August 2019.
	Alterra and Markel Bermuda are party to a secured credit facility (the senior credit facility), which expires on December 15, 2015. On August 1, 2014, the Company reduced the capacity of the senior credit facility from $900 million to $650 million. 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. The Company pays a fee of 0.40% for outstanding letters of credit under the senior credit facility and a commitment fee of 0.125% on the total unused portion of the senior credit facility. At December 31, 2014 and 2013, the Company had no borrowings outstanding under the senior credit facility. At December 31, 2014, $374.5 million of letters of credit were issued and outstanding under the senior credit facility.
	At December 31, 2014, 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.
	The Company paid $125.8 million, $114.5 million and $92.9 million in interest on its senior long-term debt and other debt during the years ended December 31, 2014, 2013 and 2012, respectively.

Shareholders_Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Shareholders' Equity	Shareholders' Equity
	a)The Company had 50,000,000 shares of no par value common stock authorized of which 13,961,675 shares and 13,985,620 shares were issued and outstanding at December 31, 2014 and 2013, respectively. The Company also has 10,000,000 shares of no par value preferred stock authorized, none of which was issued or outstanding at December 31, 2014 or 2013.
	The Company's Board of Directors has approved the repurchase of up to $300 million of common stock under a share repurchase program (the Program). Under the Program, the Company may repurchase outstanding shares of common stock from time to time, primarily through open-market transactions. The Program has no expiration date but may be terminated by the Board of Directors at any time. As of December 31, 2014, the Company had repurchased 34,385 shares of common stock at a cost of $20.5 million under the Program.
	b)Net income per share was determined by dividing adjusted net income to shareholders by the applicable weighted average shares outstanding. 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. Average closing common stock market prices are used to calculate the dilutive effect attributable to restricted stock.
		Years Ended December 31,
	(in thousands, except per share amounts)	2014				2013			2012
	Net income to shareholders	$	321,182			$	281,021		$	253,385
	Adjustment of redeemable noncontrolling interests	(8,186		)		1,963			(3,101		)
	Adjusted net income to shareholders	$	312,996			$	282,984		$	250,284
	Basic common shares outstanding	13,984				12,538			9,640
	Dilutive potential common shares from conversion of options	11				12			6
	Dilutive potential common shares from conversion of restricted stock	62				36			20
	Diluted shares outstanding	14,057				12,586			9,666
	Basic net income per share	$	22.38			$	22.57		$	25.96
	Diluted net income per share	$	22.27			$	22.48		$	25.89
	c)The Company's Employee Stock Purchase and Bonus Plan provides a method for employees and directors to purchase shares of the Company's common stock on the open market. The plan encourages share ownership by providing for the award of bonus shares to participants equal to 10% of the net increase in the number of shares owned under the plan in a given year, excluding shares acquired through the plan's loan program component. Under the loan program, the Company offers subsidized unsecured loans so participants may purchase shares and awards bonus shares equal to 5% of the shares purchased with a loan. The Company has authorized 100,000 shares for purchase under this plan, of which 20,740 and 33,454 shares were available for purchase at December 31, 2014 and 2013, respectively. At December 31, 2014 and 2013, loans outstanding under the plan, which are included in receivables on the consolidated balance sheets, totaled $15.0 million and $12.8 million, respectively.
	d)In April 2012, the Company adopted the 2012 Equity Incentive Compensation Plan (2012 Compensation Plan), which replaced the Markel Corporation Omnibus Incentive Plan (Omnibus Incentive Plan). The 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 and is administered by the Compensation Committee of the Company's Board of Directors (Compensation Committee). At December 31, 2014, there were 145,253 shares reserved for issuance under the 2012 Compensation Plan.
	Restricted stock units are awarded to certain associates and executive officers based upon meeting performance conditions determined by the Compensation Committee. These awards generally vest at the end of the third year following the year for which the Compensation Committee determines performance conditions have been met. At the end of the vesting period, recipients are entitled to receive one share of the Company's common stock for each vested restricted stock unit. During 2014, the Company awarded 17,917 restricted stock units to associates and executive officers based on performance conditions being met.
	Restricted stock units also are awarded to associates and executive officers to assist the Company in securing or retaining the services of key employees. During 2014, the Company awarded 8,950 restricted stock units to associates and executive officers as a hiring or retention incentive. The restricted stock units had a grant-date fair value of $5.4 million. These awards generally 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.
	During 2014, the Company awarded 1,413 shares of restricted stock to its non-employee directors. The shares awarded to non-employee directors will vest in 2015.
	The following table summarizes nonvested share-based awards.
		Number			Weighted Average
		of Awards			Grant-Date
					Fair Value
	Nonvested awards at January 1, 2014	98,711			$	449.93
	Granted	28,280			583.74
	Vested	(8,479	)		491.51
	Forfeited	(380	)		411
	Nonvested awards at December 31, 2014	118,132			$	479.11
	The fair value of the Company's share-based awards issued under the Omnibus Incentive Plan was determined based on the average price of the Company's common shares on the grant date. The fair value of the Company's share-based awards granted under the 2012 Compensation Plan is determined based on the closing price of the Company's common shares on the grant date. The weighted average grant-date fair value of the Company's share-based awards granted in 2014, 2013 and 2012 was $583.74, $517.24 and $412.04, respectively. As of December 31, 2014, unrecognized compensation cost related to nonvested share-based awards issued under the Omnibus Incentive Plan and 2012 Compensation plan was $22.8 million, which is expected to be recognized over a weighted average period of 2.2 years. The fair value of the Company's share-based awards that vested during 2014, 2013 and 2012 was $4.2 million, $2.5 million and $4.7 million, respectively.
	e)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			Weighted			Weighted Average		Intrinsic Value
		of			Average			Remaining		(in millions)
		Shares			Exercise Price			Contractual Term
								(years)
	Outstanding and exercisable, January 1, 2014	35,941			$	412.39
	Exercised	13,636			$	413.06
	Outstanding and exercisable, December 31, 2014	22,305			$	411.98		1.7		$	6.1
	During 2014, 13,636 options were exercised under the Alterra Equity Award Plans, resulting in cash proceeds of $5.6 million and a current tax benefit of $0.8 million. The intrinsic value of options exercised in 2014 was $2.5 million. From the Acquisition Date through December 31, 2013, 65,934 options were exercised under the Alterra Equity Award Plans, resulting in cash proceeds of $24.3 million and a current tax benefit of $0.8 million. The intrinsic value of options exercised in 2013 was $9.4 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 is being 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 at January 1, 2014	68,411			$	529.59
	Vested	(34,119	)		529.59
	Forfeited	(377	)		529.59
	Nonvested awards at December 31, 2014	33,915			$	529.59
	The Company recognized compensation expense totaling $3.9 million and $12.7 million for the years ended December 31, 2014 and 2013, respectively, related to these restricted stock awards. As of December 31, 2014, unrecognized compensation cost related to the nonvested restricted stock awards was $0.6 million, which is expected to be recognized over a weighted average period of less than one year. The fair value of the restricted stock awards that vested during 2014 and 2013 was $18.1 million and $41.3 million, respectively.

Other_Comprehensive_Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2014
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.
	(dollars in thousands)	Unrealized			Foreign			Net Actuarial			Total
		Holding Gains			Currency			Pension Loss
		on Available-for-
		Sale Securities
	31-Dec-11	$	704,719		$	(2,614	)	$	(41,185	)	$	660,920
	Other comprehensive income before reclassifications	266,265			1,539			4,670			272,474
	Amounts reclassified from accumulated other comprehensive income	(24,051		)	—			1,994			(22,057		)
	Total other comprehensive income	242,214			1,539			6,664			250,417
	31-Dec-12	$	946,933		$	(1,075	)	$	(34,521	)	$	911,337
	Other comprehensive income (loss) before reclassifications	225,404			(10,171		)	2,517			217,750
	Amounts reclassified from accumulated other comprehensive income	(40,830		)	—			1,548			(39,282		)
	Total other comprehensive income (loss)	184,574			(10,171		)	4,065			178,468
	31-Dec-13	$	1,131,507		$	(11,246	)	$	(30,456	)	$	1,089,805
	Other comprehensive income (loss) before reclassifications	687,908			(32,245		)	(16,516		)	639,147
	Amounts reclassified from accumulated other comprehensive income	(26,161		)	—			1,766			(24,395		)
	Total other comprehensive income (loss)	661,747			(32,245		)	(14,750		)	614,752
	31-Dec-14	$	1,793,254		$	(43,491	)	$	(45,206	)	$	1,704,557
	The following table summarizes the tax expense (benefit) associated with each component of other comprehensive income.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Change in net unrealized gains on investments:
	Net holding gains arising during the period	$	328,564		$	93,837		$	122,524
	Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	614			(34		)	(49		)
	Reclassification adjustments for net gains included in net income	(9,890		)	(16,382		)	(10,881		)
	Change in net unrealized gains on investments	319,288			77,421			111,594
	Change in foreign currency translation adjustments	1,918			(1,619		)	(446		)
	Change in net actuarial pension loss	(3,687		)	1,015			1,991
	Total	$	317,519		$	76,817		$	113,139
	The following table presents the details of amounts reclassified from accumulated other comprehensive income into income, by component.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Unrealized holding gains on available-for-sale securities:
	Other-than-temporary impairment losses	$	(4,784	)	$	(4,706	)	$	(12,078	)
	Net realized investment gains, excluding other-than-temporary impairment losses	40,835			61,918			47,010
	Total before taxes	36,051			57,212			34,932
	Income taxes	(9,890		)	(16,382		)	(10,881		)
	Reclassification of unrealized holding gains, net of taxes	$	26,161		$	40,830		$	24,051
	Net actuarial pension loss:
	Underwriting, acquisition and insurance expenses	$	(2,084	)	$	(1,934	)	$	(2,590	)
	Income taxes	318			386			596
	Reclassification of net actuarial pension loss, net of taxes	$	(1,766	)	$	(1,548	)	$	(1,994	)

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
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, commercial mortgage-backed securities, residential mortgage-backed securities, asset-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 commercial mortgage-backed securities, residential mortgage-backed securities and asset-backed securities include the type of underlying assets, 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, prior to its scheduled termination in December 2014, included a credit default swap. The fair value of the credit default swap was measured by the Company using an external valuation model with any changes in fair value recorded in net investment income. Due to the significance of unobservable inputs required in measuring the fair value of the credit default swap, the credit default swap was classified as Level 3 within the fair value hierarchy. As of December 31, 2013, the fair value of the credit default swap was included in other liabilities on the consolidated balance sheet. Net investment income in 2014, 2013 and 2012 included favorable changes in the fair value of the credit default swap of $2.2 million, $10.5 million and $16.6 million, respectively. The Company had no other material derivative instruments at December 31, 2014 or 2013.
	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.
		December 31, 2014
	(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	$	—	$	673,262	$	—	$	673,262
	Obligations of states, municipalities and political subdivisions	—		4,317,547		—		4,317,547
	Foreign governments	—		1,611,921		—		1,611,921
	Commercial mortgage-backed securities	—		430,627		—		430,627
	Residential mortgage-backed securities	—		982,847		—		982,847
	Asset-backed securities	—		99,490		—		99,490
	Corporate bonds	—		2,307,188		—		2,307,188
	Total fixed maturities	—		10,422,882		—		10,422,882
	Equity securities:
	Insurance, banks and other financial institutions	1,311,925		—		—		1,311,925
	Industrial, consumer and all other	2,825,651		—		—		2,825,651
	Total equity securities	4,137,576		—		—		4,137,576
	Short-term investments	1,469,975		124,874		—		1,594,849
	Total investments available-for-sale	$	5,607,551	$	10,547,756	$	—	$	16,155,307
		December 31, 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	$	—	$	1,194,231	$	—	$	1,194,231
	Obligations of states, municipalities and political subdivisions	—		3,075,715		—		3,075,715
	Foreign governments	—		1,461,054		—		1,461,054
	Commercial mortgage-backed securities	—		367,821		—		367,821
	Residential mortgage-backed securities	—		870,248		—		870,248
	Asset-backed securities	—		188,289		—		188,289
	Corporate bonds	—		2,985,178		—		2,985,178
	Total fixed maturities	—		10,142,536		—		10,142,536
	Equity securities:
	Insurance, banks and other financial institutions	1,015,083		—		—		1,015,083
	Industrial, consumer and all other	2,236,715		—		—		2,236,715
	Total equity securities	3,251,798		—		—		3,251,798
	Short-term investments	1,312,561		139,727		—		1,452,288
	Total investments available-for-sale	$	4,564,359	$	10,282,263	$	—	$	14,846,622
	Liabilities:
	Derivative contracts	$	—	$	—	$	2,230	$	2,230
	There were no transfers into or out of Level 1 and Level 2 during 2014 or 2013.
	Except as disclosed in note 2, the Company did not have other any assets or liabilities measured at fair value on a non-recurring basis during the years ended December 31, 2014 and 2013.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance
	The Company uses reinsurance and retrocessional 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. Historically, the Company's products were written with limits that did not require significant reinsurance. Following the acquisition of Alterra, the Company has certain insurance and reinsurance products that use higher levels of reinsurance. In a reinsurance transaction, an insurance company transfers, or cedes, all or part of its exposure in return for a portion of the premium. In a retrocessional reinsurance transaction, a reinsurance 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 or retrocessional agreement.
	A credit risk exists with ceded reinsurance to the extent that any reinsurer is unable to meet the obligations assumed under the reinsurance or retrocessional contracts. 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 December 31, 2014 and 2013, balances recoverable from the Company's ten largest reinsurers, by group, represented approximately 63% and 62%, respectively, of the reinsurance recoverable on paid and unpaid losses, before considering reinsurance allowances. At December 31, 2014, the Company's largest reinsurance balance was due from the Fairfax Financial Group and represented 10% of the reinsurance recoverable on paid and unpaid losses, before considering reinsurance allowances.
	To further reduce credit exposure to reinsurance recoverable balances, the Company has received collateral, including letters of credit and trust accounts, from certain reinsurers. Collateral related to these reinsurance agreements is available, without restriction, when the Company pays losses covered by the reinsurance agreements.
	The following table summarizes the Company's reinsurance allowance for doubtful accounts.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Reinsurance allowance, beginning of year	$	76,210		$	71,148		$	69,067
	Additions	10,316			13,621			24,179
	Deductions	(26,713		)	(8,559		)	(22,098		)
	Reinsurance allowance, end of year	$	59,813		$	76,210		$	71,148
	Management believes the Company's reinsurance allowance for doubtful accounts is adequate at December 31, 2014; however, the deterioration in the credit quality of existing reinsurers or disputes over reinsurance and retrocessional contracts could result in additional charges.
	The following table summarizes the effect of reinsurance and retrocessional reinsurance on premiums written and earned.
		Years Ended December 31,
		2014						2013						2012
	(dollars in thousands)	Written			Earned			Written			Earned			Written			Earned
	Direct	$	3,478,273		$	3,443,912		$	3,143,957		$	2,947,812		$	2,115,353		$	2,057,735
	Assumed	1,327,240			1,298,371			776,269			1,016,853			398,328			376,186
	Ceded	(888,498		)	(901,371		)	(683,543		)	(733,049		)	(299,555		)	(286,793		)
	Net premiums	$	3,917,015		$	3,840,912		$	3,236,683		$	3,231,616		$	2,214,126		$	2,147,128
	Incurred losses and loss adjustment expenses were net of reinsurance recoverables (ceded incurred losses and loss adjustment expenses) of $423.1 million, $269.4 million and $165.8 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	The percentage of ceded earned premiums to gross earned premiums was 19%, 18% and 12% for the years ended December 31, 2014, 2013 and 2012, respectively. The percentage of assumed earned premiums to net earned premiums was 34%, 31% and 18% for the years ended December 31, 2014, 2013 and 2012, respectively.

Commitments_And_Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments And Contingencies	Commitments and Contingencies
	a)The Company leases substantially all of its facilities and certain furniture and equipment under noncancelable operating leases with remaining terms up to 20 years.
	The following table summarizes the Company's minimum annual rental commitments, excluding taxes, insurance and other operating costs payable directly by the Company, for noncancelable operating leases at December 31, 2014.
	Years Ending December 31,	(dollars in
		thousands)
	2015	$	28,434
	2016	23,636
	2017	28,124
	2018	26,549
	2019	25,169
	2020 and thereafter	151,000
	Total	$	282,912
	Rental expense was $42.7 million, $35.3 million and $28.1 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	b)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.

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions
	The Company engages in certain related party transactions in the normal course of business. These transactions are at arm's length and are not material to the Company's consolidated financial statements.

Statutory_Financial_Informatio

Statutory Financial Information	12 Months Ended
	Dec. 31, 2014
Statutory Financial Information [Abstract]
Statutory Financial Information	Statutory Financial Information
	a) Statutory capital and surplus and statutory net income (loss) for the Company's wholly-owned insurance subsidiaries as of December 31, 2014 and 2013 and for the years ended December 31, 2014, 2013 and 2012, respectively, is summarized below.
	Statutory Capital and Surplus
	(dollars in thousands)	2014		2013
	United States	$	2,619,001	$	2,396,530
	United Kingdom	$	604,564	$	510,426
	Bermuda	$	1,890,920	$	1,503,004
	Other	$	177,738	$	172,447
	As of December 31, 2014, the amount of statutory capital and surplus necessary to satisfy regulatory requirements is not significant in relation to actual statutory capital and surplus.
	Statutory Net Income (Loss)
		Years Ended December 31,
	(dollars in thousands)	2014		2013			2012
	United States	$	212,909	$	235,009		$	127,179
	United Kingdom	$	70,632	$	109,983		$	82,573
	Bermuda	$	162,809	$	249,772		N/A
	Other	$	1,271	$	(12,617	)	N/A
	United States
	The laws of the domicile states of the Company's U.S. insurance subsidiaries govern the amount of dividends that may be paid to the Company. Generally, statutes in the domicile states of the Company's U.S. insurance subsidiaries require prior approval for payment of extraordinary, as opposed to ordinary, dividends. At December 31, 2014, the Company's U.S. insurance subsidiaries could pay up to $310.6 million during the following 12 months under the ordinary dividend regulations.
	In converting from U.S. statutory accounting principles to U.S. GAAP, typical adjustments include deferral of policy acquisition costs, differences in the calculation of deferred income taxes and the inclusion of net unrealized gains or losses relating to fixed maturities in shareholders' equity. The Company does not use any permitted statutory accounting practices that are different from prescribed statutory accounting practices which impact statutory capital and surplus.
	United Kingdom
	The Company's United Kingdom insurance subsidiary and its Lloyd's managing agent are authorized by the Prudential Regulation Authority (PRA) and regulated by both the PRA and the Financial Conduct Authority (FCA). The PRA oversees compliance with established periodic auditing and reporting requirements, minimum solvency margins and individual capital assessment requirements and dividend restrictions, while both the PRA and the FCA oversee compliance with risk assessment reviews and various other requirements. Markel International Insurance Company Limited (MIICL) is required to give 14 days advance notice to the PRA for any dividends from MIICL. Markel Syndicate Management Limited, the managing agent of the Company's syndicate at Lloyd's, is required to satisfy the solvency requirements of Lloyd's. In addition, the Company's United Kingdom subsidiaries must comply with the United Kingdom Companies Act of 2006, which provides that dividends may only be paid out of profits available for that purpose. As of December 31, 2014, earnings of the Company's United Kingdom subsidiaries are considered reinvested indefinitely for U.S. income tax purposes and will not be made available for distributions to the holding company.
	Bermuda
	Markel Bermuda is subject to enhanced capital requirements in addition to minimum solvency and liquidity requirements. The enhanced capital requirement is determined by reference to a risk-based capital model that determines a control threshold for statutory capital and surplus by taking into account the risk characteristics of different aspects of the insurer's business. At December 31, 2014, Markel Bermuda satisfied both the enhanced capital requirements and the minimum solvency and liquidity requirements.
	Under the Bermuda Insurance Act, Markel Bermuda is prohibited from paying or declaring dividends during a fiscal year if it is in breach of its enhanced capital requirement, solvency margin or minimum liquidity ratio or if the declaration or payment of the dividend would cause a breach. If an insurer fails to meet its solvency margin or minimum liquidity ratio on the last day of any financial year, it is prohibited from declaring or paying any dividends during the next financial year without the approval of the Bermuda Monetary Authority (BMA). Further, Markel Bermuda is prohibited from declaring or paying, in any financial year, dividends of more than 25% of its total statutory capital and surplus as set forth in its previous year's statutory balance sheet unless at least seven days before payment of those dividends it files with the BMA an affidavit stating that it will continue to meet its solvency margin and minimum liquidity ratio. Markel Bermuda must obtain the BMA's prior approval for a reduction by 15% or more of the total statutory capital as set forth in its previous year's financial statements. In addition, as a long-term insurer, Markel Bermuda may not declare or pay a dividend to any person other than a policyholder unless the value of the assets in its long-term business fund, as certified by Markel Bermuda's approved actuary, exceeds the liabilities of its long-term business by the amount of the dividend and at least the prescribed minimum solvency margin. As of December 31, 2014, Markel Bermuda could pay up to $472.7 million during the following 12 months without making any additional filings with the BMA.
	Other Jurisdictions
	The Company's other foreign subsidiaries are subject to capital and solvency requirements in their respective jurisdictions of domicile that govern their ability to declare and pay dividends. As of December 31, 2014, earnings of our foreign subsidiaries are considered reinvested indefinitely for U.S. income tax purposes and will not be made available for distributions to the holding company.
	b)    Lloyd's sets the corporate members' required capital annually based on each syndicates' business plans, rating environment, reserving environment and input arising from Lloyd's discussions with, inter alia, regulatory and rating agencies. Such required capital is referred to as Funds at Lloyd's (FAL), and comprises cash and investments. The amount of cash and investments held as FAL as of December 31, 2014 was $784.6 million. The amount which the Company provides as FAL is not available for distribution to the holding company. The Company's corporate members may also be required to maintain funds under the control of Lloyd's in excess of their capital requirements and such funds also may not be available for distribution to the holding company.

Segment_Reporting_Disclosures

Segment Reporting Disclosures	12 Months Ended
	Dec. 31, 2014
Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract]
Segment Reporting Disclosures	Segment Reporting Disclosures
	In conjunction with the continued integration of Alterra into the Company's insurance operations, during the first quarter of 2014, the Company changed the way it aggregates and monitors its ongoing underwriting results. Effective January 1, 2014, the Company monitors and reports its ongoing underwriting operations in the following three segments: U.S. Insurance, International Insurance and Reinsurance. In determining how to aggregate and monitor its underwriting results, the Company considers many factors, including the geographic location and regulatory environment of the insurance entity underwriting the risk, the nature of the insurance product sold, the type of account written and the type of customer served. The U.S. Insurance segment includes all direct business and facultative placements written by the Company's insurance subsidiaries domiciled in the United States. The International Insurance segment includes all direct business and facultative placements written by the Company's insurance subsidiaries domiciled outside of the United States, including the Company's syndicate at Lloyd's. The Reinsurance segment includes all treaty reinsurance written across the Company. Results for lines of business discontinued prior to, or in conjunction with, acquisitions, including the results attributable to the run-off of life and annuity reinsurance business previously written by Alterra, are reported in the Other Insurance (Discontinued Lines) segment. All investing activities related to the Company's insurance operations are included in the Investing segment.
	The Company's non-insurance operations include its Markel Ventures operations, which primarily consist of controlling interests in various industrial and service businesses. The Company's non-insurance operations also include the results of the Company's legal and professional consulting services, which were acquired through the acquisition of Abbey in January 2014. For purposes of segment reporting, the Company's non-insurance operations are not considered to be a reportable segment.
	The following table summarizes the Company's gross written premiums by country. Gross written premiums are attributed to individual countries based upon location of risk.
		Years Ended December 31,
	(dollars in thousands)	2014			% of			2013				% of			2012				% of
					Total							Total							Total
	United States	$	3,523,239		73	%		$	2,934,868			75	%		$	1,768,011			70	%
	United Kingdom	441,669			9			245,143				6			147,891				6
	Canada	125,617			3			128,420				3			120,542				5
	Other countries	714,988			15			611,795				16			477,237				19
	Total	$	4,805,513		100	%		$	3,920,226			100	%		$	2,513,681			100	%
	Most of the Company's gross written premiums are placed through insurance and reinsurance brokers. During the years ended December 31, 2014 and 2013, the top three independent brokers accounted for approximately 28% and 24% of consolidated gross premiums written. During the years ended December 31, 2014 and 2013, the top three independent brokers accounted for approximately 68%, of gross premiums written in the Reinsurance segment and 41% and 42%, respectively, of gross premiums written in the International Insurance 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 each of the Company's underwriting segments is 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 underwriting 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 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 U.S. Insurance, International Insurance, Reinsurance or Other Insurance (Discontinued Lines) segments since the Company does not manage its assets by underwriting segment. The Company does not allocate capital expenditures for long-lived assets to any of its underwriting segments for management reporting purposes.
	All segment disclosures for the prior periods have been revised to be consistent with the new segment structure.
	a)	The following tables summarize the Company's segment disclosures.
		Year Ended December 31, 2014
	(dollars in thousands)	U.S. Insurance			International Insurance				Reinsurance				Other				Investing				Consolidated
													Insurance
													(Discontinued
													Lines)
	Gross premium volume	$	2,493,823		$	1,200,403			$	1,112,728			$	(1,441	)		$	—			$	4,805,513
	Net written premiums	2,071,466			889,336				956,584				(371		)		—				3,917,015
	Earned premiums	2,022,860			909,679				908,385				(12		)		—				3,840,912
	Losses and loss adjustment expenses:
	Current accident year	(1,340,129		)	(660,409		)		(637,474		)		—				—				(2,638,012		)
	Prior accident years	216,557			166,615				79,951				(27,578		)		—				435,545
	Amortization of policy acquisition costs	(403,233		)	(141,394		)		(110,289		)		—				—				(654,916		)
	Other operating expenses	(396,737		)	(207,175		)		(201,673		)		(381		)		—				(805,966		)
	Underwriting profit (loss)	99,318			67,316				38,900				(27,971		)		—				177,563
	Net investment income	—			—				—				—				363,230				363,230
	Net realized investment gains	—			—				—				—				46,000				46,000
	Other revenues (insurance)	2,478			21,827				2,696				1,631				—				28,632
	Other expenses (insurance)	(5,149		)	(18,706		)		(1,847		)		(37,132		)		—				(62,834		)
	Segment profit (loss)	$	96,647		$	70,437			$	39,749			$	(63,472	)		$	409,230			$	552,591
	Other revenues (non-insurance)																				854,893
	Other expenses (non-insurance)																				(792,037		)
	Amortization of intangible assets																				(57,627		)
	Interest expense																				(117,442		)
	Income before income taxes																				$	440,378
	U.S. GAAP combined ratio (1)	95		%	93		%		96		%		NM			(2)					95		%
	(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.
	(2)	NM — Ratio is not meaningful.
		Year Ended December 31, 2013
	(dollars in thousands)	U.S. Insurance			International Insurance				Reinsurance				Other				Investing				Consolidated
													Insurance
													(Discontinued
													Lines)
	Gross premium volume	$	2,252,739		$	1,101,099			$	566,348			$	40			$	—			$	3,920,226
	Net written premiums	1,915,770			840,050				480,822				41				—				3,236,683
	Earned premiums	1,727,766			833,984				669,826				40				—				3,231,616
	Losses and loss adjustment expenses:
	Current accident year	(1,173,258		)	(588,759		)		(465,385		)		—				—				(2,227,402		)
	Prior accident years	298,113			130,660				12,938				(30,582		)		—				411,129
	Transaction costs and other acquisition-related expenses (1)	(12,724		)	(13,366		)		(49,050		)		—				—				(75,140		)
	Amortization of policy acquisition costs	(287,795		)	(138,626		)		(45,494		)		—				—				(471,915		)
	Other operating expenses	(409,886		)	(171,666		)		(183,817		)		112				—				(765,257		)
	Underwriting profit (loss)	142,216			52,227				(60,982		)		(30,430		)		—				103,031
	Net investment income	—			—				—				—				317,373				317,373
	Net realized investment gains	—			—				—				—				63,152				63,152
	Other revenues (insurance)	13,648			4,284				5,432				1,130				—				24,494
	Other expenses (insurance)	(17,087		)	(5,065		)		—				(28,126		)		—				(50,278		)
	Segment profit (loss)	$	138,777		$	51,446			$	(55,550	)		$	(57,426	)		$	380,525			$	457,772
Other revenues (non-insurance)																				686,448
Other expenses (non-insurance)																				(613,250		)
Amortization of intangible assets																				(55,223		)
Interest expense																				(114,004		)
Income before income taxes																				$	361,743
U.S. GAAP combined ratio (2)	92		%	94		%		109		%		NM			(3)					97		%
(1)	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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.
	Year Ended December 31, 2012
(dollars in thousands)	U.S. Insurance			International Insurance				Reinsurance				Other				Investing				Consolidated
												Insurance
												(Discontinued
												Lines)
Gross premium volume	$	1,595,849		$	788,092			$	129,744			$	(4	)		$	—			$	2,513,681
Net written premiums	1,418,579			677,344				118,208				(5		)		—				2,214,126
Earned premiums	1,360,229			672,405				114,499				(5		)		—				2,147,128
Losses and loss adjustment expenses:
Current accident year	(984,018		)	(492,177		)		(76,875		)		—				—				(1,553,070		)
Prior accident years	225,870			172,758				21,449				(21,075		)		—				399,002
Prospective adoption of FASB ASU No. 2010-26 (1)	(31,033		)	(10,597		)		(1,463		)		—				—				(43,093		)
Other amortization of policy acquisition costs	(243,466		)	(130,637		)		(10,913		)		—				—				(385,016		)
Other operating expenses	(327,682		)	(153,545		)		(19,933		)		(203		)		—				(501,363		)
Underwriting profit (loss)	(100		)	58,207				26,764				(21,283		)		—				63,588
Net investment income	—			—				—				—				282,107				282,107
Net realized investment gains	—			—				—				—				31,593				31,593
Other revenues (insurance)	44,968			4,964				—				—				—				49,932
Other expenses (insurance)	(41,425		)	(3,867		)		—				—				—				(45,292		)
Segment profit (loss)	$	3,443		$	59,304			$	26,764			$	(21,283	)		$	313,700			$	381,928
Other revenues (non-insurance)																				489,352
Other expenses (non-insurance)																				(432,956		)
Amortization of intangible assets																				(33,512		)
Interest expense																				(92,762		)
Income before income taxes																				$	312,050
U.S. GAAP combined ratio(2)	100		%	91		%		77		%		NM			(3)					97		%
(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.
(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 summarizes deferred policy acquisition costs, unearned premiums and unpaid losses and loss adjustment expenses by segment.
(dollars in thousands)	Deferred Policy			Unearned				Unpaid Losses and
	Acquisition Costs			Premiums				Loss Adjustment Expenses
December 31, 2014
U.S. Insurance	$	165,333		$	1,110,910			$	3,577,166
International Insurance	47,618			491,708				3,353,417
Reinsurance	140,459			643,072				2,818,792
Other Insurance (Discontinued Lines)	—			—				654,777
Total	$	353,410		$	2,245,690			$	10,404,152
December 31, 2013
U.S. Insurance	$	151,015		$	1,058,529			$	3,572,667
International Insurance	50,425			516,689				3,399,228
Reinsurance	59,527			551,897				2,811,362
Other Insurance (Discontinued Lines)	—			—				478,799
Total	$	260,967		$	2,127,115			$	10,262,056
c)The following table summarizes segment earned premiums by major product grouping.
	Years Ended December 31,
(dollars in thousands)	2014			2013				2012
U.S. Insurance:
General liability	$	491,645		$	431,798			$	354,003
Professional liability	321,005			268,203				216,234
Property	266,019			190,530				141,568
Personal lines	299,442			185,935				86,217
Programs	244,216			205,004				135,037
Workers compensation	263,164			250,790				241,208
Other	137,369			195,506				185,962
Total U.S. Insurance	2,022,860			1,727,766				1,360,229
International Insurance:
General liability	146,178			128,171				102,875
Professional liability	285,300			252,816				174,590
Property	76,691			91,497				48,483
Marine and energy	287,263			287,745				288,605
Other	114,247			73,755				57,852
Total International Insurance	909,679			833,984				672,405
Reinsurance:
Property	270,461			227,394				71,172
Casualty	323,390			244,981				476
Auto	152,645			84,042				21,163
Other	161,889			113,409				21,688
Total Reinsurance	908,385			669,826				114,499
Other Insurance (Discontinued Lines)	(12		)	40				(5		)
Total earned premiums	$	3,840,912		$	3,231,616			$	2,147,128
The Company does not manage products at this level of aggregation. The Company offers a diverse portfolio of products and manages these products in logical groupings within each operating segment.
d)The following table reconciles segment assets to the Company's consolidated balance sheets.
	December 31,
(dollars in thousands)	2014			2013				2012
Segment assets:
Investing	$	18,531,150		$	17,550,332			$	9,277,697
Underwriting	5,422,445			5,468,731				2,387,305
Total segment assets	23,953,595			23,019,063				11,665,002
Non-insurance operations	1,246,762			936,448				891,586
Total assets	$	25,200,357		$	23,955,511			$	12,556,588

Other_Revenues_And_Other_Expen

Other Revenues And Other Expenses	12 Months Ended
	Dec. 31, 2014
Other Income and Expenses [Abstract]
Other Revenues And Other Expenses	Other Revenues and Other Expenses
	The following table summarizes the components of other revenues and other expenses.
		Years Ended December 31,
		2014				2013				2012
	(dollars in thousands)	Other		Other		Other		Other		Other		Other
		Revenues		Expenses		Revenues		Expenses		Revenues		Expenses
	Insurance:
	Managing general agent operations	$	23,324	$	22,527	$	17,399	$	20,382	$	48,056	$	43,069
	Life and annuity	1,631		37,132		1,130		28,126		—		—
	Other	3,677		3,175		5,965		1,770		1,876		2,223
		28,632		62,834		24,494		50,278		49,932		45,292
	Non-Insurance:
	Markel Ventures: Manufacturing	575,353		513,668		495,138		437,712		366,886		328,484
	Markel Ventures: Non-Manufacturing	262,767		261,551		191,310		175,538		122,466		104,472
	Other	16,773		16,818		—		—		—		—
		854,893		792,037		686,448		613,250		489,352		432,956
	Total	$	883,525	$	854,871	$	710,942	$	663,528	$	539,284	$	478,248
	The Company's Markel Ventures operations primarily consist of controlling interests in various industrial and service businesses and are viewed by management as separate and distinct from the Company's insurance operations. While each of the companies is operated independently from one another, management aggregates financial results into two industry groups: manufacturing and non-manufacturing.

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Employee Benefit Plans	Employee Benefit Plans
	a)The Company maintains defined contribution plans for employees of its United States insurance operations in accordance with Section 401(k) of the IRC. Employees of the Company's Markel Ventures subsidiaries are provided post-retirement benefits under separate plans. The Company also provides various defined contribution plans for employees of its international insurance operations, which are in line with local market terms and conditions of employment. Expenses relating to the Company's defined contribution plans, including the defined contribution plans of Alterra effective May 1, 2013, were $27.2 million, $24.3 million and $19.1 million in 2014, 2013 and 2012, respectively.
	b)The Terra Nova Pension Plan is a defined benefit plan which covers certain employees in our international insurance operations who meet the eligibility conditions set out in the plan. The plan has been closed to new participants since 2001. The cost of providing pensions for employees is charged to earnings over the average working life of employees according to actuarial recommendations. Final benefits are based on the employee's years of credited service and the higher of pensionable compensation received in the calendar year preceding retirement or the best average pensionable compensation received in any three consecutive years in the ten years preceding retirement. Effective April 1, 2012, employees are no longer accruing benefits for future service in the Terra Nova Pension Plan. The Company uses December 31 as the measurement date for the Terra Nova Pension Plan.
	The following table summarizes the funded status of the Terra Nova Pension Plan and the amounts recognized on the accompanying consolidated balance sheets of the Company.
		Years Ended December 31,
	(dollars in thousands)	2014			2013
	Change in projected benefit obligation:
	Projected benefit obligation at beginning of period	$	163,010		$	151,327
	Interest cost	7,572			6,533
	Plan amendments	495			—
	Benefits paid	(4,424		)	(3,542		)
	Actuarial loss	29,609			5,459
	Effect of foreign currency rate changes	(10,706		)	3,233
	Projected benefit obligation at end of year	$	185,556		$	163,010
	Change in plan assets:
	Fair value of plan assets at beginning of period	$	189,437		$	164,090
	Actual gain on plan assets	22,395			19,430
	Employer contributions	5,610			5,338
	Benefits paid	(4,424		)	(3,542		)
	Effect of foreign currency rate changes	(11,619		)	4,121
	Fair value of plan assets at end of year	$	201,399		$	189,437
	Funded status of the plan	$	15,843		$	26,427
	Net actuarial pension loss	61,378			42,941
	Total	$	77,221		$	69,368
	Net actuarial pension loss is recognized as a component of accumulated other comprehensive income, net of a tax benefit of $16.2 million and $12.4 million in 2014 and 2013, respectively. The asset for pension benefits, also referred to as the funded status of the plan, at December 31, 2014 and December 31, 2013 was included in other assets on the consolidated balance sheets.
	The following table presents the changes in plan assets and projected benefit obligation recognized in accumulated other comprehensive income.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Net actuarial gain (loss)	$	(20,521	)	$	3,146		$	6,065
	Amortization of:
	Net actuarial loss	1,589			1,934			2,590
	Prior service costs	495			—			—
	Tax benefit (expense)	3,687			(1,015		)	(1,991		)
	Total other comprehensive income (loss)	$	(14,750	)	$	4,065		$	6,664
	The following table summarizes the components of net periodic benefit income and the weighted average assumptions for the Terra Nova Pension Plan.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Components of net periodic benefit income:
	Service cost	$	—		$	—		$	361
	Interest cost	7,572			6,533			6,815
	Expected return on plan assets	(12,812		)	(10,825		)	(9,788		)
	Amortization of prior service cost	495			—			—
	Amortization of net actuarial pension loss	1,589			1,934			2,590
	Net periodic benefit income	$	(3,156	)	$	(2,358	)	$	(22	)
	Weighted average assumptions as of December 31:
	Discount rate	3.8		%	4.7		%	4.5		%
	Expected return on plan assets	6		%	6.6		%	6.6		%
	Rate of compensation increase	2.9		%	3.2		%	3.1		%
	The projected benefit obligation and the net periodic benefit income are determined by independent actuaries using assumptions provided by the Company. In determining the discount rate, the Company uses the current yield on high-quality, fixed-income investments that have maturities corresponding to the anticipated timing of estimated defined benefit payments. The expected return on plan assets is estimated based upon the anticipated average yield on plan assets and reflects expected changes in the allocation of plan assets. Asset returns reflect management's belief that 4.5% is a reasonable rate of return to anticipate for fixed maturities given current market conditions and future expectations. In addition, the expected return on plan assets includes an assumption that equity securities will outperform fixed maturities by approximately 3.5% over the long term. The rate of compensation increase is based upon historical experience and management's expectation of future compensation.
	Management's discount rate and rate of compensation increase assumptions at December 31, 2014 were used to calculate the Company's projected benefit obligation. Management's discount rate, expected return on plan assets and rate of compensation increase assumptions at December 31, 2013 were used to calculate the net periodic benefit income for 2014. The Company estimates that net periodic benefit income in 2015 will include an expense of $2.4 million resulting from the amortization of the net actuarial pension loss included as a component of accumulated other comprehensive income at December 31, 2014.
	The fair values of each of the plan's assets are measured using quoted prices in active markets for identical assets, which represent Level 1 inputs within the fair value hierarchy established in FASB ASC 820-10. The following table summarizes the fair value of plan assets as of December 31, 2014 and 2013.
		December 31,
	(dollars in thousands)	2014			2013
	Plan assets:
	Fixed maturity index funds	$	114,243		$	70,997
	Equity security index funds	87,148			118,431
	Cash and cash equivalents	8			9
	Total	$	201,399		$	189,437
	During 2014, the Company revised the target asset allocation and adjusted the investment balances to reduce risk while maintaining long-term return objectives. The Company's target asset allocation for the plan is 46% equity securities and 54% fixed maturities. At December 31, 2014, the actual allocation of assets in the plan was 43% equity securities and 57% fixed maturities. At December 31, 2013, the actual allocation of plan assets was 63% equity securities and 37% fixed maturities.
	Investments are managed by a third party investment manager. Equity securities are invested in an index fund where 30% is indexed to United Kingdom equities and 70% is indexed to other markets. Assets are also invested in a mutual fund with a diversified global portfolio of equities, government bonds and investment grade debt. The primary objective of investing in these funds is to earn rates of return that are consistently in excess of inflation. Investing in equity securities, historically, has provided rates of return that are higher than investments in fixed maturities. Fixed maturity investments are allocated between five mutual funds; two index funds that include United Kingdom government securities, one index fund that includes securities issued by other foreign governments, one mutual fund that includes investment grade corporate bonds from the United Kingdom and foreign markets and one index fund that includes United Kingdom corporate securities. The assets in these funds are invested to meet the Company's obligations for current pensioners and those individuals nearing retirement. The plan does not invest in the Company's common shares.
	At December 31, 2014 and 2013, the fair value of plan assets exceeded the plan's accumulated benefit obligation of $166.9 million and $146.0 million, respectively. The Company does not expect to have any required contributions or make any voluntary plan contributions in 2015.
	The benefits expected to be paid in each year from 2015 to 2019 are $4.0 million, $4.1 million, $4.1 million, $4.2 million and $4.3 million, respectively. The aggregate benefits expected to be paid in the five years from 2020 to 2024 are $23.1 million. The expected benefits to be paid are based on the same assumptions used to measure the Company's projected benefit obligation at December 31, 2014 and include estimated future employee service.

Markel_Corporation_Parent_Comp

Markel Corporation (Parent Company Only) Financial Information	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Markel Corporation (Parent Company Only) Financial Information	Markel Corporation (Parent Company Only) Financial Information
	The following parent company only condensed financial information reflects the financial position, results of operations and cash flows of Markel Corporation.
	CONDENSED BALANCE SHEETS
		December 31,
		2014			2013
		(dollars in thousands)
	ASSETS
	Investments, available-for-sale, at estimated fair value:
	Fixed maturities (amortized cost of $47,346 in 2014 and $66,154 in 2013)	$	48,807		$	67,363
	Equity securities (cost of $193,864 in 2014 and $197,549 in 2013)	434,714			392,123
	Short-term investments (estimated fair value approximates cost)	764,953			654,971
	Total Investments	1,248,474			1,114,457
	Cash and cash equivalents	243,702			207,352
	Restricted cash and cash equivalents	959			1,010
	Receivables	16,110			14,326
	Investments in consolidated subsidiaries	7,560,862			6,826,790
	Notes receivable from subsidiaries	212,631			168,611
	Income taxes receivable	10,951			5,320
	Other assets	93,434			102,193
	Total Assets	$	9,387,123		$	8,440,059
	LIABILITIES AND SHAREHOLDERS' EQUITY
	Senior long-term debt	$	1,635,173		$	1,633,873
	Net deferred tax liability	74,534			40,443
	Other liabilities	82,598			92,166
	Total Liabilities	1,792,305			1,766,482
	Total Shareholders' Equity	7,594,818			6,673,577
	Total Liabilities and Shareholders' Equity	$	9,387,123		$	8,440,059
	CONDENSED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME
		Years Ended December 31,
		2014			2013			2012
		(dollars in thousands)
	REVENUES
	Net investment income	$	5,354		$	21,946		$	30,619
	Dividends on common stock of consolidated subsidiaries	217,121			806,233			337,585
	Net realized investment gains:
	Other-than-temporary impairment losses	(120		)	(15		)	(38		)
	Net realized investment gains, excluding other-than-temporary impairment losses	3,873			67,232			14,926
	Net realized investment gains	3,753			67,217			14,888
	Other	—			1			3
	Total Revenues	226,228			895,397			383,095
	EXPENSES
	Interest expense	94,097			92,743			87,391
	Other expenses	2,685			2,617			1,166
	Total Expenses	96,782			95,360			88,557
	Income Before Equity in Undistributed Earnings of Consolidated Subsidiaries and Income Taxes	129,446			800,037			294,538
	Equity in undistributed earnings of consolidated subsidiaries	163,341			(520,323		)	(61,663		)
	Income tax benefit	(28,395		)	(1,307		)	(20,510		)
	Net Income to Shareholders	$	321,182		$	281,021		$	253,385
	OTHER COMPREHENSIVE INCOME TO SHAREHOLDERS
	Change in net unrealized gains on investments, net of taxes:
	Net holding gains arising during the period	$	32,118		$	66,623		$	10,897
	Consolidated subsidiaries' net holding gains arising during the period	655,617			158,922			255,528
	Consolidated subsidiaries' change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	173			(141		)	(160		)
	Reclassification adjustments for net gains (losses) included in net income to shareholders	(1,874		)	(43,220		)	11,847
	Consolidated subsidiaries' reclassification adjustments for net gains (losses) included in net income to shareholders	(24,287		)	2,390			(35,898		)
	Change in net unrealized gains on investments, net of taxes	661,747			184,574			242,214
	Change in foreign currency translation adjustments, net of taxes	1,949			(2,670		)	(242		)
	Consolidated subsidiaries' change in foreign currency translation adjustments, net of taxes	(34,194		)	(7,501		)	1,781
	Consolidated subsidiaries' change in net actuarial pension loss, net of taxes	(14,750		)	4,065			6,664
	Total Other Comprehensive Income to Shareholders	614,752			178,468			250,417
	Comprehensive Income to Shareholders	$	935,934		$	459,489		$	503,802
	CONDENSED STATEMENTS OF CASH FLOWS
		Years Ended December 31,
		2014			2013			2012
	(dollars in thousands)
OPERATING ACTIVITIES
Net income to shareholders	$	321,182		$	281,021		$	253,385
Adjustments to reconcile net income to shareholders to net cash provided by operating activities	(218,396		)	186,574			(153,773		)
Net Cash Provided By Operating Activities	102,786			467,595			99,612
INVESTING ACTIVITIES
Proceeds from sales of fixed maturities and equity securities	9,306			142,259			149,314
Proceeds from maturities, calls and prepayments of fixed maturities	15,710			2,819			64,340
Cost of fixed maturities and equity securities purchased	(687		)	(23,412		)	(139,681		)
Net change in short-term investments	(109,728		)	10,251			(374,801		)
Securities received from subsidiaries as dividends or repayment of notes receivable	89,996			249,996			260,088
Decrease in notes receivable due from subsidiaries	28,506			5,302			66,802
Capital contributions to subsidiaries	(74,788		)	(67,878		)	(198,349		)
Acquisitions	—			(1,017,988		)	(100,409		)
Cost of equity method investments	—			(5,291		)	(38,250		)
Change in restricted cash and cash equivalents	51			(348		)	(204		)
Additions to property and equipment	(342		)	(3,653		)	(9,437		)
Other	(2,150		)	3,207			(4,369		)
Net Cash Used By Investing Activities	(44,126		)	(704,736		)	(324,956		)
FINANCING ACTIVITIES
Additions to senior long-term debt	—			491,235			347,207
Repayment and retirement of senior long-term debt	—			(246,665		)	(157,359		)
Repurchases of common stock	(26,053		)	(57,388		)	(16,873		)
Issuance of common stock	5,691			24,518			9,145
Other	(1,948		)	(5,023		)	436
Net Cash Provided (Used) By Financing Activities	(22,310		)	206,677			182,556
Increase (decrease) in cash and cash equivalents	36,350			(30,464		)	(42,788		)
Cash and cash equivalents at beginning of year	207,352			237,816			280,604
CASH AND CASH EQUIVALENTS AT END OF YEAR	$	243,702		$	207,352		$	237,816

Quarterly_Financial_Informatio

Quarterly Financial Information	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information	Quarterly Financial Information (unaudited)
	The following table presents the unaudited quarterly results of consolidated operations for 2014, 2013 and 2012.
		Quarters Ended
	(dollars in thousands, except per share amounts)	Mar. 31		30-Jun			Sept. 30		Dec. 31
	2014
	Operating revenues	$	1,239,655	$	1,258,971		$	1,299,286	$	1,335,755
	Net income	87,501		41,141			76,824		118,222
	Net income to shareholders	87,716		40,068			75,803		117,595
	Comprehensive income to shareholders	230,273		250,588			36,502		418,571
	Net income per share:
	Basic	$	6.28	$	2.67		$	5.33	$	8.1
	Diluted	6.25		2.66			5.3		8.05
	Common stock price ranges:
	High	$	596.87	$	655.75		$	666	$	707.36
	Low	527.17		593.76			623.9		632.65
	2013
	Operating revenues	$	819,864	$	1,031,769		$	1,191,665	$	1,279,785
	Net income	89,263		28,676			66,967		98,939
	Net income to shareholders	88,902		27,756			65,599		98,764
	Comprehensive income (loss) to shareholders	257,684		(149,054		)	144,409		206,450
	Net income per share:
	Basic	$	9.53	$	2.24		$	4.69	$	6.98
	Diluted	9.5		2.24			4.67		6.95
	Common stock price ranges:
	High	$	510.05	$	546.94		$	549.09	$	582.59
	Low	434.98		501.76			506.64		511.06
	2012
	Operating revenues	$	733,135	$	693,247		$	765,775	$	807,955
	Net income	57,713		90,768			51,674		58,093
	Net income to shareholders	57,253		89,687			49,653		56,792
	Comprehensive income to shareholders	205,945		73,416			147,454		76,987
	Net income per share:
	Basic	$	5.94	$	8.44		$	5.33	$	6.25
	Diluted	5.92		8.42			5.32		6.23
	Common stock price ranges:
	High	$	451.9	$	453.5		$	459.9	$	502.2
	Low	398.65		421			420		425.17

Summary_Of_Significant_Account1

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation. 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 December 31, 2014 and not in any prior periods, except with respect to the Supplemental Pro Forma Information included in note 2. The Company consolidates the results of its Markel Ventures subsidiaries on a one-month lag. Certain prior year amounts have been reclassified to conform to the current presentation.
Use of Estimates	Use of Estimates. 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. Management periodically reviews its estimates and assumptions. Quarterly reviews include evaluating the adequacy of reserves for unpaid losses and loss adjustment expenses, life and annuity reinsurance benefit reserves, litigation contingencies, the reinsurance allowance for doubtful accounts and income tax liabilities, as well as analyzing the recoverability of deferred tax assets, estimating reinsurance premiums written and earned and evaluating the investment portfolio for other-than-temporary declines in estimated fair value. Estimates and assumptions for goodwill and intangible assets are reviewed in conjunction with an acquisition, and goodwill and indefinite-lived intangible assets are reassessed at least annually for impairment. Actual results may differ materially from the estimates and assumptions used in preparing the consolidated financial statements.
Investments	Investments. Available-for-sale investments are recorded at estimated fair value. Unrealized gains and losses on investments, net of deferred income taxes, are included in accumulated other comprehensive income in shareholders' equity. 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.
	Premiums and discounts are amortized or accreted over the lives of the related fixed maturities as an adjustment to the yield using the effective interest method. Dividend and interest income are recognized when earned. Realized investment gains or losses are included in earnings. Realized gains or losses from sales of investments are derived using the first-in, first-out method.
	Investments accounted for under the equity method of accounting are recorded at cost within other assets on the consolidated balance sheets and subsequently increased or decreased by the Company's proportionate share of the net income or loss of the investee. The Company records its proportionate share of net income or loss of the investee in net investment income. The Company records its proportionate share of other comprehensive income or loss of the investee as a component of other comprehensive income. Dividends or other equity distributions are recorded as a reduction of the investment. The Company reviews equity method investments for impairment when events or circumstances indicate that a decline in the fair value of the investment below its carrying value is other-than-temporary.
Cash and Cash Equivalents	Cash and Cash Equivalents. The Company considers all investments with original maturities of 90 days or less to be cash equivalents. The carrying value of the Company's cash and cash equivalents and restricted cash and cash equivalents approximates fair value.
Receivables	Receivables. Receivables include amounts receivable from agents, brokers and insureds, which represent premiums that are both currently due and amounts not yet due on insurance and reinsurance policies. Premiums for insurance policies are generally due at inception. Premiums for reinsurance policies generally become due over the period of coverage based on the policy terms. The Company monitors the credit risk associated with premiums receivable, taking into consideration the fact that in certain instances credit risk may be reduced by the Company's right to offset loss obligations or unearned premiums against premiums receivable. Amounts deemed uncollectible are charged to net income in the period they are determined. Changes in the estimate of reinsurance premiums written will result in an adjustment to premiums receivable in the period they are determined.
Reinsurance Recoverables	Reinsurance Recoverables. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured business. Allowances are established for amounts deemed uncollectible and reinsurance recoverables are recorded net of these allowances. The Company evaluates the financial condition of its reinsurers and monitors concentration risk to minimize its exposure to significant losses from individual reinsurers.
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs. Costs directly related to the acquisition of insurance premiums are deferred and amortized over the related policy period, generally one year. Concurrent with the adoption of Financial Accounting Standards Board (FASB) Accounting Standards Update (ASU) No. 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts, effective January 1, 2012, the Company only defers acquisition costs incurred that are related directly to the successful acquisition of new or renewal insurance contracts, including commissions to agents and brokers and premium taxes. Commissions received related to reinsurance premiums ceded are netted against broker commissions in determining acquisition costs eligible for deferral. To the extent that future policy revenues on existing policies are not adequate to cover related costs and expenses, deferred policy acquisition costs are charged to earnings. The Company does not consider anticipated investment income in determining whether a premium deficiency exists.
Goodwill and Intangible Assets	Goodwill and Intangible Assets. Goodwill and intangible assets are recorded as a result of business acquisitions. Goodwill represents the excess of the amount paid to acquire a business over the net fair value of assets acquired and liabilities assumed at the date of acquisition. Indefinite-lived and other intangible assets are recorded at fair value as of the acquisition date. The determination of the fair value of certain assets acquired and liabilities assumed involves significant judgment and the use of valuation models and other estimates, which require assumptions that are inherently subjective. Goodwill and indefinite-lived intangible assets are tested for impairment at least annually. The Company completes an annual test during the fourth quarter of each year based upon the results of operations through September 30. Intangible assets with definite lives are amortized using the straight-line method over their estimated useful lives, generally five to 20 years, and are reviewed for impairment when events or circumstances indicate that their carrying value may not be recoverable.
Property and Equipment	Property and Equipment. Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization of property and equipment are calculated using the straight-line method over the estimated useful lives (generally, the life of the lease for leasehold improvements, ten to 40 years for buildings, seven to 40 years for land improvements, three to ten years for furniture and equipment and three to 25 years for other property and equipment).
Redeemable Noncontrolling Interests	Redeemable Noncontrolling Interests. The Company owns controlling interests in various companies through its Markel Ventures operations. In some cases, the Company has the option to acquire the remaining equity interests, and the remaining equity interests have the option to sell their interests to the Company, in the future. The redemption value of the remaining equity interests is generally based on the respective company's earnings in specified periods preceding the redemption date. The redeemable noncontrolling interests generally become redeemable through 2018.
	The Company recognizes changes in the redemption value that exceed the carrying value of redeemable noncontrolling interests to retained earnings as if the balance sheet date were also the redemption date. Changes in the redemption value also result in an adjustment to net income to shareholders in the calculation of basic and diluted net income per share. The adjustment recorded to retained earnings during 2014, 2013 and 2012 was an increase of $8.2 million, a decrease of $2.0 million, and an increase of $3.1 million, respectively.
Income Taxes	Income Taxes. The Company records deferred income taxes to reflect the net tax effect of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and their tax bases. Deferred tax assets are reduced by a valuation allowance when management believes it is more likely than not that some, or all, of the deferred tax assets will not be realized. The Company recognizes the tax benefit from an uncertain tax position taken or expected to be taken in income tax returns only if it is more likely than not that the tax position will be sustained upon examination by tax authorities, based on the technical merits of the position. Tax positions that meet the more likely than not threshold are then measured using a probability weighted approach, whereby the largest amount of tax benefit that is greater than 50% likely of being realized upon ultimate settlement is recognized. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense.
Unpaid Losses and Loss Adjustment Expenses	Unpaid Losses and Loss Adjustment Expenses. Unpaid losses and loss adjustment expenses on our property and casualty insurance business are based on evaluations of reported claims and estimates for losses and loss adjustment expenses incurred but not reported. Estimates for losses and loss adjustment expenses incurred but not reported are based on reserve development studies, among other things. The Company does not discount reserves for losses and loss adjustment expenses to reflect estimated present value, except for reserves assumed in connection with an acquisition, which are recorded at fair value at the acquisition date. Recorded reserves are estimates, and the ultimate liability may be greater or less than the estimates.
Life and Annuity Benefits	Life and Annuity Benefits. Prior to its acquisition by the Company, Alterra entered into long duration reinsurance contracts for life and annuity benefits which subject the Company to mortality, longevity and morbidity risks. 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 an unlocking event occurs. To the extent existing policy reserves, together with the present value of future gross premiums and expected investment income earned thereon, are not adequate to cover the present value of future benefits, settlement and maintenance costs, the locked-in assumptions are revised to current best estimate assumptions and a charge to earnings for life and annuity benefits is recognized at that time. Because of the assumptions and estimates used in establishing reserves for life and annuity benefit obligations and the long-term nature of these reinsurance contracts, the ultimate liability may be greater or less than the estimates.
	Results attributable to the run-off of 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.
Revenue Recognition	Revenue Recognition.
	Property and Casualty Premiums
	Insurance premiums are generally earned on a pro rata basis over the policy period, typically one year. The cost of reinsurance ceded is initially recorded as prepaid reinsurance premiums and is amortized over the reinsurance contract period in proportion to the amount of insurance protection provided. Premiums ceded are netted against premiums written.
	Assumed reinsurance premiums are recorded at the inception of each contract based upon contract terms and information received from cedents and brokers and are earned on a pro rata basis over the coverage period, or for multi-year contracts, in proportion with the underlying risk exposure to the extent there is variability in the exposure through the coverage period. Changes in reinsurance premium estimates are expected and may result in significant adjustments in any period. These estimates change over time as additional information regarding changes in underlying exposures is obtained. Any subsequent differences arising on such estimates are recorded as premiums written in the period they are determined. The Company uses the periodic method to account for assumed reinsurance from foreign reinsurers. The Company's foreign reinsurers provide sufficient information to record foreign assumed business in the same manner as the Company records assumed business from United States reinsurers.
	Certain contracts that the Company writes provide for reinstatement of coverage. Reinstatement premiums are the premiums for the restoration of the insurance or reinsurance limit of a contract to its full amount after a loss occurrence by the insured or reinsured. The Company accrues for reinstatement premiums resulting from losses recorded. Such accruals are based upon contractual terms and management judgment is involved with respect to the amount of losses recorded. Changes in estimates of losses recorded on contracts with reinstatement premium features will result in changes in reinstatement premiums based on contractual terms. Reinstatement premiums are recognized at the time losses are recorded and are earned on a pro-rata basis over the coverage period.
	Other Revenues
	Other revenues primarily relate to the Company's Markel Ventures operations and consist of revenues from the sale of manufactured products and service revenues. Revenues from manufactured products are generally recognized at the time title transfers to the customer, which typically occurs at the point of shipment or delivery to the customer, depending on the terms of the sales arrangement. Revenues from services are generally recognized as the services are performed. Services provided pursuant to a contract are recognized either over the contract period or upon completion of the elements specified in the contract, depending on the terms of the contract.
Stock-based Compensation	Stock-based Compensation. Stock-based compensation expense is recognized as part of underwriting, acquisition and insurance expenses over the requisite service period. Stock-based compensation expense, net of taxes, was $18.7 million in 2014, $18.4 million in 2013 and $4.4 million in 2012. See note 12.
Foreign Currency Translation	Foreign Currency Translation. The functional currencies of the Company's foreign operations are the currencies in which the majority of their business is transacted. Assets and liabilities of foreign operations are translated into the United States Dollar using the exchange rates in effect at the balance sheet date. Revenues and expenses of foreign operations are translated using the average exchange rate for the period. Gains or losses from translating the financial statements of foreign operations are included, net of taxes, in shareholders' equity as a component of accumulated other comprehensive income. Gains and losses arising from transactions denominated in a foreign currency, other than a functional currency, are included in net income.
	The Company manages its exposure to foreign currency risk primarily by matching assets, other than goodwill and intangible assets, and liabilities denominated in the same currency. To the extent that assets and liabilities in foreign currencies are not matched, the Company is exposed to foreign currency risk. For functional currencies, the related exchange rate fluctuations are reflected in other comprehensive income. The cumulative foreign currency translation adjustment, net of taxes, was a loss of $43.5 million and $11.2 million at December 31, 2014 and 2013, respectively.
Derivative Financial Instruments	Derivative Financial Instruments. Derivative instruments, including derivative instruments resulting from hedging activities, are measured at fair value and recognized as either assets or liabilities on the consolidated balance sheets. The changes in fair value of derivatives are recognized in earnings unless the derivative is designated as a hedge and qualifies for hedge accounting.
	The Company's foreign currency forward contracts are generally designated and qualify as hedges of a net investment in a foreign operation. The effective portion of the change in fair value resulting from these hedges is reported in currency translation adjustments as part of other comprehensive income. The ineffective portion of the change in fair value is recognized in earnings.
Comprehensive Income	Comprehensive Income. Comprehensive income represents all changes in equity that result from recognized transactions and other economic events during the period. Other comprehensive income refers to revenues, expenses, gains and losses that under U.S. GAAP are included in comprehensive income but excluded from net income, such as unrealized gains or losses on investments, foreign currency translation adjustments and changes in net actuarial pension loss.
Net Income Per Share	Net Income Per Share. Basic net income per share is computed by dividing adjusted net income to shareholders by the weighted average number of common shares outstanding during the year. 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. See note 12(b).
Recent Accounting Pronouncements	Recent Accounting Pronouncements. Effective January 1, 2014, the Company adopted FASB ASU No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. ASU No. 2013-11 requires that a liability related to an unrecognized tax benefit be offset against a deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward if such settlement is required or expected in the event the uncertain tax position is disallowed. In that case, the liability associated with the unrecognized tax benefit is presented in the financial statements as a reduction to the related deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward. Otherwise, the unrecognized tax benefit should be presented in the financial statements as a liability and should not be combined with deferred tax assets. The adoption of this guidance did not have an impact on the Company's financial position, results of operations or cash flows.
	In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which creates a new comprehensive revenue recognition standard that will serve as a single source of revenue guidance for all companies in all industries. The guidance applies to all companies that either enter into contracts with customers to transfer goods or services or enter into contracts for the transfer of nonfinancial assets, unless those contracts are within the scope of other standards, such as insurance contracts. ASU No. 2014-09's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. In doing so, companies will need to use more judgment and make more estimates than under the current guidance. These may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU No. 2014-09 becomes effective for the Company during the first quarter of 2017 and may be applied retrospectively or under a modified retrospective method where the cumulative effect is recognized at the date of initial application. Early application is not permitted. The Company is currently evaluating ASU No. 2014-09 to determine the potential impact that adopting this standard will have on its consolidated financial statements.

Acquisitions_Tables

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2014
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 Fair Value Of Assets Acquired And Liabilities Assumed	The following table summarizes the 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	859,884
	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	223,108
	Net assets	2,801,103
	Goodwill	295,690
	Intangible assets	207,500
	Acquisition date fair value	$	3,304,293
Summary Of Intangible Assets Acquired	The following table summarizes the intangible assets recorded in connection with the acquisition.
	(dollars in thousands)	Amount			Economic
					Useful Life
	Customer relationships	$	132,000		18 years
	Broker relationships	19,000			18 years
	Technology	18,000			Ten years
	Trade names	1,000			One year
	Lloyd's syndicate capacity	12,000			Indefinite
	Insurance licenses	25,500			Indefinite
	Intangible assets as of the Acquisition Date	$	207,500
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)	Year Ended
		31-Dec-13
	Transaction costs	$	15,981
	Acquisition-related costs:
	Severance costs	31,734
	Stay bonuses	14,804
	Acceleration of Alterra long-term incentive compensation awards and restricted stock awards	12,621
	Total transaction and acquisition-related costs	$	75,140
Summary Of Results Of Operations	The following table summarizes the results of Alterra from the Acquisition Date through December 31, 2013 that have been included within the Company's consolidated statements of income and comprehensive income.
	(dollars in thousands)	Year Ended
		December 31, 2013
	Operating revenues	$	912,387
	Net loss to shareholders	$	(93,074	)
Unaudited Pro Forma Consolidated Financial Information	The following table presents unaudited pro forma consolidated information for the years ended December 31, 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 year ended December 31, 2013, severance costs attributable to the acquisition totaling $31.7 million for the year ended December 31, 2013, and stay bonuses of $14.8 million for the year ended December 31, 2013. The acceleration of compensation expense during the year ended December 31, 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.
		Unaudited
		Consolidated Pro Forma
		Years Ended December 31,
	(in thousands, except per share amounts)	2013			2012
	Earned premiums	$	3,680,220		$	3,509,834
	Operating revenues	4,899,628			4,561,107
	Net income to shareholders	422,829			308,496
	U.S. GAAP combined ratio (1)	95		%	99		%
	Basic net income per share	$	30.33		$	21.79
	Diluted net income per share	$	30.19		$	21.71
	Weighted average common shares outstanding:
	Basic	14,007			14,014
	Diluted	14,069			14,065
	(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)	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Available-For-Sale Investments	The following tables summarize the Company's available-for-sale investments.
		December 31, 2014
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized Other-			Estimated
		Cost			Unrealized			Unrealized			Than-Temporary			Fair
					Holding			Holding			Impairment Losses			Value
					Gains			Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	662,462		$	12,963		$	(2,163	)	$	—		$	673,262
	Obligations of states, municipalities and political subdivisions	4,075,748			245,158			(3,359		)	—			4,317,547
	Foreign governments	1,458,255			154,707			(1,041		)	—			1,611,921
	Commercial mortgage-backed securities	427,904			5,325			(2,602		)	—			430,627
	Residential mortgage-backed securities	954,263			34,324			(3,482		)	(2,258		)	982,847
	Asset-backed securities	100,073			99			(682		)	—			99,490
	Corporate bonds	2,250,432			69,016			(10,441		)	(1,819		)	2,307,188
	Total fixed maturities	9,929,137			521,592			(23,770		)	(4,077		)	10,422,882
	Equity securities:
	Insurance, banks and other financial institutions	523,739			789,717			(1,531		)	—			1,311,925
	Industrial, consumer and all other	1,427,919			1,403,566			(5,834		)	—			2,825,651
	Total equity securities	1,951,658			2,193,283			(7,365		)	—			4,137,576
	Short-term investments	1,594,819			36			(6		)	—			1,594,849
	Investments, available-for-sale	$	13,475,614		$	2,714,911		$	(31,141	)	$	(4,077	)	$	16,155,307
		December 31, 2013
	(dollars in thousands)	Amortized			Gross			Gross			Unrealized Other-			Estimated
		Cost			Unrealized			Unrealized			Than-Temporary			Fair
					Holding			Holding			Impairment Losses			Value
					Gains			Losses
	Fixed maturities:
	U.S. Treasury securities and obligations of U.S. government agencies	$	1,215,522		$	9,051		$	(30,342	)	$	—		$	1,194,231
	Obligations of states, municipalities and political subdivisions	2,986,758			116,341			(27,384		)	—			3,075,715
	Foreign governments	1,484,818			30,647			(54,411		)	—			1,461,054
	Commercial mortgage-backed securities	379,555			62			(11,796		)	—			367,821
	Residential mortgage-backed securities	875,902			13,046			(16,442		)	(2,258		)	870,248
	Asset-backed securities	189,646			257			(1,614		)	—			188,289
	Corporate bonds	2,996,940			54,777			(61,650		)	(4,889		)	2,985,178
	Total fixed maturities	10,129,141			224,181			(203,639		)	(7,147		)	10,142,536
	Equity securities:
	Insurance, banks and other financial institutions	422,975			592,112			(4		)	—			1,015,083
	Industrial, consumer and all other	1,143,578			1,094,251			(1,114		)	—			2,236,715
	Total equity securities	1,566,553			1,686,363			(1,118		)	—			3,251,798
	Short-term investments	1,452,270			18			—			—			1,452,288
	Investments, available-for-sale	$	13,147,964		$	1,910,562		$	(204,757	)	$	(7,147	)	$	14,846,622
Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position	The following tables summarize gross unrealized investment losses by the length of time that securities have continuously been in an unrealized loss position.
		December 31, 2014
		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	$	108,250		$	(62	)	$	163,359		$	(2,101	)	$	271,609	$	(2,163	)
	Obligations of states, municipalities and political subdivisions	58,583			(542		)	92,441			(2,817		)	151,024		(3,359		)
	Foreign governments	18,856			(386		)	56,217			(655		)	75,073		(1,041		)
	Commercial mortgage-backed securities	45,931			(210		)	147,558			(2,392		)	193,489		(2,602		)
	Residential mortgage-backed securities	9,613			(2,285		)	207,374			(3,455		)	216,987		(5,740		)
	Asset-backed securities	30,448			(20		)	45,160			(662		)	75,608		(682		)
	Corporate bonds	141,176			(2,263		)	621,821			(9,997		)	762,997		(12,260		)
	Total fixed maturities	412,857			(5,768		)	1,333,930			(22,079		)	1,746,787		(27,847		)
	Equity securities:
	Insurance, banks and other financial institutions	16,219			(1,531		)	—			—			16,219		(1,531		)
	Industrial, consumer and all other	86,062			(5,834		)	—			—			86,062		(5,834		)
	Total equity securities	102,281			(7,365		)	—			—			102,281		(7,365		)
	Short-term investments	181,964			(6		)	—			—			181,964		(6		)
	Total	$	697,102		$	(13,139	)	$	1,333,930		$	(22,079	)	$	2,031,032	$	(35,218	)
		December 31, 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	$	587,929		$	(30,342	)	$	—		$	—		$	587,929	$	(30,342	)
	Obligations of states, municipalities and political subdivisions	513,608			(27,238		)	3,512			(146		)	517,120		(27,384		)
	Foreign governments	950,040			(54,411		)	—			—			950,040		(54,411		)
	Commercial mortgage-backed securities	357,737			(11,796		)	—			—			357,737		(11,796		)
	Residential mortgage-backed securities	437,675			(18,700		)	—			—			437,675		(18,700		)
	Asset-backed securities	142,011			(1,614		)	—			—			142,011		(1,614		)
	Corporate bonds	1,817,737			(66,539		)	—			—			1,817,737		(66,539		)
	Total fixed maturities	4,806,737			(210,640		)	3,512			(146		)	4,810,249		(210,786		)
	Equity securities:
	Insurance, banks and other financial institutions	144			(4		)	—			—			144		(4		)
	Industrial, consumer and all other	20,943			(714		)	27,735			(400		)	48,678		(1,114		)
	Total equity securities	21,087			(718		)	27,735			(400		)	48,822		(1,118		)
	Total	$	4,827,824		$	(211,358	)	$	31,247		$	(546	)	$	4,859,071	$	(211,904	)
Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturities By Contractual Maturity	The amortized cost and estimated fair value of fixed maturities at December 31, 2014 are shown below by contractual maturity.
	(dollars in thousands)	Amortized			Estimated
		Cost			Fair Value
	Due in one year or less	$	747,997		$	752,715
	Due after one year through five years	2,007,798			2,059,192
	Due after five years through ten years	2,026,628			2,142,582
	Due after ten years	3,664,474			3,955,429
		8,446,897			8,909,918
	Commercial mortgage-backed securities	427,904			430,627
	Residential mortgage-backed securities	954,263			982,847
	Asset-backed securities	100,073			99,490
	Total fixed maturities	$	9,929,137		$	10,422,882
Components Of Net Investment Income	The following table presents the components of net investment income.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Interest:
	Municipal bonds (tax-exempt)	$	98,262		$	82,308		$	90,316
	Municipal bonds (taxable)	49,345			28,041			22,663
	Other taxable bonds	152,789			134,377			107,270
	Short-term investments, including overnight deposits	5,959			3,573			2,755
	Dividends on equity securities	65,031			48,641			50,416
	Change in fair value of credit default swap	2,230			10,460			16,641
	Income from equity method investments	4,766			21,898			1,961
	Other	108			355			(41		)
		378,490			329,653			291,981
	Investment expenses	(15,260		)	(12,280		)	(9,874		)
	Net investment income	$	363,230		$	317,373		$	282,107
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.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Realized gains:
	Sales of fixed maturities	$	8,417		$	13,772		$	18,337
	Sales of equity securities	51,356			73,592			29,578
	Other	15,205			5,940			749
	Total realized gains	74,978			93,304			48,664
	Realized losses:
	Sales of fixed maturities	(18,136		)	(25,168		)	(563		)
	Sales of equity securities	(802		)	(278		)	(342		)
	Other-than-temporary impairments	(4,784		)	(4,706		)	(12,078		)
	Other	(5,256		)	—			(4,088		)
	Total realized losses	(28,978		)	(30,152		)	(17,071		)
	Net realized investment gains	$	46,000		$	63,152		$	31,593
	Change in net unrealized gains on investments:
	Fixed maturities	$	480,350		$	(403,610	)	$	51,783
	Equity securities	500,673			665,599			302,013
	Short-term investments	12			6			12
	Net increase	$	981,035		$	261,995		$	353,808
Components Of Restricted Assets	The following table presents the components of restricted assets.
		December 31,
	(dollars in thousands)	2014			2013
	Restricted assets held in trust or on deposit to support underwriting activities	$	4,961,061		$	5,161,871
	Investments and cash and cash equivalents pledged as security for letters of credit	635,340			695,072
	Investments and cash pledged as collateral for credit default swap	—			33,430
	Cash held in escrow for Abbey acquisition	—			199,006
	Total	$	5,596,401		$	6,089,379
Schedule Of Restricted Assets	Total restricted assets are included on the Company's consolidated balance sheets as follows.
		December 31,
	(dollars in thousands)	2014			2013
	Investments, available-for-sale	$	5,040,413		$	5,225,701
	Restricted cash and cash equivalents	522,225			786,926
	Other assets	33,763			76,752
	Total	$	5,596,401		$	6,089,379

Receivables_Tables

Receivables (Tables)	12 Months Ended
	Dec. 31, 2014
Receivables [Abstract]
Components Of Receivables	The following table presents the components of receivables.
		December 31,
	(dollars in thousands)	2014			2013
	Amounts receivable from agents, brokers and insureds	$	1,031,519		$	1,058,021
	Trade accounts receivable	97,225			85,203
	Employee stock loans receivable (see note 12(c))	15,044			12,822
	Other	8,601			5,420
		1,152,389			1,161,466
	Allowance for doubtful receivables	(17,172		)	(19,693		)
	Receivables	$	1,135,217		$	1,141,773

Deferred_Policy_Acquisition_Co1

Deferred Policy Acquisition Costs (Tables)	12 Months Ended
	Dec. 31, 2014
Deferred Policy Acquisition Costs Disclosures [Abstract]
Amounts Of Policy Acquisition Costs Acquired, Deferred And Amortized	The following table presents the amounts of policy acquisition costs deferred and amortized.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Balance, beginning of year	$	260,967		$	157,465		$	194,674
	Policy acquisition costs deferred	754,303			577,620			390,360
	Amortization of policy acquisition costs	(654,916		)	(471,915		)	(428,109		)
	Foreign currency movements	(6,944		)	(2,203		)	540
	Deferred policy acquisition costs	$	353,410		$	260,967		$	157,465
Components Of Underwriting, Acquisition And Insurance Expenses	The following table presents the components of underwriting, acquisition and insurance expenses.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Transaction costs and other acquisition-related expenses (1)	$	—		$	75,140		$	—
	Prospective adoption of FASB ASU No. 2010-26 (2)	—			—			43,093
	Other amortization of policy acquisition costs	654,916			471,915			385,016
	Other operating expenses	805,966			765,257			501,363
	Underwriting, acquisition and insurance expenses	$	1,460,882		$	1,312,312		$	929,472
	(1)	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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)	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.

Property_And_Equipment_Tables

Property And Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Components Of Property And Equipment	The following table presents the components of property and equipment, which are included in other assets on the consolidated balance sheets.
		December 31,
	(dollars in thousands)	2014			2013
	Land	$	56,848		$	48,036
	Buildings	78,786			59,307
	Leasehold improvements	98,098			68,363
	Land improvements	70,596			57,673
	Furniture and equipment	255,566			217,528
	Other	116,884			104,690
		676,778			555,597
	Accumulated depreciation and amortization	(255,388		)	(207,688		)
	Property and equipment	$	421,390		$	347,909

Goodwill_And_Intangible_Assets1

Goodwill And Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Components Of Goodwill	The following table presents the components of goodwill. As described in note 19, effective January 1, 2014, the Company redefined its segments. As a result, goodwill was reallocated as of December 31, 2012 using a relative fair value allocation approach. The following table presents the components of goodwill by reportable segment.
	(dollars in thousands)	U.S. Insurance		International Insurance			Reinsurance		Other(1)			Total
	January 1, 2013	$	172,824	$	309,662		$	—	$	192,444		$	674,930
	Acquisitions (see note 2)	107,755		65,190			122,745		1,185			296,875
	Foreign currency movements and other adjustments	—		(2,088		)	—		(2,000		)	(4,088		)
	December 31, 2013	$	280,579	$	372,764		$	122,745	$	191,629		$	967,717
	Acquisitions (see note 2)	—		42,989			—		61,539			104,528
	Impairment loss	—		—			—		(13,737		)	(13,737		)
	Foreign currency movements and other adjustments	—		(7,570		)	—		(1,823		)	(9,393		)
	December 31, 2014 (2)	$	280,579	$	408,183		$	122,745	$	237,608		$	1,049,115
	(1)	Amounts included in Other above are related to the Company's non-insurance operations, which are not included in a reportable segment.
	(2)	Goodwill is net of accumulated impairment losses of $13.7 million included in Other.
Components Of Intangible Assets	The following table presents the components of intangible assets with a net carrying amount.
		December 31,
		2014					2013
	(dollars in thousands)	Gross		Accumulated			Gross		Accumulated
		Carrying		Amortization			Carrying		Amortization
		Amount					Amount
	Customer relationships	$	452,157	$	(69,483	)	$	298,886	$	(43,286	)
	Broker relationships	175,681		(34,827		)	178,693		(23,255		)
	Trade names	94,795		(17,673		)	65,880		(12,666		)
	Technology	62,288		(22,671		)	56,429		(16,222		)
	Insurance licenses	39,985		—			40,185		(133		)
	Lloyd's syndicate capacity	12,000		—			12,000		—
	Other	18,903		(8,408		)	14,197		(5,625		)
	Total	$	855,809	$	(153,062	)	$	666,270	$	(101,187	)

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Components Of Income Before Income Taxes	Income before income taxes includes the following components.
		Years Ended December 31,
	(dollars in thousands)	2014				2013				2012
	Domestic operations	$	240,279			$	325,133			$	126,466
	Foreign operations	200,099				36,610				185,584
	Income before income taxes	$	440,378			$	361,743			$	312,050
Components Of Income Tax Expense	Income tax expense includes the following components.
		Years Ended December 31,
	(dollars in thousands)	2014				2013				2012
	Current:
	Domestic	$	7,573			$	50,683			$	14,340
	Foreign	24,574				23,165				1,814
	Total current tax expense	32,147				73,848				16,154
	Deferred:
	Domestic	43,673				23,906				(3,734		)
	Foreign	40,870				(19,856		)		41,382
	Total deferred tax expense	84,543				4,050				37,648
	Income tax expense	$	116,690			$	77,898			$	53,802
Reconciliations Of United States Corporate Income Tax Rate To Effective Tax Rate On Income Before Income Taxes	Reconciliations of the United States corporate income tax rate to the effective tax rate on income before income taxes are presented in the following table.
		Years Ended December 31,
		2014			2013			2012
	United States corporate tax rate	35	%		35	%		35	%
	Tax-exempt investment income	(9	)		(9	)		(12	)
	Tax credits	(1	)		—			—
	Foreign operations	—			(4	)		(5	)
	Other	1			—			(1	)
	Effective tax rate	26	%		22	%		17	%
Components Of Domestic And Foreign Deferred Tax Assets And Liabilities	The following table presents the components of domestic and foreign deferred tax assets and liabilities.
		December 31,
	(dollars in thousands)	2014				2013
	Assets:
	Unpaid losses and loss adjustment expenses	$	239,588			$	283,365
	Life and annuity benefits	143,102				161,209
	Unearned premiums recognized for income tax purposes	108,960				76,862
	Net operating loss carryforwards	36,359				43,010
	Tax credit carryforwards	32,525				33,773
	Other-than-temporary impairments not yet deductible for income tax purposes	28,106				34,978
	Differences between financial reporting and tax bases	132,878				139,357
	Total gross deferred tax assets	721,518				772,554
	Less valuation allowance	(4,801		)		—
	Total gross deferred tax assets, net of allowance	716,717				772,554
	Liabilities:
	Net unrealized gains on investments	759,212				459,015
	Amortization of goodwill and other intangible assets	106,927				97,580
	Deferred policy acquisition costs	101,766				65,543
	Differences between financial reporting and tax bases	59,359				46,699
	Total gross deferred tax liabilities	1,027,264				668,837
	Net deferred tax asset (liability)	$	(310,547	)		$	103,717
Reconciliation Of Beginning And Ending Unrecognized Tax Benefits	The following table presents a reconciliation of unrecognized tax benefits.
		Years Ended December 31,
	(dollars in thousands)	2014				2013
	Unrecognized tax benefits, beginning of year	$	18,219			$	18,870
	Increases for tax positions taken in prior years	3				—
	Lapse of statute of limitations	(522		)		(651		)
	Unrecognized tax benefits, end of year	$	17,700			$	18,219

Unpaid_Losses_And_Loss_Adjustm1

Unpaid Losses And Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses	The following table presents a reconciliation of consolidated beginning and ending reserves for losses and loss adjustment expenses.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Net reserves for losses and loss adjustment expenses, beginning of year	$	8,407,642		$	4,592,652		$	4,607,767
	Foreign currency movements, commutations and other	(137,385		)	(780		)	30,359
	Adjusted net reserves for losses and loss adjustment expenses, beginning of year	8,270,257			4,591,872			4,638,126
	Incurred losses and loss adjustment expenses:
	Current year	2,638,012			2,227,402			1,553,070
	Prior years	(435,545		)	(411,129		)	(399,002		)
	Total incurred losses and loss adjustment expenses	2,202,467			1,816,273			1,154,068
	Payments:
	Current year	502,107			670,928			268,745
	Prior years	1,436,851			906,302			931,955
	Total payments	1,938,958			1,577,230			1,200,700
	Effect of foreign currency rate changes	(19,476		)	(7,915		)	1,158
	Net reserves for losses and loss adjustment expenses of acquired insurance companies	21,193			3,584,642			—
	Net reserves for losses and loss adjustment expenses, end of year	8,535,483			8,407,642			4,592,652
	Reinsurance recoverable on unpaid losses	1,868,669			1,854,414			778,774
	Gross reserves for losses and loss adjustment expenses, end of year	$	10,404,152		$	10,262,056		$	5,371,426
Reconciliation Of Asbestos And Environmental Reserves For Losses And Loss Adjustment Expenses	The following table provides a reconciliation of beginning and ending A&E reserves for losses and loss adjustment expenses, which are a component of consolidated unpaid losses and loss adjustment expenses. Amounts included in the following table are presented before consideration of reinsurance allowances.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Net reserves for A&E losses and loss adjustment expenses, beginning of year	$	272,194		$	260,791		$	244,772
	Commutations and other	115			(5,067		)	(897		)
	Adjusted net reserves for A&E losses and loss adjustment expenses, beginning of year	272,309			255,724			243,875
	Incurred losses and loss adjustment expenses	32,840			30,128			38,179
	Payments	(17,426		)	(13,658		)	(21,263		)
	Net reserves for A&E losses and loss adjustment expenses, end of year	287,723			272,194			260,791
	Reinsurance recoverable on unpaid losses	102,719			100,784			100,063
	Gross reserves for A&E losses and loss adjustment expenses, end of year	$	390,442		$	372,978		$	360,854

Life_And_Annuity_Benefits_Tabl

Life And Annuity Benefits (Tables)	12 Months Ended
	Dec. 31, 2014
Liability for Future Policy Benefits [Abstract]
Schedule Of Life And Annuity Benefits	The following table presents life and annuity benefits.
		December 31,
	(dollars in thousands)	2014		2013
	Life	$	182,604	$	190,765
	Annuities	1,031,946		1,194,558
	Accident and health	91,268		101,251
	Total	$	1,305,818	$	1,486,574

Senior_LongTerm_Debt_And_Other1

Senior Long-Term Debt And Other Debt (Tables)	12 Months Ended
	Dec. 31, 2014
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.
		December 31,
	(dollars in thousands)	2014		2013
	7.20% unsecured senior notes, due April 14, 2017, interest payable semi-annually, net of unamortized premium of $3,526 in 2014 and $4,822 in 2013	$	94,155	$	95,451
	7.125% unsecured senior notes, due September 30, 2019, interest payable semi-annually, net of unamortized discount of $1,343 in 2014 and $1,626 in 2013	348,657		348,374
	6.25% unsecured senior notes, due September 30, 2020, interest payable semi-annually, net of unamortized premium of $53,172 in 2014 and $61,273 in 2013	403,172		411,273
	5.35% unsecured senior notes, due June 1, 2021, interest payable semi-annually, net of unamortized discount of $1,325 in 2014 and $1,531 in 2013	248,675		248,469
	4.90% unsecured senior notes, due July 1, 2022, interest payable semi-annually, net of unamortized discount of $2,095 in 2014 and $2,374 in 2013	347,905		347,626
	3.625% unsecured senior notes, due March 30, 2023, interest payable semi-annually, net of unamortized discount of $1,659 in 2014 and $1,860 in 2013	248,341		248,140
	7.35% unsecured senior notes, due August 15, 2034, interest payable semi-annually, net of unamortized discount of $2,078 in 2014 and $2,185 in 2013	197,922		197,815
	5.0% unsecured senior notes, due March 30, 2043, interest payable semi-annually, net of unamortized discount of $6,327 in 2014 and $6,551 in 2013	243,673		243,449
	Other debt, at various interest rates ranging from 1.6% to 6.5%	121,094		115,630
	Senior long-term debt and other debt	$	2,253,594	$	2,256,227
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 December 31, 2014.
	Years Ending December 31,	(dollars in
		thousands)
	2015	$	23,340
	2016	21,850
	2017	112,942
	2018	2,348
	2019	352,199
	2020 and thereafter	1,699,044
	Total principal payments	$	2,211,723
	Net unamortized premium	41,871
	Senior long-term debt and other debt	$	2,253,594

Shareholders_Equity_Tables

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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. 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. Average closing common stock market prices are used to calculate the dilutive effect attributable to restricted stock.
		Years Ended December 31,
	(in thousands, except per share amounts)	2014				2013			2012
	Net income to shareholders	$	321,182			$	281,021		$	253,385
	Adjustment of redeemable noncontrolling interests	(8,186		)		1,963			(3,101		)
	Adjusted net income to shareholders	$	312,996			$	282,984		$	250,284
	Basic common shares outstanding	13,984				12,538			9,640
	Dilutive potential common shares from conversion of options	11				12			6
	Dilutive potential common shares from conversion of restricted stock	62				36			20
	Diluted shares outstanding	14,057				12,586			9,666
	Basic net income per share	$	22.38			$	22.57		$	25.96
	Diluted net income per share	$	22.27			$	22.48		$	25.89
Summary Of Options	The following table summarizes additional information with respect to these options.
		Number			Weighted			Weighted Average		Intrinsic Value
		of			Average			Remaining		(in millions)
		Shares			Exercise Price			Contractual Term
								(years)
	Outstanding and exercisable, January 1, 2014	35,941			$	412.39
	Exercised	13,636			$	413.06
	Outstanding and exercisable, December 31, 2014	22,305			$	411.98		1.7		$	6.1
2012 Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Nonvested Share-Based Awards	The following table summarizes nonvested share-based awards.
		Number			Weighted Average
		of Awards			Grant-Date
					Fair Value
	Nonvested awards at January 1, 2014	98,711			$	449.93
	Granted	28,280			583.74
	Vested	(8,479	)		491.51
	Forfeited	(380	)		411
	Nonvested awards at December 31, 2014	118,132			$	479.11
Alterra Equity Award Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Nonvested Share-Based Awards	The following table summarizes activity related to these nonvested restricted stock awards.
		Number			Weighted Average
		of Awards			Grant-Date
					Fair Value
	Nonvested awards at January 1, 2014	68,411			$	529.59
	Vested	(34,119	)		529.59
	Forfeited	(377	)		529.59
	Nonvested awards at December 31, 2014	33,915			$	529.59

Other_Comprehensive_Income_Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Other Comprehensive Income (Loss), Tax [Abstract]
Change In Accumulated Other Comprehensive Income By Component, Net Of Taxes And Noncontrolling Interests	The following table presents the change in accumulated other comprehensive income by component, net of taxes and noncontrolling interests.
	(dollars in thousands)	Unrealized			Foreign			Net Actuarial			Total
		Holding Gains			Currency			Pension Loss
		on Available-for-
		Sale Securities
	31-Dec-11	$	704,719		$	(2,614	)	$	(41,185	)	$	660,920
	Other comprehensive income before reclassifications	266,265			1,539			4,670			272,474
	Amounts reclassified from accumulated other comprehensive income	(24,051		)	—			1,994			(22,057		)
	Total other comprehensive income	242,214			1,539			6,664			250,417
	31-Dec-12	$	946,933		$	(1,075	)	$	(34,521	)	$	911,337
	Other comprehensive income (loss) before reclassifications	225,404			(10,171		)	2,517			217,750
	Amounts reclassified from accumulated other comprehensive income	(40,830		)	—			1,548			(39,282		)
	Total other comprehensive income (loss)	184,574			(10,171		)	4,065			178,468
	31-Dec-13	$	1,131,507		$	(11,246	)	$	(30,456	)	$	1,089,805
	Other comprehensive income (loss) before reclassifications	687,908			(32,245		)	(16,516		)	639,147
	Amounts reclassified from accumulated other comprehensive income	(26,161		)	—			1,766			(24,395		)
	Total other comprehensive income (loss)	661,747			(32,245		)	(14,750		)	614,752
	31-Dec-14	$	1,793,254		$	(43,491	)	$	(45,206	)	$	1,704,557
Summary Of Tax Expense (Benefit) Of Other Comprehensive Income	The following table summarizes the tax expense (benefit) associated with each component of other comprehensive income.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Change in net unrealized gains on investments:
	Net holding gains arising during the period	$	328,564		$	93,837		$	122,524
	Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	614			(34		)	(49		)
	Reclassification adjustments for net gains included in net income	(9,890		)	(16,382		)	(10,881		)
	Change in net unrealized gains on investments	319,288			77,421			111,594
	Change in foreign currency translation adjustments	1,918			(1,619		)	(446		)
	Change in net actuarial pension loss	(3,687		)	1,015			1,991
	Total	$	317,519		$	76,817		$	113,139
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.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Unrealized holding gains on available-for-sale securities:
	Other-than-temporary impairment losses	$	(4,784	)	$	(4,706	)	$	(12,078	)
	Net realized investment gains, excluding other-than-temporary impairment losses	40,835			61,918			47,010
	Total before taxes	36,051			57,212			34,932
	Income taxes	(9,890		)	(16,382		)	(10,881		)
	Reclassification of unrealized holding gains, net of taxes	$	26,161		$	40,830		$	24,051
	Net actuarial pension loss:
	Underwriting, acquisition and insurance expenses	$	(2,084	)	$	(1,934	)	$	(2,590	)
	Income taxes	318			386			596
	Reclassification of net actuarial pension loss, net of taxes	$	(1,766	)	$	(1,548	)	$	(1,994	)

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
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.
		December 31, 2014
	(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	$	—	$	673,262	$	—	$	673,262
	Obligations of states, municipalities and political subdivisions	—		4,317,547		—		4,317,547
	Foreign governments	—		1,611,921		—		1,611,921
	Commercial mortgage-backed securities	—		430,627		—		430,627
	Residential mortgage-backed securities	—		982,847		—		982,847
	Asset-backed securities	—		99,490		—		99,490
	Corporate bonds	—		2,307,188		—		2,307,188
	Total fixed maturities	—		10,422,882		—		10,422,882
	Equity securities:
	Insurance, banks and other financial institutions	1,311,925		—		—		1,311,925
	Industrial, consumer and all other	2,825,651		—		—		2,825,651
	Total equity securities	4,137,576		—		—		4,137,576
	Short-term investments	1,469,975		124,874		—		1,594,849
	Total investments available-for-sale	$	5,607,551	$	10,547,756	$	—	$	16,155,307
		December 31, 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	$	—	$	1,194,231	$	—	$	1,194,231
	Obligations of states, municipalities and political subdivisions	—		3,075,715		—		3,075,715
	Foreign governments	—		1,461,054		—		1,461,054
	Commercial mortgage-backed securities	—		367,821		—		367,821
	Residential mortgage-backed securities	—		870,248		—		870,248
	Asset-backed securities	—		188,289		—		188,289
	Corporate bonds	—		2,985,178		—		2,985,178
	Total fixed maturities	—		10,142,536		—		10,142,536
	Equity securities:
	Insurance, banks and other financial institutions	1,015,083		—		—		1,015,083
	Industrial, consumer and all other	2,236,715		—		—		2,236,715
	Total equity securities	3,251,798		—		—		3,251,798
	Short-term investments	1,312,561		139,727		—		1,452,288
	Total investments available-for-sale	$	4,564,359	$	10,282,263	$	—	$	14,846,622
	Liabilities:
	Derivative contracts	$	—	$	—	$	2,230	$	2,230

Reinsurance_Tables

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Reinsurance Allowance For Doubtful Accounts	The following table summarizes the Company's reinsurance allowance for doubtful accounts.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Reinsurance allowance, beginning of year	$	76,210		$	71,148		$	69,067
	Additions	10,316			13,621			24,179
	Deductions	(26,713		)	(8,559		)	(22,098		)
	Reinsurance allowance, end of year	$	59,813		$	76,210		$	71,148
Effect Of Reinsurance On Premiums Written And Earned	The following table summarizes the effect of reinsurance and retrocessional reinsurance on premiums written and earned.
		Years Ended December 31,
		2014						2013						2012
	(dollars in thousands)	Written			Earned			Written			Earned			Written			Earned
	Direct	$	3,478,273		$	3,443,912		$	3,143,957		$	2,947,812		$	2,115,353		$	2,057,735
	Assumed	1,327,240			1,298,371			776,269			1,016,853			398,328			376,186
	Ceded	(888,498		)	(901,371		)	(683,543		)	(733,049		)	(299,555		)	(286,793		)
	Net premiums	$	3,917,015		$	3,840,912		$	3,236,683		$	3,231,616		$	2,214,126		$	2,147,128

Commitments_And_Contingencies_

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Minimum Annual Rental Commitments	The following table summarizes the Company's minimum annual rental commitments, excluding taxes, insurance and other operating costs payable directly by the Company, for noncancelable operating leases at December 31, 2014.
	Years Ending December 31,	(dollars in
		thousands)
	2015	$	28,434
	2016	23,636
	2017	28,124
	2018	26,549
	2019	25,169
	2020 and thereafter	151,000
	Total	$	282,912

Statutory_Financial_Informatio1

Statutory Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
Statutory Financial Information [Abstract]
Actual Statutory Capital And Surplus	Statutory capital and surplus and statutory net income (loss) for the Company's wholly-owned insurance subsidiaries as of December 31, 2014 and 2013 and for the years ended December 31, 2014, 2013 and 2012, respectively, is summarized below.
	Statutory Capital and Surplus
	(dollars in thousands)	2014		2013
	United States	$	2,619,001	$	2,396,530
	United Kingdom	$	604,564	$	510,426
	Bermuda	$	1,890,920	$	1,503,004
	Other	$	177,738	$	172,447
Statutory Net Income (Loss)
	Statutory Net Income (Loss)
		Years Ended December 31,
	(dollars in thousands)	2014		2013			2012
	United States	$	212,909	$	235,009		$	127,179
	United Kingdom	$	70,632	$	109,983		$	82,573
	Bermuda	$	162,809	$	249,772		N/A
	Other	$	1,271	$	(12,617	)	N/A

Segment_Reporting_Disclosures_

Segment Reporting Disclosures (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting, Disclosure of Entity's Reportable Segments [Abstract]
Summary Of Gross Written Premiums By Country	The following table summarizes the Company's gross written premiums by country. Gross written premiums are attributed to individual countries based upon location of risk.
		Years Ended December 31,
	(dollars in thousands)	2014			% of			2013				% of			2012				% of
					Total							Total							Total
	United States	$	3,523,239		73	%		$	2,934,868			75	%		$	1,768,011			70	%
	United Kingdom	441,669			9			245,143				6			147,891				6
	Canada	125,617			3			128,420				3			120,542				5
	Other countries	714,988			15			611,795				16			477,237				19
	Total	$	4,805,513		100	%		$	3,920,226			100	%		$	2,513,681			100	%
Company's Segment Disclosures	The following tables summarize the Company's segment disclosures.
		Year Ended December 31, 2014
	(dollars in thousands)	U.S. Insurance			International Insurance				Reinsurance				Other				Investing				Consolidated
													Insurance
													(Discontinued
													Lines)
	Gross premium volume	$	2,493,823		$	1,200,403			$	1,112,728			$	(1,441	)		$	—			$	4,805,513
	Net written premiums	2,071,466			889,336				956,584				(371		)		—				3,917,015
	Earned premiums	2,022,860			909,679				908,385				(12		)		—				3,840,912
	Losses and loss adjustment expenses:
	Current accident year	(1,340,129		)	(660,409		)		(637,474		)		—				—				(2,638,012		)
	Prior accident years	216,557			166,615				79,951				(27,578		)		—				435,545
	Amortization of policy acquisition costs	(403,233		)	(141,394		)		(110,289		)		—				—				(654,916		)
	Other operating expenses	(396,737		)	(207,175		)		(201,673		)		(381		)		—				(805,966		)
	Underwriting profit (loss)	99,318			67,316				38,900				(27,971		)		—				177,563
	Net investment income	—			—				—				—				363,230				363,230
	Net realized investment gains	—			—				—				—				46,000				46,000
	Other revenues (insurance)	2,478			21,827				2,696				1,631				—				28,632
	Other expenses (insurance)	(5,149		)	(18,706		)		(1,847		)		(37,132		)		—				(62,834		)
	Segment profit (loss)	$	96,647		$	70,437			$	39,749			$	(63,472	)		$	409,230			$	552,591
	Other revenues (non-insurance)																				854,893
	Other expenses (non-insurance)																				(792,037		)
	Amortization of intangible assets																				(57,627		)
	Interest expense																				(117,442		)
	Income before income taxes																				$	440,378
	U.S. GAAP combined ratio (1)	95		%	93		%		96		%		NM			(2)					95		%
	(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.
	(2)	NM — Ratio is not meaningful.
		Year Ended December 31, 2013
	(dollars in thousands)	U.S. Insurance			International Insurance				Reinsurance				Other				Investing				Consolidated
													Insurance
													(Discontinued
													Lines)
	Gross premium volume	$	2,252,739		$	1,101,099			$	566,348			$	40			$	—			$	3,920,226
	Net written premiums	1,915,770			840,050				480,822				41				—				3,236,683
	Earned premiums	1,727,766			833,984				669,826				40				—				3,231,616
	Losses and loss adjustment expenses:
	Current accident year	(1,173,258		)	(588,759		)		(465,385		)		—				—				(2,227,402		)
	Prior accident years	298,113			130,660				12,938				(30,582		)		—				411,129
	Transaction costs and other acquisition-related expenses (1)	(12,724		)	(13,366		)		(49,050		)		—				—				(75,140		)
	Amortization of policy acquisition costs	(287,795		)	(138,626		)		(45,494		)		—				—				(471,915		)
	Other operating expenses	(409,886		)	(171,666		)		(183,817		)		112				—				(765,257		)
	Underwriting profit (loss)	142,216			52,227				(60,982		)		(30,430		)		—				103,031
	Net investment income	—			—				—				—				317,373				317,373
	Net realized investment gains	—			—				—				—				63,152				63,152
	Other revenues (insurance)	13,648			4,284				5,432				1,130				—				24,494
	Other expenses (insurance)	(17,087		)	(5,065		)		—				(28,126		)		—				(50,278		)
	Segment profit (loss)	$	138,777		$	51,446			$	(55,550	)		$	(57,426	)		$	380,525			$	457,772
	Other revenues (non-insurance)																				686,448
	Other expenses (non-insurance)																				(613,250		)
	Amortization of intangible assets																				(55,223		)
	Interest expense																				(114,004		)
	Income before income taxes																				$	361,743
	U.S. GAAP combined ratio (2)	92		%	94		%		109		%		NM			(3)					97		%
	(1)	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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.
		Year Ended December 31, 2012
	(dollars in thousands)	U.S. Insurance			International Insurance				Reinsurance				Other				Investing				Consolidated
													Insurance
													(Discontinued
													Lines)
	Gross premium volume	$	1,595,849		$	788,092			$	129,744			$	(4	)		$	—			$	2,513,681
	Net written premiums	1,418,579			677,344				118,208				(5		)		—				2,214,126
Earned premiums	1,360,229			672,405				114,499				(5		)		—				2,147,128
Losses and loss adjustment expenses:
Current accident year	(984,018		)	(492,177		)		(76,875		)		—				—				(1,553,070		)
Prior accident years	225,870			172,758				21,449				(21,075		)		—				399,002
Prospective adoption of FASB ASU No. 2010-26 (1)	(31,033		)	(10,597		)		(1,463		)		—				—				(43,093		)
Other amortization of policy acquisition costs	(243,466		)	(130,637		)		(10,913		)		—				—				(385,016		)
Other operating expenses	(327,682		)	(153,545		)		(19,933		)		(203		)		—				(501,363		)
Underwriting profit (loss)	(100		)	58,207				26,764				(21,283		)		—				63,588
Net investment income	—			—				—				—				282,107				282,107
Net realized investment gains	—			—				—				—				31,593				31,593
Other revenues (insurance)	44,968			4,964				—				—				—				49,932
Other expenses (insurance)	(41,425		)	(3,867		)		—				—				—				(45,292		)
Segment profit (loss)	$	3,443		$	59,304			$	26,764			$	(21,283	)		$	313,700			$	381,928
Other revenues (non-insurance)																				489,352
Other expenses (non-insurance)																				(432,956		)
Amortization of intangible assets																				(33,512		)
Interest expense																				(92,762		)
Income before income taxes																				$	312,050
U.S. GAAP combined ratio(2)	100		%	91		%		77		%		NM			(3)					97		%
(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.
(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.
Summary Of Deferred Policy Acquisition Costs, Unearned Premiums And Unpaid Losses And Loss Adjustment Expenses	The following table summarizes deferred policy acquisition costs, unearned premiums and unpaid losses and loss adjustment expenses by segment.
	(dollars in thousands)	Deferred Policy			Unearned				Unpaid Losses and
		Acquisition Costs			Premiums				Loss Adjustment Expenses
	December 31, 2014
	U.S. Insurance	$	165,333		$	1,110,910			$	3,577,166
	International Insurance	47,618			491,708				3,353,417
	Reinsurance	140,459			643,072				2,818,792
	Other Insurance (Discontinued Lines)	—			—				654,777
	Total	$	353,410		$	2,245,690			$	10,404,152
	December 31, 2013
	U.S. Insurance	$	151,015		$	1,058,529			$	3,572,667
	International Insurance	50,425			516,689				3,399,228
	Reinsurance	59,527			551,897				2,811,362
	Other Insurance (Discontinued Lines)	—			—				478,799
	Total	$	260,967		$	2,127,115			$	10,262,056
Summary Of Segment Earned Premiums	The following table summarizes segment earned premiums by major product grouping.
		Years Ended December 31,
	(dollars in thousands)	2014			2013				2012
	U.S. Insurance:
	General liability	$	491,645		$	431,798			$	354,003
	Professional liability	321,005			268,203				216,234
	Property	266,019			190,530				141,568
	Personal lines	299,442			185,935				86,217
	Programs	244,216			205,004				135,037
	Workers compensation	263,164			250,790				241,208
	Other	137,369			195,506				185,962
	Total U.S. Insurance	2,022,860			1,727,766				1,360,229
	International Insurance:
	General liability	146,178			128,171				102,875
	Professional liability	285,300			252,816				174,590
	Property	76,691			91,497				48,483
	Marine and energy	287,263			287,745				288,605
	Other	114,247			73,755				57,852
	Total International Insurance	909,679			833,984				672,405
	Reinsurance:
	Property	270,461			227,394				71,172
	Casualty	323,390			244,981				476
	Auto	152,645			84,042				21,163
	Other	161,889			113,409				21,688
	Total Reinsurance	908,385			669,826				114,499
	Other Insurance (Discontinued Lines)	(12		)	40				(5		)
	Total earned premiums	$	3,840,912		$	3,231,616			$	2,147,128
Segment Assets To The Company's Consolidated Balance Sheets	The following table reconciles segment assets to the Company's consolidated balance sheets.
		December 31,
	(dollars in thousands)	2014			2013				2012
	Segment assets:
	Investing	$	18,531,150		$	17,550,332			$	9,277,697
	Underwriting	5,422,445			5,468,731				2,387,305
	Total segment assets	23,953,595			23,019,063				11,665,002
	Non-insurance operations	1,246,762			936,448				891,586
	Total assets	$	25,200,357		$	23,955,511			$	12,556,588

Other_Revenues_And_Other_Expen1

Other Revenues And Other Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Other Income and Expenses [Abstract]
Summary Of Other Revenues And Other Expenses By Component	The following table summarizes the components of other revenues and other expenses.
		Years Ended December 31,
		2014				2013				2012
	(dollars in thousands)	Other		Other		Other		Other		Other		Other
		Revenues		Expenses		Revenues		Expenses		Revenues		Expenses
	Insurance:
	Managing general agent operations	$	23,324	$	22,527	$	17,399	$	20,382	$	48,056	$	43,069
	Life and annuity	1,631		37,132		1,130		28,126		—		—
	Other	3,677		3,175		5,965		1,770		1,876		2,223
		28,632		62,834		24,494		50,278		49,932		45,292
	Non-Insurance:
	Markel Ventures: Manufacturing	575,353		513,668		495,138		437,712		366,886		328,484
	Markel Ventures: Non-Manufacturing	262,767		261,551		191,310		175,538		122,466		104,472
	Other	16,773		16,818		—		—		—		—
		854,893		792,037		686,448		613,250		489,352		432,956
	Total	$	883,525	$	854,871	$	710,942	$	663,528	$	539,284	$	478,248

Employee_Benefit_Plans_Tables

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Funded Status Of The Terra Nova Pension Plan	The following table summarizes the funded status of the Terra Nova Pension Plan and the amounts recognized on the accompanying consolidated balance sheets of the Company.
		Years Ended December 31,
	(dollars in thousands)	2014			2013
	Change in projected benefit obligation:
	Projected benefit obligation at beginning of period	$	163,010		$	151,327
	Interest cost	7,572			6,533
	Plan amendments	495			—
	Benefits paid	(4,424		)	(3,542		)
	Actuarial loss	29,609			5,459
	Effect of foreign currency rate changes	(10,706		)	3,233
	Projected benefit obligation at end of year	$	185,556		$	163,010
	Change in plan assets:
	Fair value of plan assets at beginning of period	$	189,437		$	164,090
	Actual gain on plan assets	22,395			19,430
	Employer contributions	5,610			5,338
	Benefits paid	(4,424		)	(3,542		)
	Effect of foreign currency rate changes	(11,619		)	4,121
	Fair value of plan assets at end of year	$	201,399		$	189,437
	Funded status of the plan	$	15,843		$	26,427
	Net actuarial pension loss	61,378			42,941
	Total	$	77,221		$	69,368
Schedule of Defined Benefit Plan Amounts Recognized in Accumulated Other Comprehensive Income	The following table presents the changes in plan assets and projected benefit obligation recognized in accumulated other comprehensive income.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Net actuarial gain (loss)	$	(20,521	)	$	3,146		$	6,065
	Amortization of:
	Net actuarial loss	1,589			1,934			2,590
	Prior service costs	495			—			—
	Tax benefit (expense)	3,687			(1,015		)	(1,991		)
	Total other comprehensive income (loss)	$	(14,750	)	$	4,065		$	6,664
Schedule Of Net Periodic Benefit Income And Weighted Average Assumptions	The following table summarizes the components of net periodic benefit income and the weighted average assumptions for the Terra Nova Pension Plan.
		Years Ended December 31,
	(dollars in thousands)	2014			2013			2012
	Components of net periodic benefit income:
	Service cost	$	—		$	—		$	361
	Interest cost	7,572			6,533			6,815
	Expected return on plan assets	(12,812		)	(10,825		)	(9,788		)
	Amortization of prior service cost	495			—			—
	Amortization of net actuarial pension loss	1,589			1,934			2,590
	Net periodic benefit income	$	(3,156	)	$	(2,358	)	$	(22	)
	Weighted average assumptions as of December 31:
	Discount rate	3.8		%	4.7		%	4.5		%
	Expected return on plan assets	6		%	6.6		%	6.6		%
	Rate of compensation increase	2.9		%	3.2		%	3.1		%
Summary Of Fair Value Of Plan Assets	The following table summarizes the fair value of plan assets as of December 31, 2014 and 2013.
		December 31,
	(dollars in thousands)	2014			2013
	Plan assets:
	Fixed maturity index funds	$	114,243		$	70,997
	Equity security index funds	87,148			118,431
	Cash and cash equivalents	8			9
	Total	$	201,399		$	189,437

Markel_Corporation_Parent_Comp1

Markel Corporation (Parent Company Only) Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule Of Condensed Balance Sheets	CONDENSED BALANCE SHEETS
		December 31,
		2014			2013
		(dollars in thousands)
	ASSETS
	Investments, available-for-sale, at estimated fair value:
	Fixed maturities (amortized cost of $47,346 in 2014 and $66,154 in 2013)	$	48,807		$	67,363
	Equity securities (cost of $193,864 in 2014 and $197,549 in 2013)	434,714			392,123
	Short-term investments (estimated fair value approximates cost)	764,953			654,971
	Total Investments	1,248,474			1,114,457
	Cash and cash equivalents	243,702			207,352
	Restricted cash and cash equivalents	959			1,010
	Receivables	16,110			14,326
	Investments in consolidated subsidiaries	7,560,862			6,826,790
	Notes receivable from subsidiaries	212,631			168,611
	Income taxes receivable	10,951			5,320
	Other assets	93,434			102,193
	Total Assets	$	9,387,123		$	8,440,059
	LIABILITIES AND SHAREHOLDERS' EQUITY
	Senior long-term debt	$	1,635,173		$	1,633,873
	Net deferred tax liability	74,534			40,443
	Other liabilities	82,598			92,166
	Total Liabilities	1,792,305			1,766,482
	Total Shareholders' Equity	7,594,818			6,673,577
	Total Liabilities and Shareholders' Equity	$	9,387,123		$	8,440,059
Schedule Of Condensed Statements Of Income And Comprehensive Income	CONDENSED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME
		Years Ended December 31,
		2014			2013			2012
		(dollars in thousands)
	REVENUES
	Net investment income	$	5,354		$	21,946		$	30,619
	Dividends on common stock of consolidated subsidiaries	217,121			806,233			337,585
	Net realized investment gains:
	Other-than-temporary impairment losses	(120		)	(15		)	(38		)
	Net realized investment gains, excluding other-than-temporary impairment losses	3,873			67,232			14,926
	Net realized investment gains	3,753			67,217			14,888
	Other	—			1			3
	Total Revenues	226,228			895,397			383,095
	EXPENSES
	Interest expense	94,097			92,743			87,391
	Other expenses	2,685			2,617			1,166
	Total Expenses	96,782			95,360			88,557
	Income Before Equity in Undistributed Earnings of Consolidated Subsidiaries and Income Taxes	129,446			800,037			294,538
	Equity in undistributed earnings of consolidated subsidiaries	163,341			(520,323		)	(61,663		)
	Income tax benefit	(28,395		)	(1,307		)	(20,510		)
	Net Income to Shareholders	$	321,182		$	281,021		$	253,385
	OTHER COMPREHENSIVE INCOME TO SHAREHOLDERS
	Change in net unrealized gains on investments, net of taxes:
	Net holding gains arising during the period	$	32,118		$	66,623		$	10,897
	Consolidated subsidiaries' net holding gains arising during the period	655,617			158,922			255,528
	Consolidated subsidiaries' change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	173			(141		)	(160		)
	Reclassification adjustments for net gains (losses) included in net income to shareholders	(1,874		)	(43,220		)	11,847
	Consolidated subsidiaries' reclassification adjustments for net gains (losses) included in net income to shareholders	(24,287		)	2,390			(35,898		)
	Change in net unrealized gains on investments, net of taxes	661,747			184,574			242,214
	Change in foreign currency translation adjustments, net of taxes	1,949			(2,670		)	(242		)
	Consolidated subsidiaries' change in foreign currency translation adjustments, net of taxes	(34,194		)	(7,501		)	1,781
	Consolidated subsidiaries' change in net actuarial pension loss, net of taxes	(14,750		)	4,065			6,664
	Total Other Comprehensive Income to Shareholders	614,752			178,468			250,417
	Comprehensive Income to Shareholders	$	935,934		$	459,489		$	503,802
Schedule Of Condensed Statements Of Cash Flows	CONDENSED STATEMENTS OF CASH FLOWS
		Years Ended December 31,
		2014			2013			2012
		(dollars in thousands)
	OPERATING ACTIVITIES
	Net income to shareholders	$	321,182		$	281,021		$	253,385
	Adjustments to reconcile net income to shareholders to net cash provided by operating activities	(218,396		)	186,574			(153,773		)
	Net Cash Provided By Operating Activities	102,786			467,595			99,612
	INVESTING ACTIVITIES
	Proceeds from sales of fixed maturities and equity securities	9,306			142,259			149,314
	Proceeds from maturities, calls and prepayments of fixed maturities	15,710			2,819			64,340
	Cost of fixed maturities and equity securities purchased	(687		)	(23,412		)	(139,681		)
	Net change in short-term investments	(109,728		)	10,251			(374,801		)
	Securities received from subsidiaries as dividends or repayment of notes receivable	89,996			249,996			260,088
	Decrease in notes receivable due from subsidiaries	28,506			5,302			66,802
	Capital contributions to subsidiaries	(74,788		)	(67,878		)	(198,349		)
	Acquisitions	—			(1,017,988		)	(100,409		)
	Cost of equity method investments	—			(5,291		)	(38,250		)
	Change in restricted cash and cash equivalents	51			(348		)	(204		)
	Additions to property and equipment	(342		)	(3,653		)	(9,437		)
	Other	(2,150		)	3,207			(4,369		)
	Net Cash Used By Investing Activities	(44,126		)	(704,736		)	(324,956		)
	FINANCING ACTIVITIES
	Additions to senior long-term debt	—			491,235			347,207
	Repayment and retirement of senior long-term debt	—			(246,665		)	(157,359		)
	Repurchases of common stock	(26,053		)	(57,388		)	(16,873		)
	Issuance of common stock	5,691			24,518			9,145
	Other	(1,948		)	(5,023		)	436
	Net Cash Provided (Used) By Financing Activities	(22,310		)	206,677			182,556
	Increase (decrease) in cash and cash equivalents	36,350			(30,464		)	(42,788		)
	Cash and cash equivalents at beginning of year	207,352			237,816			280,604
	CASH AND CASH EQUIVALENTS AT END OF YEAR	$	243,702		$	207,352		$	237,816

Quarterly_Financial_Informatio1

Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results Of Consolidated Operations	The following table presents the unaudited quarterly results of consolidated operations for 2014, 2013 and 2012.
		Quarters Ended
	(dollars in thousands, except per share amounts)	Mar. 31		30-Jun			Sept. 30		Dec. 31
	2014
	Operating revenues	$	1,239,655	$	1,258,971		$	1,299,286	$	1,335,755
	Net income	87,501		41,141			76,824		118,222
	Net income to shareholders	87,716		40,068			75,803		117,595
	Comprehensive income to shareholders	230,273		250,588			36,502		418,571
	Net income per share:
	Basic	$	6.28	$	2.67		$	5.33	$	8.1
	Diluted	6.25		2.66			5.3		8.05
	Common stock price ranges:
	High	$	596.87	$	655.75		$	666	$	707.36
	Low	527.17		593.76			623.9		632.65
	2013
	Operating revenues	$	819,864	$	1,031,769		$	1,191,665	$	1,279,785
	Net income	89,263		28,676			66,967		98,939
	Net income to shareholders	88,902		27,756			65,599		98,764
	Comprehensive income (loss) to shareholders	257,684		(149,054		)	144,409		206,450
	Net income per share:
	Basic	$	9.53	$	2.24		$	4.69	$	6.98
	Diluted	9.5		2.24			4.67		6.95
	Common stock price ranges:
	High	$	510.05	$	546.94		$	549.09	$	582.59
	Low	434.98		501.76			506.64		511.06
	2012
	Operating revenues	$	733,135	$	693,247		$	765,775	$	807,955
	Net income	57,713		90,768			51,674		58,093
	Net income to shareholders	57,253		89,687			49,653		56,792
	Comprehensive income to shareholders	205,945		73,416			147,454		76,987
	Net income per share:
	Basic	$	5.94	$	8.44		$	5.33	$	6.25
	Diluted	5.92		8.42			5.32		6.23
	Common stock price ranges:
	High	$	451.9	$	453.5		$	459.9	$	502.2
	Low	398.65		421			420		425.17

Summary_Of_Significant_Account2

Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	1-May-13
Summary of Significant Accounting Policies [Line Items]
Deferred policy acquisition cost amortization period	1 year
Adjustment of redeemable noncontrolling interests	$8,200,000	($2,000,000)	$3,100,000
Tax benefit greater than 50%	50.00%
Insurance premiums revenue recognition period	1 year
Stock-based compensation expense, net of taxes	18,700,000	18,400,000	4,400,000
Cumulative foreign currency translation loss	($43,500,000)	($11,200,000)
Alterra Capital Holdings [Member]
Summary of Significant Accounting Policies [Line Items]
Percentage of issued and outstanding common stock acquired				100.00%

Recovered_Sheet1

Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) (Maximum [Member], Cash And Cash Equivalents [Member])	12 Months Ended
	Dec. 31, 2014
Maximum [Member] | Cash And Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Investment maturity period	90 days

Recovered_Sheet2

Summary of Significant Accounting Policies (Goodwill and Intangible Assets) (Details)	12 Months Ended
	Dec. 31, 2014
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, estimated useful life	5 years
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, estimated useful life	20 years

Recovered_Sheet3

Summary of Significant Accounting Policies (Property and Equipment) (Details)	12 Months Ended
	Dec. 31, 2014
Building [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	10 years
Building [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	40 years
Land Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Land Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	40 years
Furniture and Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	3 years
Furniture and Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	10 years
Other Property And Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	3 years
Other Property And Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life	25 years

Acquisitions_Narrative_Details

Acquisitions (Narrative) (Details) (USD $)	12 Months Ended				0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012	Jan. 01, 2013	1-May-13	Jan. 17, 2014	Jul. 23, 2014
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period	16 years 9 months 22 days
United States corporate tax rate	35.00%	35.00%		35.00%
Provision for United States income taxes on earnings of the Company's foreign subsidiaries	$0
Acceleration of Alterra long-term incentive compensation awards and restricted stock awards		12,621,000	[1]
Transaction costs		15,981,000	[1]
Pro forma transaction costs, total		39,000,000
Other acquisition related costs, severance costs		31,734,000	[1]
Other acquisition related costs, stay bonuses		14,804,000	[1]
Customer Relationships [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period	18 years 7 months 9 days
Trade Names [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period	12 years 6 months 2 days
Essentia Insurance Company [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Percentage of outstanding shares acquired					100.00%
Other intangible assets recognized, asset acquisition					35,400,000
Percentage of outstanding shares of Essentia purchased by Hagerty due to the exercise of options					9.90%
Company's ownership interest in Essentia					90.10%
Essentia Insurance Company [Member] | Customer Relationships [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Definite-lived intangible assets, weighted-average amortization period					6 years
Other intangible assets recognized, asset acquisition					25,000,000
Alterra Capital Holdings [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Percentage acquired						100.00%
Number of Markel Corporation shares issued for each outstanding share of Alterra common stock						0.04315
Cash paid for each oustanding share of Alterra common stock						$10
Total purchase consideration						3,304,293,000
Cash consideration						964,330,000
Stock consideration, shares						4,300,000
Options issued, taxes						1,900,000
Restricted stock issued, taxes						10,100,000
Restricted stock units issued, taxes						700,000
Goodwill recognized						295,690,000
Goodwill, tax deductible						0
Indefinite-lived intangible assets						37,500,000
Finite-lived intangible assets						170,000,000
Definite-lived intangible assets, weighted-average amortization period						17 years
Investments acquired, adjustment to fair value						223,100,000
Investments acquired, incremental premium to be amortized to net investment income						495,500,000
Investments, unamortized incremental premium	281,100,000	398,100,000
Investments, amortization expense on incremental premium	59,300,000	58,300,000
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
Unpaid losses and loss adjustment expenses, unamortized fair value adjustment	114,600,000	136,500,000
Life and annuity benefits acquired, adjustment to fair value						329,600,000
Senior long-term debt acquired, adjustment to fair value						71,900,000
Senior long-term debt acquired, unamortized premium	56,700,000	66,100,000
United States corporate tax rate	35.00%
Net deferred tax assets recognized						310,100,000
Deferred tax assets recognized, accrued losses and loss adjustment expenses and life and annuity benefits						343,900,000
Deferred tax liabilities recognized, intangible assets						64,600,000
Provision for United States income taxes on earnings of the Company's foreign subsidiaries	0
Pro forma transaction costs, Alterra		23,000,000
Alterra Capital Holdings [Member] | Customer Relationships [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Finite-lived intangible assets						132,000,000
Definite-lived intangible assets, weighted-average amortization period						18 years
Alterra Capital Holdings [Member] | Trade Names [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Finite-lived intangible assets						1,000,000
Definite-lived intangible assets, weighted-average amortization period						1 year
Abbey Protection plc [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Percentage acquired							100.00%
Total purchase consideration							190,700,000
Cash consideration							190,700,000
Goodwill recognized							65,800,000
Goodwill, tax deductible							0
Finite-lived intangible assets							113,400,000
Abbey Protection plc [Member] | Customer Relationships [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Finite-lived intangible assets							103,500,000
Definite-lived intangible assets, weighted-average amortization period							20 years
Abbey Protection plc [Member] | Trade Names [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Finite-lived intangible assets							9,900,000
Definite-lived intangible assets, weighted-average amortization period							14 years
Abbey Protection plc [Member] | International Insurance [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Goodwill recognized							43,000,000
Abbey Protection plc [Member] | Non-Insurance Operations [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Goodwill recognized							22,800,000
Cottrell Inc [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Percentage acquired								100.00%
Non-Insurance Acquisitions [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Total purchase consideration	187,000,000
Goodwill recognized	38,700,000
Finite-lived intangible assets	78,700,000
Non-Insurance Acquisitions [Member] | Customer Relationships [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Finite-lived intangible assets	53,700,000
Definite-lived intangible assets, weighted-average amortization period	17 years
Non-Insurance Acquisitions [Member] | Trade Names [Member]
Schedule of Asset and Business Acquisitions, by Acquisition [Line Items]
Finite-lived intangible assets	$13,000,000
Definite-lived intangible assets, weighted-average amortization period	10 years
[1]	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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) (Alterra Capital Holdings [Member], USD $)	0 Months Ended
In Thousands, except Share data, unless otherwise specified	1-May-13		1-May-13
Business Acquisition [Line Items]
Exchange ratio per the Merger Agreement	0.04315		0.04315
Markel share issuance to Alterra shareholders	4,300,000
Incentive award ratio per the Merger Agreement	0.06252		0.06252
Multiplied by Markel's weighted average stock price on April 30, 2013	$529.59	[1]	$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		96,433,000
Multiplied by cash price per share component per the Merger Agreement	$10		$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		-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		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		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_Fair_V

Acquisitions (Summary Of Fair Value Of Assets Acquired And Liabilities Assumed) (Details) (Alterra Capital Holdings [Member], USD $)	1-May-13
In Thousands, unless otherwise specified
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	859,884
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	223,108
Net assets	2,801,103
Goodwill	295,690
Intangible assets	207,500
Acquisition date fair value	$3,304,293

Acquisitions_Summary_Of_Intang

Acquisitions (Summary Of Intangible Assets Acquired) (Details) (USD $)	12 Months Ended	0 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	1-May-13
Schedule of Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years	16 years 9 months 22 days
Customer Relationships [Member]
Schedule of Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years	18 years 7 months 9 days
Technology [Member]
Schedule of Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years	7 years
Trade Names [Member]
Schedule of Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years	12 years 6 months 2 days
Alterra Capital Holdings [Member]
Schedule of Intangible Assets [Line Items]
Finite-lived intangible assets		$170,000
Indefinite-lived intangible assets		37,500
Economic useful lives of finite-lived intangible assets, years		17 years
Intangible assets as of the Acquisition Date		207,500
Alterra Capital Holdings [Member] | Lloyd's Syndicate Capacity [Member]
Schedule of Intangible Assets [Line Items]
Indefinite-lived intangible assets		12,000
Economic useful lives of indefinite-lived intangible assets, years		Indefinite
Alterra Capital Holdings [Member] | Insurance Licenses [Member]
Schedule of Intangible Assets [Line Items]
Indefinite-lived intangible assets		25,500
Economic useful lives of indefinite-lived intangible assets, years		Indefinite
Alterra Capital Holdings [Member] | Customer Relationships [Member]
Schedule of Intangible Assets [Line Items]
Finite-lived intangible assets		132,000
Economic useful lives of finite-lived intangible assets, years		18 years
Alterra Capital Holdings [Member] | Broker Relationships [Member]
Schedule of Intangible Assets [Line Items]
Finite-lived intangible assets		19,000
Economic useful lives of finite-lived intangible assets, years		18 years
Alterra Capital Holdings [Member] | Technology [Member]
Schedule of Intangible Assets [Line Items]
Finite-lived intangible assets		18,000
Economic useful lives of finite-lived intangible assets, years		10 years
Alterra Capital Holdings [Member] | Trade Names [Member]
Schedule of Intangible Assets [Line Items]
Finite-lived intangible assets		$1,000
Economic useful lives of finite-lived intangible assets, years		1 year

Acquisitions_Summary_Of_Transa

Acquisitions (Summary Of Transaction And Acquisition-Related Costs) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Acquisitions [Abstract]
Transaction costs			$15,981	[1]
Severance costs			31,734	[1]
Stay bonuses			14,804	[1]
Acceleration of Alterra long-term incentive compensation awards and restricted stock awards			12,621	[1]
Total transaction and acquisition-related costs	$0	[1]	$75,140	[1],[2]	$0	[1]
[1]	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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 transaction costs of $16.0 million for the year ended DecemberB 31, 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 $14.8 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_Summary_Of_Result

Acquisitions (Summary Of Results Of Operations) (Details) (Alterra Capital Holdings [Member], USD $)	8 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013
Alterra Capital Holdings [Member]
Business Acquisition [Line Items]
Operating revenues	$912,387
Net loss to shareholders	($93,074)

Acquisitions_Unaudited_Pro_For

Acquisitions (Unaudited Pro Forma Consolidated Financial Information) (Details) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Acquisitions [Abstract]
Earned premiums	$3,680,220		$3,509,834
Operating revenues	4,899,628		4,561,107
Net income to shareholders	$422,829		$308,496
U.S. GAAP combined ratio	95.00%	[1]	99.00%	[1]
Basic net income per share	$30.33		$21.79
Diluted net income per share	$30.19		$21.71
Weighted average common shares outstanding, basic	14,007		14,014
Weighted average common shares outstanding, diluted	14,069		14,065
[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 $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	securities	securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities in unrealized loss position, number of positions	552	1,364
Available-for-sale securities, estimated fair value	$2,031,032,000	$4,859,071,000
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-35,218,000	-211,904,000
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	396	9
Available-for-sale securities, estimated fair value, 12 months or longer	1,333,930,000	31,247,000
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-22,079,000	-546,000
Estimated average duration of the fixed maturities	5 years 9 months 22 days
Cumulative credit loss recognized in income	12,700,000	12,700,000	21,400,000
Other-than-temporary impairment losses recognized in net income	-4,784,000	-4,706,000	-12,078,000
Percentage threshold of shareholders' equity used to define concentration of investments	10.00%	10.00%
Ten largest equity holdings	1,900,000,000
Ten largest equity holdings, percentage of equity portfolio	45.00%
Investments in property and casualty	756,300,000
Investments in property and casualty, percentage of equity portfolio	18.00%
Berkshire Hathaway Inc. [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments in property and casualty	470,700,000
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value	1,746,787,000	4,810,249,000
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-27,847,000	-210,786,000
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	396	8
Available-for-sale securities, estimated fair value, 12 months or longer	1,333,930,000	3,512,000
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-22,079,000	-146,000
Other-than-temporary impairment losses recognized in net income	-300,000	-1,900,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value	102,281,000	48,822,000
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-7,365,000	-1,118,000
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end		1
Available-for-sale securities, estimated fair value, 12 months or longer	0	27,735,000
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	-400,000
Other-than-temporary impairment losses recognized in net income	($4,500,000)	($2,800,000)	($12,100,000)
Alterra Capital Holdings [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	391

Investments_AvailableForSale_I

Investments (Available-For-Sale Investments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	$13,475,614	$13,147,964
Available-for-sale investments, gross unrealized holding gains	2,714,911	1,910,562
Available-for-sale investments, gross unrealized holding losses	-31,141	-204,757
Available-for-sale investments, unrealized other-than-temporary impairment losses	-4,077	-7,147
Available-for-sale investments, estimated fair value	16,155,307	14,846,622
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	662,462	1,215,522
Available-for-sale investments, gross unrealized holding gains	12,963	9,051
Available-for-sale investments, gross unrealized holding losses	-2,163	-30,342
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	673,262	1,194,231
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	4,075,748	2,986,758
Available-for-sale investments, gross unrealized holding gains	245,158	116,341
Available-for-sale investments, gross unrealized holding losses	-3,359	-27,384
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,317,547	3,075,715
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,458,255	1,484,818
Available-for-sale investments, gross unrealized holding gains	154,707	30,647
Available-for-sale investments, gross unrealized holding losses	-1,041	-54,411
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,611,921	1,461,054
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	427,904	379,555
Available-for-sale investments, gross unrealized holding gains	5,325	62
Available-for-sale investments, gross unrealized holding losses	-2,602	-11,796
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	430,627	367,821
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	954,263	875,902
Available-for-sale investments, gross unrealized holding gains	34,324	13,046
Available-for-sale investments, gross unrealized holding losses	-3,482	-16,442
Available-for-sale investments, unrealized other-than-temporary impairment losses	-2,258	-2,258
Available-for-sale investments, estimated fair value	982,847	870,248
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	100,073	189,646
Available-for-sale investments, gross unrealized holding gains	99	257
Available-for-sale investments, gross unrealized holding losses	-682	-1,614
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	99,490	188,289
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	2,250,432	2,996,940
Available-for-sale investments, gross unrealized holding gains	69,016	54,777
Available-for-sale investments, gross unrealized holding losses	-10,441	-61,650
Available-for-sale investments, unrealized other-than-temporary impairment losses	-1,819	-4,889
Available-for-sale investments, estimated fair value	2,307,188	2,985,178
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	9,929,137	10,129,141
Available-for-sale investments, gross unrealized holding gains	521,592	224,181
Available-for-sale investments, gross unrealized holding losses	-23,770	-203,639
Available-for-sale investments, unrealized other-than-temporary impairment losses	-4,077	-7,147
Available-for-sale investments, estimated fair value	10,422,882	10,142,536
Insurance, Banks And Other Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	523,739	422,975
Available-for-sale investments, gross unrealized holding gains	789,717	592,112
Available-for-sale investments, gross unrealized holding losses	-1,531	-4
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,311,925	1,015,083
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,427,919	1,143,578
Available-for-sale investments, gross unrealized holding gains	1,403,566	1,094,251
Available-for-sale investments, gross unrealized holding losses	-5,834	-1,114
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	2,825,651	2,236,715
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,951,658	1,566,553
Available-for-sale investments, gross unrealized holding gains	2,193,283	1,686,363
Available-for-sale investments, gross unrealized holding losses	-7,365	-1,118
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,137,576	3,251,798
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,594,819	1,452,270
Available-for-sale investments, gross unrealized holding gains	36	18
Available-for-sale investments, gross unrealized holding losses	-6	0
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	$1,594,849	$1,452,288

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 $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	$697,102	$4,827,824
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-13,139	-211,358
Available-for-sale securities, estimated fair value, 12 months or longer	1,333,930	31,247
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-22,079	-546
Available-for-sale securities, estimated fair value	2,031,032	4,859,071
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-35,218	-211,904
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	108,250	587,929
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-62	-30,342
Available-for-sale securities, estimated fair value, 12 months or longer	163,359	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-2,101	0
Available-for-sale securities, estimated fair value	271,609	587,929
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-2,163	-30,342
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	58,583	513,608
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-542	-27,238
Available-for-sale securities, estimated fair value, 12 months or longer	92,441	3,512
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-2,817	-146
Available-for-sale securities, estimated fair value	151,024	517,120
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-3,359	-27,384
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	18,856	950,040
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-386	-54,411
Available-for-sale securities, estimated fair value, 12 months or longer	56,217	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-655	0
Available-for-sale securities, estimated fair value	75,073	950,040
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-1,041	-54,411
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	45,931	357,737
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-210	-11,796
Available-for-sale securities, estimated fair value, 12 months or longer	147,558	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-2,392	0
Available-for-sale securities, estimated fair value	193,489	357,737
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-2,602	-11,796
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	9,613	437,675
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-2,285	-18,700
Available-for-sale securities, estimated fair value, 12 months or longer	207,374	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-3,455	0
Available-for-sale securities, estimated fair value	216,987	437,675
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-5,740	-18,700
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	30,448	142,011
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-20	-1,614
Available-for-sale securities, estimated fair value, 12 months or longer	45,160	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-662	0
Available-for-sale securities, estimated fair value	75,608	142,011
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-682	-1,614
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	141,176	1,817,737
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-2,263	-66,539
Available-for-sale securities, estimated fair value, 12 months or longer	621,821	0
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-9,997	0
Available-for-sale securities, estimated fair value	762,997	1,817,737
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-12,260	-66,539
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	412,857	4,806,737
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-5,768	-210,640
Available-for-sale securities, estimated fair value, 12 months or longer	1,333,930	3,512
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	-22,079	-146
Available-for-sale securities, estimated fair value	1,746,787	4,810,249
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-27,847	-210,786
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	16,219	144
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-1,531	-4
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	16,219	144
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-1,531	-4
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	86,062	20,943
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-5,834	-714
Available-for-sale securities, estimated fair value, 12 months or longer	0	27,735
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	-400
Available-for-sale securities, estimated fair value	86,062	48,678
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-5,834	-1,114
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	102,281	21,087
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-7,365	-718
Available-for-sale securities, estimated fair value, 12 months or longer	0	27,735
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	0	-400
Available-for-sale securities, estimated fair value	102,281	48,822
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	-7,365	-1,118
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, estimated fair value, less than 12 months	181,964
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, less than 12 months	-6
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	181,964
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	($6)

Investments_Schedule_Of_Amorti

Investments (Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturities By Contractual Maturity) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, amortized cost	$747,997
Due after one year through five years, amortized cost	2,007,798
Due after five years through ten years, amortized cost	2,026,628
Due after ten years, amortized cost	3,664,474
Amortized cost, sub-total	8,446,897
Fixed maturities, amortized cost	9,929,137	10,129,141
Due in one year or less, estimated fair value	752,715
Due after one year through five years, estimated fair value	2,059,192
Due after five years through ten years, estimated fair value	2,142,582
Due after ten years, estimated fair value	3,955,429
Estimated fair value, sub-total	8,909,918
Fixed maturities, estimated fair value	10,422,882	10,142,536
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	427,904
Estimated fair value	430,627
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	954,263
Estimated fair value	982,847
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	100,073
Estimated fair value	$99,490

Investments_Components_Of_Net_

Investments (Components Of Net Investment Income) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Municipal bonds (tax-exempt)	$98,262	$82,308	$90,316
Short-term investments, including overnight deposits	5,959	3,573	2,755
Dividends on equity securities	65,031	48,641	50,416
Change in fair value of credit default swap	2,230	10,460	16,641
Income from equity method investments	4,766	21,898	1,961
Other	108	355	-41
Total investment income	378,490	329,653	291,981
Investment expenses	-15,260	-12,280	-9,874
Net investment income	363,230	317,373	282,107
Taxable Municipal Bonds [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Taxable bonds	49,345	28,041	22,663
Other Taxable Bonds [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Taxable bonds	$152,789	$134,377	$107,270

Investments_Summary_Of_Net_Rea

Investments (Summary Of Net Realized Investment Gains And The Change In Net Unrealized Gains On Investments) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Realized gains	$74,978	$93,304	$48,664
Realized losses	-28,978	-30,152	-17,071
Other-than-temporary impairment losses recognized in net income	-4,784	-4,706	-12,078
Net realized investment gains	46,000	63,152	31,593
Change in net unrealized gains on investments	981,035	261,995	353,808
Sales Of Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Realized gains	8,417	13,772	18,337
Realized losses	-18,136	-25,168	-563
Sales Of Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Realized gains	51,356	73,592	29,578
Realized losses	-802	-278	-342
Other [Member]
Gain (Loss) on Investments [Line Items]
Realized gains	15,205	5,940	749
Realized losses	-5,256	0	-4,088
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses recognized in net income	-300	-1,900
Change in net unrealized gains on investments	480,350	-403,610	51,783
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses recognized in net income	-4,500	-2,800	-12,100
Change in net unrealized gains on investments	500,673	665,599	302,013
Short-Term Investments [Member]
Gain (Loss) on Investments [Line Items]
Change in net unrealized gains on investments	$12	$6	$12

Investments_Investments_Compon

Investments Investments (Components Of Restricted Assets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Components Of Restricted Assets [Line Items]
Restricted assets	$5,596,401	$6,089,379
Held In Trust Or On Deposit [Member]
Components Of Restricted Assets [Line Items]
Restricted assets	4,961,061	5,161,871
Cash And Cash Equivalents And Securities Pledged As Collateral [Member]
Components Of Restricted Assets [Line Items]
Restricted assets	635,340	695,072
Pledged Investments And Cash And Cash Equivalents As Collateral For Credit Default Swap [Member]
Components Of Restricted Assets [Line Items]
Restricted assets	0	33,430
Cash Held In Escrow [Member]
Components Of Restricted Assets [Line Items]
Restricted assets	$0	$199,006

Investments_Schedule_Of_Restri

Investments (Schedule Of Restricted Assets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]
Investments, available-for-sale	$5,040,413	$5,225,701
Restricted cash and cash equivalents	522,225	786,926
Other assets	33,763	76,752
Total	$5,596,401	$6,089,379

Receivables_Components_Of_Rece

Receivables (Components Of Receivables) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Receivables [Abstract]
Amounts receivable from agents, brokers and insureds	$1,031,519	$1,058,021
Trade accounts receivable	97,225	85,203
Employee stock loans receivable (see note 12(c))	15,044	12,822
Other	8,601	5,420
Receivables, gross	1,152,389	1,161,466
Allowance for doubtful receivables	-17,172	-19,693
Receivables	$1,135,217	$1,141,773

Deferred_Policy_Acquisition_Co2

Deferred Policy Acquisition Costs (Amounts Of Policy Acquisition Costs Acquired, Deferred And Amortized) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of year	$260,967	$157,465	$194,674
Policy acquisition costs deferred	754,303	577,620	390,360
Amortization of policy acquisition costs	-654,916	-471,915	-428,109
Foreign currency movements	-6,944	-2,203	540
Deferred policy acquisition costs	$353,410	$260,967	$157,465

Deferred_Policy_Acquisition_Co3

Deferred Policy Acquisition Costs (Components Of Underwriting, Acquisition And Insurance Expenses) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Deferred Policy Acquisition Costs Disclosures [Abstract]
Transaction costs and other acquisition-related expenses	$0	[1]	$75,140	[1],[2]	$0	[1]
Transaction costs			15,981	[1]
Other acquisition related costs, severance costs			31,734	[1]
Other acquisition related costs, stay bonuses			14,804	[1]
Other acquisition related costs, other compensation costs			12,621	[1]
Prospective adoption of FASB ASU No. 2010-26	0	[3]	0	[3]	43,093	[3]
Other amortization of policy acquisition costs	654,916		471,915		385,016
Other operating expenses	805,966		765,257		501,363
Underwriting, acquisition and insurance expenses	$1,460,882		$1,312,312		$929,472
[1]	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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 transaction costs of $16.0 million for the year ended DecemberB 31, 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 $14.8 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.

Property_And_Equipment_Compone

Property And Equipment (Components Of Property And Equipment) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Abstract]
Land	$56,848,000	$48,036,000
Buildings	78,786,000	59,307,000
Leasehold improvements	98,098,000	68,363,000
Land improvements	70,596,000	57,673,000
Furniture and equipment	255,566,000	217,528,000
Other	116,884,000	104,690,000
Property and equipment, gross	676,778,000	555,597,000
Accumulated depreciation and amortization	-255,388,000	-207,688,000
Property and equipment	421,390,000	347,909,000
Depreciation and amortization expense of property and equipment	$51,200,000	$51,500,000	$36,000,000

Goodwill_And_Intangible_Assets2

Goodwill And Intangible Assets (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill And Intangible Assets [Line Items]
Goodwill impairment	($13,737,000)	$0	$0
Amortization of intangible assets	57,627,000	55,223,000	33,512,000
Estimated amortization of intangible assets for 2015	58,300,000
Estimated amortization of intangible assets for 2016	55,200,000
Estimated amortization of intangible assets for 2017	52,800,000
Estimated amortization of intangible assets for 2018	52,500,000
Estimated amortization of intangible assets for 2019	46,700,000
Indefinite-lived intangible assets	58,000,000	58,000,000
Intangible assets acquired during period	$204,400,000
Definite-lived intangible assets, weighted-average amortization period	16 years 9 months 22 days
Customer Relationships [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	18 years 7 months 9 days
Trade Names [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	12 years 6 months 2 days
Technology [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	7 years

Goodwill_And_Intangible_Assets3

Goodwill And Intangible Assets (Components Of Goodwill) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Goodwill [Line Items]
Goodwill, beginning balance	$967,717,000		$674,930,000
Acquisitions (see note 2)	104,528,000		296,875,000
Impairment loss	-13,737,000		0		0
Foreign currency movements and other adjustments	-9,393,000		-4,088,000
Goodwill, ending balance	1,049,115,000	[1]	967,717,000		674,930,000
U.S. Insurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance	280,579,000		172,824,000
Acquisitions (see note 2)	0		107,755,000
Impairment loss	0
Foreign currency movements and other adjustments	0		0
Goodwill, ending balance	280,579,000	[1]	280,579,000
International Insurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance	372,764,000		309,662,000
Acquisitions (see note 2)	42,989,000		65,190,000
Impairment loss	0
Foreign currency movements and other adjustments	-7,570,000		-2,088,000
Goodwill, ending balance	408,183,000	[1]	372,764,000
Reinsurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance	122,745,000		0
Acquisitions (see note 2)	0		122,745,000
Impairment loss	0
Foreign currency movements and other adjustments	0		0
Goodwill, ending balance	122,745,000	[1]	122,745,000
Other [Member]
Goodwill [Line Items]
Goodwill, beginning balance	191,629,000	[2]	192,444,000	[2]
Acquisitions (see note 2)	61,539,000	[2]	1,185,000	[2]
Impairment loss	-13,737,000	[2]
Foreign currency movements and other adjustments	-1,823,000	[2]	-2,000,000	[2]
Accumulated impairment losses	13,700,000	[1]
Goodwill, ending balance	$237,608,000	[1],[2]	$191,629,000	[2]
[1]	Goodwill is net of accumulated impairment losses of $13.7 million included in Other.
[2]	Amounts included in Other above are related to the Company's non-insurance operations, which are not included in a reportable segment.

Goodwill_And_Intangible_Assets4

Goodwill And Intangible Assets (Components Of Intangible Assets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	$855,809	$666,270
Intangible assets, accumulated amortization	-153,062	-101,187
Customer Relationships [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	452,157	298,886
Intangible assets, accumulated amortization	-69,483	-43,286
Broker Relationships [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	175,681	178,693
Intangible assets, accumulated amortization	-34,827	-23,255
Trade Names [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	94,795	65,880
Intangible assets, accumulated amortization	-17,673	-12,666
Technology [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	62,288	56,429
Intangible assets, accumulated amortization	-22,671	-16,222
Insurance Licenses [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	39,985	40,185
Intangible assets, accumulated amortization	0	-133
Lloyd's Syndicate Capacity [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	12,000	12,000
Intangible assets, accumulated amortization	0	0
Other [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross carrying amount	18,903	14,197
Intangible assets, accumulated amortization	($8,408)	($5,625)

Income_Taxes_Narrative_Details

Income Taxes (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Income tax payments	$89,500,000	$35,700,000	$30,000,000
Income taxes payable	37,600,000	84,200,000
Tax credit carryforwards	32,525,000	33,773,000
Tax credit, expiration date	31-Dec-19
Net operating loss carryforwards	61,200,000
Estimated gross deferred tax assets including net operating losses realized through reversal of existing temporary differences attributable to gross deferred tax liabilities	716,700,000
Unrecognized tax benefits	17,700,000	18,219,000	18,870,000
Unrecognized tax benefits decrease on effective tax rate	15,600,000
Provision for United States income taxes on earnings of the Company's foreign subsidiaries	0
Earliest Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards, expiration date	31-Dec-27
Latest Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards, expiration date	31-Dec-33
Markel Capital Limited [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$61,900,000
Markel Capital Limited [Member] | Earliest Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards, expiration date	31-Dec-21
Markel Capital Limited [Member] | Latest Tax Year [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards, expiration date	31-Dec-32

Income_Taxes_Components_Of_Inc

Income Taxes (Components Of Income Before Income Taxes) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Domestic operations	$240,279	$325,133	$126,466
Foreign operations	200,099	36,610	185,584
Income Before Income Taxes	$440,378	$361,743	$312,050

Income_Taxes_Components_Of_Inc1

Income Taxes (Components Of Income Tax Expense) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Domestic, current	$7,573	$50,683	$14,340
Foreign, current	24,574	23,165	1,814
Total current tax expense	32,147	73,848	16,154
Domestic, deferred	43,673	23,906	-3,734
Foreign, deferred	40,870	-19,856	41,382
Total deferred tax expense	84,543	4,050	37,648
Income tax expense	$116,690	$77,898	$53,802

Income_Taxes_Reconciliations_O

Income Taxes (Reconciliations Of United States Corporate Income Tax Rate To Effective Tax Rate On Income Before Income Taxes) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
United States corporate tax rate	35.00%	35.00%	35.00%
Tax-exempt investment income	-9.00%	-9.00%	-12.00%
Tax credits	-1.00%	0.00%	0.00%
Foreign operations	0.00%	-4.00%	-5.00%
Other	1.00%	0.00%	-1.00%
Effective tax rate	26.00%	22.00%	17.00%

Income_Taxes_Components_Of_Dom

Income Taxes (Components Of Domestic And Foreign Deferred Tax Assets And Liabilities) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Income Tax Disclosure [Abstract]
Unpaid losses and loss adjustment expenses	$239,588	$283,365
Life and annuity benefits	143,102	161,209
Unearned premiums recognized for income tax purposes	108,960	76,862
Net operating loss carryforwards	36,359	43,010
Tax credit carryforwards	32,525	33,773
Other-than-temporary impairments not yet deductible for income tax purposes	28,106	34,978
Differences between financial reporting and tax bases	132,878	139,357
Total gross deferred tax assets	721,518	772,554
Less valuation allowance	-4,801	0
Total gross deferred tax assets, net of allowance	716,717	772,554
Net unrealized gains on investments	759,212	459,015
Amortization of goodwill and other intangible assets	106,927	97,580
Deferred policy acquisition costs	101,766	65,543
Differences between financial reporting and tax bases	59,359	46,699
Total gross deferred tax liabilities	1,027,264	668,837
Net deferred tax liability	-310,547
Net deferred tax asset		$103,717

Income_Taxes_Reconciliation_Of

Income Taxes (Reconciliation Of Beginning And Ending Unrecognized Tax Benefits) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning of year	$18,219	$18,870
Increases for tax positions taken in prior years	3	0
Lapse of statute of limitations	-522	-651
Unrecognized tax benefits, end of year	$17,700	$18,219

Unpaid_Losses_And_Loss_Adjustm2

Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Increase (decrease) in reserves for losses and loss adjustment expenses due to movement in foreign currency rates of exchange	($127,700,000)	$700,000	$23,400,000
Net reserves for losses and loss adjustment expenses of acquired insurance companies	21,193,000	3,584,642,000	0
Favorable development on prior years' loss reserves	-435,545,000	-411,129,000	-399,002,000
Prior years' development on asbestos and environmental exposures	32,840,000	30,128,000	38,179,000
Liability for asbestos and environmental claims net incurred loss annual claim review	27,200,000	28,400,000	31,100,000
Incurred losses and loss adjustment expenses, current year	2,638,012,000	2,227,402,000	1,553,070,000
Reinsurance recoverables	423,100,000	269,400,000	165,800,000
Asbestos-related reserves, gross loss incurred	390,442,000	372,978,000	360,854,000
Asbestos-related reserves, net loss incurred	287,723,000	272,194,000	260,791,000	244,772,000
Asbestos and environmental net reserves for reported claims	173,800,000
Asbestos and environmental reserves incurred but not reported, net	113,900,000
Liability for asbestos and environmental claims, net claims paid since inception	415,700,000
Liability for asbestos and environmental claims, net litigation paid since inception	85,700,000
Hurricane Sandy [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current year			99,600,000
Reinsurance recoverables			77,600,000
Asbestos [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Asbestos-related reserves, gross loss incurred	290,100,000
Asbestos-related reserves, net loss incurred	218,900,000
Long-Tail Casualty And Professional Liability Witihin U.S. Insurance Segment And Professional Liability And Marine And Energy International Segment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development on prior years' loss reserves	($250,400,000)	($255,200,000)	($292,300,000)

Unpaid_Losses_And_Loss_Adjustm3

Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Net reserves for losses and loss adjustment expenses, beginning of year	$8,407,642	$4,592,652	$4,607,767
Foreign currency movements, commutations and other	-137,385	-780	30,359
Adjusted net reserves for losses and loss adjustment expenses, beginning of year	8,270,257	4,591,872	4,638,126
Incurred losses and loss adjustment expenses, current year	2,638,012	2,227,402	1,553,070
Incurred losses and loss adjustment expenses, prior years	-435,545	-411,129	-399,002
Total incurred losses and loss adjustment expenses	2,202,467	1,816,273	1,154,068
Payments, current year	502,107	670,928	268,745
Payments, prior years	1,436,851	906,302	931,955
Total payments	1,938,958	1,577,230	1,200,700
Effect of foreign currency rate changes	-19,476	-7,915	1,158
Net reserves for losses and loss adjustment expenses of acquired insurance companies	21,193	3,584,642	0
Net reserves for losses and loss adjustment expenses, end of year	8,535,483	8,407,642	4,592,652
Reinsurance recoverable on unpaid losses	1,868,669	1,854,414	778,774
Gross reserves for losses and loss adjustment expenses, end of year	$10,404,152	$10,262,056	$5,371,426

Unpaid_Losses_And_Loss_Adjustm4

Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated A&E Reserves For Losses And Loss Adjustment Expenses) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Net reserves for A&E losses and loss adjustment expenses, beginning of year	$272,194	$260,791	$244,772
Commutations and other	115	-5,067	-897
Adjusted net reserves for A&E losses and loss adjustment expenses, beginning of year	272,309	255,724	243,875
Incurred losses and loss adjustment expenses	32,840	30,128	38,179
Payments	-17,426	-13,658	-21,263
Net reserves for A&E losses and loss adjustment expenses, end of year	287,723	272,194	260,791
Reinsurance recoverable on unpaid losses	102,719	100,784	100,063
Gross reserves for A&E losses and loss adjustment expenses, end of year	$390,442	$372,978	$360,854

Life_And_Annuity_Benefits_Narr

Life And Annuity Benefits (Narrative) (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Liability for Future Policy Benefits [Abstract]
Average reserve valuation rate	2.30%
Life and annuity benefits reserve single contract concentration	33.50%
Annuities included in life and annuity benefits subject to discretionary withdrawal	$0

Life_And_Annuity_Benefits_Sche

Life And Annuity Benefits (Schedule Of Life And Annuity Benefits) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Liability for Future Policy Benefits [Abstract]
Life	$182,604	$190,765
Annuities	1,031,946	1,194,558
Accident and health	91,268	101,251
Total life and annuity benefits	$1,305,818	$1,486,574

Senior_LongTerm_Debt_And_Other2

Senior Long-Term Debt And Other Debt (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Aug. 01, 2014	Mar. 08, 2013	1-May-13	Apr. 16, 2007	Sep. 27, 2010
Debt Instrument [Line Items]
Senior long-term debt and other debt, estimated fair value	$2,493,000,000	$2,372,000,000
Interest on senior long-term debt and other debt	125,800,000	114,500,000	92,900,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, capacity available				300,000,000
Line of credit facility, potential increased maximum capacity				500,000,000
Line of credit facility, unused capacity, commitment fee, percent	0.23%
Borrowings outstanding under the facility	0
Previous line of credit facility, borrowing capacity				300,000,000
Senior Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, capacity available	250,000,000
Line of credit facility, unused capacity, commitment fee, percent	0.13%
Borrowings outstanding under the facility	0	0
Previous line of credit facility, borrowing capacity				900,000,000
Credit facility, maximum capacity				650,000,000
Credit facility, potential increased maximum capacity	1,400,000,000
Line of credit facility, outstanding balance fee, percent	0.40%
Credit facility, letters of credit issued and outstanding	374,500,000
Secured Letters Of Credit [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, capacity available				200,000,000
Unsecured Senior Notes [Member] | 6.80% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	6.80%
Debt instrument, principal amount outstanding			246,700,000
Unsecured Senior Notes [Member] | 3.625% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	3.63%	3.63%
Debt instrument, face amount					250,000,000
Unsecured Senior Notes [Member] | 5.0% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	5.00%	5.00%
Debt instrument, face amount					250,000,000
Unsecured Senior Notes [Member] | 3.625% And 5.0% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Net proceeds from issuance of debt					491,200,000
Unsecured Senior Notes [Member] | 7.20% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	7.20%	7.20%
Debt instrument, principal amount outstanding						90,600,000
Debt instrument, face amount							100,000,000
Debt instrument, estimated fair value						95,800,000
Unsecured Senior Notes [Member] | 6.25% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	6.25%	6.25%
Debt instrument, face amount								350,000,000
Debt instrument, estimated fair value						$416,600,000
Unsecured Senior Notes [Member] | 7.35% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate	7.35%	7.35%

Senior_LongTerm_Debt_And_Other3

Senior Long-Term Debt And Other Debt (Summary Of Senior Long-Term Debt And Other Debt) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Senior long-term debt and other debt	$2,253,594	$2,256,227
Unsecured Senior Notes [Member] | 7.20% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	94,155	95,451
Debt instrument, interest rate	7.20%	7.20%
Debt Instrument, unamortized premium	3,526	4,822
Unsecured Senior Notes [Member] | 7.125% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	348,657	348,374
Debt instrument, interest rate	7.13%	7.13%
Debt instrument, unamortized discount	1,343	1,626
Unsecured Senior Notes [Member] | 6.25% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	403,172	411,273
Debt instrument, interest rate	6.25%	6.25%
Debt Instrument, unamortized premium	53,172	61,273
Unsecured Senior Notes [Member] | 5.35% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	248,675	248,469
Debt instrument, interest rate	5.35%	5.35%
Debt instrument, unamortized discount	1,325	1,531
Unsecured Senior Notes [Member] | 4.90% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	347,905	347,626
Debt instrument, interest rate	4.90%	4.90%
Debt instrument, unamortized discount	2,095	2,374
Unsecured Senior Notes [Member] | 3.625% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	248,341	248,140
Debt instrument, interest rate	3.63%	3.63%
Debt instrument, unamortized discount	1,659	1,860
Unsecured Senior Notes [Member] | 7.35% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	197,922	197,815
Debt instrument, interest rate	7.35%	7.35%
Debt instrument, unamortized discount	2,078	2,185
Unsecured Senior Notes [Member] | 5.0% Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Unsecured senior notes	243,673	243,449
Debt instrument, interest rate	5.00%	5.00%
Debt instrument, unamortized discount	6,327	6,551
Subsidiary And Other Debt [Member]
Debt Instrument [Line Items]
Senior long-term debt and other debt	$121,094	$115,630
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 $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Debt Disclosure [Abstract]
2015	$23,340
2016	21,850
2017	112,942
2018	2,348
2019	352,199
2020 and thereafter	1,699,044
Total principal payments	2,211,723
Net unamortized premium	41,871
Senior long-term debt and other debt	$2,253,594	$2,256,227

Shareholders_Equity_Narrative_

Shareholders' Equity (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended	1 Months Ended	8 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	1-May-13	31-May-13	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized	50,000,000	50,000,000				50,000,000
Common stock, par value	$0	$0				$0
Common stock, shares issued	13,961,675	13,985,620				13,985,620
Common stock, shares outstanding	13,961,675	13,985,620				13,985,620
Preferred stock, shares authorized	10,000,000	10,000,000				10,000,000
Preferred stock, par value	$0	$0				$0
Preferred stock, shares issued	0	0				0
Preferred stock, shares outstanding	0	0				0
November 2013 Share Repurchase Program [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Board of Directors approved the repurchase of common stock under a share repurchase program, maximum	$300,000,000
Cumulative number of common stock shares repurchased	34,385
Cumulative stock repurchased value	20,500,000
Employee Stock Purchase And Bonus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Bonus calculation percentage based on the value of stocks acquired by employees	10.00%
Percentage of bonus shares award to loan program participants	5.00%
Shares authorized under Employee Stock Purchase and Bonus Plan	100,000
Shares available for purchase	20,740	33,454				33,454
Loans outstanding under Employee Stock Purchase and Bonus Plan	15,000,000	12,800,000				12,800,000
2012 Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for issuance	145,253
2012 Compensation Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of restricted stock units awarded	28,280
Weighted average grant-date fair value, share-based awards	$583.74	$517.24	$412.04
Unrecognized compensation cost related to nonvested share-based awards	22,800,000
Weighted average remaining service period of share-based awards	2 years 2 months
Fair value of the vested share-based awards	4,200,000	2,500,000	4,700,000
2012 Compensation Plan [Member] | Restricted Stock Units (RSUs) [Member] | Non-Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of restricted stock units awarded	1,413
Restricted stock awards, year awards vest	2015
2012 Compensation Plan [Member] | Restricted Stock Units (RSUs) [Member] | Performance Based [Member] | Associates And Executive Officers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock issuable for each vested restricted stock unit	1
Number of restricted stock units awarded	17,917
Restricted stock awards, vesting period	3 years
2012 Compensation Plan [Member] | Restricted Stock Units (RSUs) [Member] | Rentention And Hiring [Member] | Associates And Executive Officers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock issuable for each vested restricted stock unit	1
Number of restricted stock units awarded	8,950
Restricted stock units, grant date fair value	5,400,000
Restricted stock awards, vesting period	3 years
Alterra Equity Award Plans [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, grant date fair value				81,600,000
Weighted average grant-date fair value, share-based awards				$529.59
Weighted average remaining service period of share-based awards	1 year			1 year
Fair value of the vested share-based awards	18,100,000	41,300,000
Number of awards issued in connection with acquisition					154,103
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	-20,600,000
Recognized compensation expense related to restricted stock awards	3,900,000	12,700,000
Unrecognized compensation cost related to nonvested restricted stock awards	600,000
Alterra Equity Award Plans [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards issued in connection with acquisition					101,875
Weighted average exercise price of options issued				$398.96
Weighted average grant-date fair value of options				$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	13,636					65,934
Cash proceeds resulting from exercise of stock options	5,600,000					24,300,000
Current tax benefits resulting from exercise of stock options	800,000					800,000
Intrinsic value of options exercised	$2,500,000					$9,400,000

Shareholders_Equity_Schedule_O

Shareholders' Equity (Schedule Of Net Income Per Share) (Details) (USD $)	3 Months Ended												12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Line Items]
Net income to shareholders	$117,595	$75,803	$40,068	$87,716	$98,764	$65,599	$27,756	$88,902	$56,792	$49,653	$89,687	$57,253	$321,182	$281,021	$253,385
Adjustment of redeemable noncontrolling interests													-8,186	1,963	-3,101
Adjusted net income to shareholders													$312,996	$282,984	$250,284
Basic common shares outstanding													13,984	12,538	9,640
Diluted shares outstanding													14,057	12,586	9,666
Basic net income per share (dollars per share)	$8.10	$5.33	$2.67	$6.28	$6.98	$4.69	$2.24	$9.53	$6.25	$5.33	$8.44	$5.94	$22.38	$22.57	$25.96
Diluted net income per share (dollars per share)	$8.05	$5.30	$2.66	$6.25	$6.95	$4.67	$2.24	$9.50	$6.23	$5.32	$8.42	$5.92	$22.27	$22.48	$25.89
Stock Options [Member]
Earnings Per Share [Line Items]
Dilutive potential common shares													11	12	6
Restricted Stock Units (RSUs) [Member]
Earnings Per Share [Line Items]
Dilutive potential common shares													62	36	20

Shareholders_Equity_Summary_Of

Shareholders' Equity (Summary Of Nonvested Share-Based Awards) (Details) (Restricted Stock Units (RSUs) [Member], USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	1-May-13
2012 Compensation Plan [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, beginning balance	98,711
Number of awards, granted	28,280
Number of awards, vested	-8,479
Number of awards, forfeited	-380
Number of awards, nonvested awards, ending balance	118,132	98,711
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, beginning balance	$449.93
Weighted average grant-date fair value, granted	$583.74	$517.24	$412.04
Weighted average grant-date fair value, vested	$491.51
Weighted average grant-date fair value, forfeited	$411
Weighted average grant-date fair value, nonvested awards, ending balance	$479.11	$449.93
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, beginning balance	68,411
Number of awards, vested	-34,119
Number of awards, forfeited	-377
Number of awards, nonvested awards, ending balance	33,915
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, beginning balance	$529.59
Weighted average grant-date fair value, granted				$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, ending balance	$529.59

Shareholders_Equity_Summary_Of1

Shareholders' Equity (Summary Of Options) (Details) (Alterra Equity Award Plans [Member], Stock Options [Member], USD $)	8 Months Ended	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014
Alterra Equity Award Plans [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding and exercisable, beginning balance, number of shares		35,941
Exercised, number of shares	65,934	13,636
Outstanding and exercisable, ending balance, number of shares	35,941	22,305
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding and exercisable, January 1, 2014, weighted average exercise price (dollars per share)		$412.39
Exercised, weighted average exercise price (dollars per share)		$413.06
Outstanding and exercisable, December 31, 2014, weighted average exercise price (dollars per share)	$412.39	$411.98
Outstanding and exercisable, December 31, 2014, weighted average remaining contractual term		1 year 8 months 16 days
Outstanding and exercisable, December 31, 2014, intrinsic value		$6.10

Other_Comprehensive_Income_Cha

Other Comprehensive Income (Change In Accumulated Other Comprehensive Income By Component, Net Of Taxes And Noncontrolling Interests) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income, beginning balance	$1,089,805	$911,337	$660,920
Other comprehensive income (loss) before reclassifications	639,147	217,750	272,474
Amounts reclassified from accumulated other comprehensive income	-24,395	-39,282	-22,057
Total other comprehensive income (loss)	614,752	178,468	250,417
Accumulated other comprehensive income, ending balance	1,704,557	1,089,805	911,337
Unrealized Holding Gains On Available-For-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income, beginning balance	1,131,507	946,933	704,719
Other comprehensive income (loss) before reclassifications	687,908	225,404	266,265
Amounts reclassified from accumulated other comprehensive income	-26,161	-40,830	-24,051
Total other comprehensive income (loss)	661,747	184,574	242,214
Accumulated other comprehensive income, ending balance	1,793,254	1,131,507	946,933
Foreign Currency [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income, beginning balance	-11,246	-1,075	-2,614
Other comprehensive income (loss) before reclassifications	-32,245	-10,171	1,539
Amounts reclassified from accumulated other comprehensive income	0	0	0
Total other comprehensive income (loss)	-32,245	-10,171	1,539
Accumulated other comprehensive income, ending balance	-43,491	-11,246	-1,075
Net Actuarial Pension Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income, beginning balance	-30,456	-34,521	-41,185
Other comprehensive income (loss) before reclassifications	-16,516	2,517	4,670
Amounts reclassified from accumulated other comprehensive income	1,766	1,548	1,994
Total other comprehensive income (loss)	-14,750	4,065	6,664
Accumulated other comprehensive income, ending balance	($45,206)	($30,456)	($34,521)

Other_Comprehensive_Income_Sch

Other Comprehensive Income (Schedule Of Tax Expense (Benefit) Of Other Comprehensive Income) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Income (Loss), Tax [Abstract]
Net holding gains arising during the period	$328,564	$93,837	$122,524
Change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period	614	-34	-49
Reclassification adjustments for net gains included in net income	-9,890	-16,382	-10,881
Change in net unrealized gains on investments	319,288	77,421	111,594
Change in foreign currency translation adjustments	1,918	-1,619	-446
Change in net actuarial pension loss	-3,687	1,015	1,991
Total	$317,519	$76,817	$113,139

Other_Comprehensive_Income_Amo

Other Comprehensive Income (Amounts Reclassified From Accumulated Other Comprehensive Income Into Income, By Component) (Details) (USD $)	3 Months Ended												12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Out Of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses recognized in net income													($4,784)	($4,706)	($12,078)
Net realized investment gains, excluding other-than-temporary impairment losses													50,784	67,858	43,671
Income Before Income Taxes													440,378	361,743	312,050
Underwriting, acquisition and insurance expenses													-1,460,882	-1,312,312	-929,472
Income Taxes													-116,690	-77,898	-53,802
Net Income	118,222	76,824	41,141	87,501	98,939	66,967	28,676	89,263	58,093	51,674	90,768	57,713	323,688	283,845	258,248
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]
Other-than-temporary impairment losses recognized in net income													-4,784	-4,706	-12,078
Net realized investment gains, excluding other-than-temporary impairment losses													40,835	61,918	47,010
Income Before Income Taxes													36,051	57,212	34,932
Income Taxes													-9,890	-16,382	-10,881
Net Income													26,161	40,830	24,051
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													-2,084	-1,934	-2,590
Income Taxes													318	386	596
Net Income													($1,766)	($1,548)	($1,994)

Fair_Value_Measurements_Narrat

Fair Value Measurements (Narrative) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on derivative instrument recognized in earnings	$2,230	$10,460	$16,641
Fair value, level 1 to level 2 transfers, amount	0	0
Fair value, level 2 to level 1 transfers, amount	0	0
Assets measured at fair value on a non-recurring basis	0	0
Liabilities measured at fair value on a non-recurring basis	$0	$0
Short-Term Investments [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment maturity period	1 year

Fair_Value_Measurements_Balanc

Fair Value Measurements (Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	$10,422,882	$10,142,536
Equity securities	4,137,576	3,251,798
Short-term investments	1,594,849	1,452,288
Total investments available-for-sale	16,155,307	14,846,622
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	10,422,882	10,142,536
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 investments available-for-sale	673,262	1,194,231
Obligations Of States, Municipalities And Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	4,317,547	3,075,715
Foreign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	1,611,921	1,461,054
Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	430,627	367,821
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	982,847	870,248
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	99,490	188,289
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	2,307,188	2,985,178
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	4,137,576	3,251,798
Insurance, Banks And Other Financial Institutions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	1,311,925	1,015,083
Industrial, Consumer And All Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments available-for-sale	2,825,651	2,236,715
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	10,422,882	10,142,536
Equity securities	4,137,576	3,251,798
Short-term investments	1,594,849	1,452,288
Total investments available-for-sale	16,155,307	14,846,622
Derivative contracts		2,230
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	0	0
Equity securities	4,137,576	3,251,798
Short-term investments	1,469,975	1,312,561
Total investments available-for-sale	5,607,551	4,564,359
Derivative contracts		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	10,422,882	10,142,536
Equity securities	0	0
Short-term investments	124,874	139,727
Total investments available-for-sale	10,547,756	10,282,263
Derivative contracts		0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	0	0
Equity securities	0	0
Short-term investments	0	0
Total investments available-for-sale	0	0
Derivative contracts		2,230
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]
Fixed maturities, estimated fair value	673,262	1,194,231
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]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	673,262	1,194,231
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]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	4,317,547	3,075,715
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]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	4,317,547	3,075,715
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]
Fixed maturities, estimated fair value	0	0
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	1,611,921	1,461,054
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	1,611,921	1,461,054
Fair Value, Measurements, Recurring [Member] | Foreign Governments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	430,627	367,821
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]
Fixed maturities, estimated fair value	0	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]
Fixed maturities, estimated fair value	430,627	367,821
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]
Fixed maturities, estimated fair value	0	0
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	982,847	870,248
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]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	982,847	870,248
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]
Fixed maturities, estimated fair value	0	0
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	99,490	188,289
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]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	99,490	188,289
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]
Fixed maturities, estimated fair value	0	0
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	2,307,188	2,985,178
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	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]
Fixed maturities, estimated fair value	2,307,188	2,985,178
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, estimated fair value	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]
Equity securities	1,311,925	1,015,083
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]
Equity securities	1,311,925	1,015,083
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]
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]
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]
Equity securities	2,825,651	2,236,715
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]
Equity securities	2,825,651	2,236,715
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]
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]
Equity securities	$0	$0

Reinsurance_Narrative_Details

Reinsurance (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	reinsurer	reinsurer
Reinsurance Disclosures [Line Items]
Number of largest company reinsurers	10	10
Reinsurance recoverables	$423.10	$269.40	$165.80
Percentage of ceded earned premiums to gross earned premiums	19.00%	18.00%	12.00%
Percentage of assumed earned premiums to net earned premiums	34.00%	31.00%	18.00%
Ten Largest Reinsurers [Member]
Reinsurance Disclosures [Line Items]
Recoverable balances as a percentage of the total balance	63.00%	62.00%
Fairfax Financial Group [Member]
Reinsurance Disclosures [Line Items]
Recoverable balances as a percentage of the total balance	10.00%

Reinsurance_Reinsurance_Allowa

Reinsurance (Reinsurance Allowance For Doubtful Accounts) (Details) (Allowance For Reinsurance Recoverable [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allowance For Reinsurance Recoverable [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Reinsurance allowance, beginning of year	$76,210	$71,148	$69,067
Additions	10,316	13,621	24,179
Deductions	-26,713	-8,559	-22,098
Reinsurance allowance, end of year	$59,813	$76,210	$71,148

Reinsurance_Effect_Of_Reinsura

Reinsurance (Effect Of Reinsurance On Premiums Written And Earned) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Disclosures [Abstract]
Direct premiums written	$3,478,273	$3,143,957	$2,115,353
Direct premiums earned	3,443,912	2,947,812	2,057,735
Assumed premiums written	1,327,240	776,269	398,328
Assumed premiums earned	1,298,371	1,016,853	376,186
Ceded premiums written	-888,498	-683,543	-299,555
Ceded premiums earned	-901,371	-733,049	-286,793
Net premiums written	3,917,015	3,236,683	2,214,126
Net premiums earned	$3,840,912	$3,231,616	$2,147,128

Commitments_And_Contingencies_1

Commitments And Contingencies (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Maximum remaining term of noncancelable operating leases	20 years
Rental expense	$42.70	$35.30	$28.10

Commitments_And_Contingencies_2

Commitments And Contingencies (Minimum Annual Rental Commitments) (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Commitments and Contingencies Disclosure [Abstract]
2015	$28,434
2016	23,636
2017	28,124
2018	26,549
2019	25,169
2020 and thereafter	151,000
Total	$282,912

Statutory_Financial_Informatio2

Statutory Financial Information (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Statutory Accounting Practices [Line Items]
Amount of cash and investments held as funds at Lloyd's	784.6
United States [Member]
Statutory Accounting Practices [Line Items]
Amount available for dividend payment	310.6
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Advance notice to declare and pay dividend under PRA	14 days
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Amount available for dividend payment	472.7

Statutory_Financial_Informatio3

Statutory Financial Information (Actual Statutory Capital And Surplus) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
United States [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$2,619,001	$2,396,530
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	604,564	510,426
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	1,890,920	1,503,004
Other Countries [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$177,738	$172,447

Statutory_Financial_Informatio4

Statutory Financial Information (Statutory Net Income (Loss)) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$212,909	$235,009	$127,179
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	70,632	109,983	82,573
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	162,809	249,772
Other Countries [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$1,271	($12,617)

Segment_Reporting_Disclosures_1

Segment Reporting Disclosures (Narrative) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	segment
Revenue, Major Customer [Line Items]
Number of reportable ongoing underwriting segments	3
Percentage of gross premiums written	100.00%	100.00%	100.00%
Top Three Independent Brokers [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	28.00%	24.00%
Top Three Independent Brokers [Member] | Reinsurance [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	68.00%	68.00%
Top Three Independent Brokers [Member] | International Insurance [Member]
Revenue, Major Customer [Line Items]
Percentage of gross premiums written	41.00%	42.00%

Segment_Reporting_Disclosures_2

Segment Reporting Disclosures (Summary Of Gross Written Premiums By Country) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$4,805,513	$3,920,226	$2,513,681
Percentage of gross premiums written	100.00%	100.00%	100.00%
United States [Member]
Segment Reporting Information [Line Items]
Gross written premiums	3,523,239	2,934,868	1,768,011
Percentage of gross premiums written	73.00%	75.00%	70.00%
United Kingdom [Member]
Segment Reporting Information [Line Items]
Gross written premiums	441,669	245,143	147,891
Percentage of gross premiums written	9.00%	6.00%	6.00%
Canada [Member]
Segment Reporting Information [Line Items]
Gross written premiums	125,617	128,420	120,542
Percentage of gross premiums written	3.00%	3.00%	5.00%
Other Countries [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$714,988	$611,795	$477,237
Percentage of gross premiums written	15.00%	16.00%	19.00%

Segment_Reporting_Disclosures_3

Segment Reporting Disclosures (Company's Segment Disclosures) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross premium volume	$4,805,513		$3,920,226		$2,513,681
Net written premiums	3,917,015		3,236,683		2,214,126
Earned premiums	3,840,912		3,231,616		2,147,128
Losses and loss adjustment expenses, current accident year	-2,638,012		-2,227,402		-1,553,070
Losses and loss adjustment expenses, prior accident years	435,545		411,129		399,002
Transaction costs and other acquisition-related expenses	0	[1]	-75,140	[1],[2]	0	[1]
Transaction costs			-15,981	[1]
Other acquisition related costs, severance costs			-31,734	[1]
Other acquisition related costs, stay bonuses			-14,804	[1]
Other acquisition related costs, other compensation costs			-12,621	[1]
Amortization of policy acquisition costs	-654,916		-471,915		-428,109
Prospective adoption of FASB ASU No. 2010-26	0	[3]	0	[3]	-43,093	[3]
Other amortization of policy acquisition costs	-654,916		-471,915		-385,016
Other operating expenses	-805,966		-765,257		-501,363
Underwriting profit (loss)	177,563		103,031		63,588
Net investment income	363,230		317,373		282,107
Net realized investment gains	46,000		63,152		31,593
Other revenues (insurance)	28,632		24,494		49,932
Other expenses (insurance)	-62,834		-50,278		-45,292
Segment profit (loss)	552,591		457,772		381,928
Other revenues (non-insurance)	854,893		686,448		489,352
Other expenses (non-insurance)	-792,037		-613,250		-432,956
Amortization of intangible assets	-57,627		-55,223		-33,512
Interest expense	-117,442		-114,004		-92,762
Income Before Income Taxes	440,378		361,743		312,050
U.S. GAAP combined ratio	95.00%	[4]	97.00%	[4]	97.00%	[4]
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	2,493,823		2,252,739		1,595,849
Net written premiums	2,071,466		1,915,770		1,418,579
Earned premiums	2,022,860		1,727,766		1,360,229
Losses and loss adjustment expenses, current accident year	-1,340,129		-1,173,258		-984,018
Losses and loss adjustment expenses, prior accident years	216,557		298,113		225,870
Transaction costs and other acquisition-related expenses			-12,724	[2]
Amortization of policy acquisition costs	-403,233		-287,795
Prospective adoption of FASB ASU No. 2010-26					-31,033	[3]
Other amortization of policy acquisition costs					-243,466
Other operating expenses	-396,737		-409,886		-327,682
Underwriting profit (loss)	99,318		142,216		-100
Net investment income	0		0		0
Net realized investment gains	0		0		0
Other revenues (insurance)	2,478		13,648		44,968
Other expenses (insurance)	-5,149		-17,087		-41,425
Segment profit (loss)	96,647		138,777		3,443
U.S. GAAP combined ratio	95.00%	[4]	92.00%	[4]	100.00%	[4]
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	1,200,403		1,101,099		788,092
Net written premiums	889,336		840,050		677,344
Earned premiums	909,679		833,984		672,405
Losses and loss adjustment expenses, current accident year	-660,409		-588,759		-492,177
Losses and loss adjustment expenses, prior accident years	166,615		130,660		172,758
Transaction costs and other acquisition-related expenses			-13,366	[2]
Amortization of policy acquisition costs	-141,394		-138,626
Prospective adoption of FASB ASU No. 2010-26					-10,597	[3]
Other amortization of policy acquisition costs					-130,637
Other operating expenses	-207,175		-171,666		-153,545
Underwriting profit (loss)	67,316		52,227		58,207
Net investment income	0		0		0
Net realized investment gains	0		0		0
Other revenues (insurance)	21,827		4,284		4,964
Other expenses (insurance)	-18,706		-5,065		-3,867
Segment profit (loss)	70,437		51,446		59,304
U.S. GAAP combined ratio	93.00%	[4]	94.00%	[4]	91.00%	[4]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	1,112,728		566,348		129,744
Net written premiums	956,584		480,822		118,208
Earned premiums	908,385		669,826		114,499
Losses and loss adjustment expenses, current accident year	-637,474		-465,385		-76,875
Losses and loss adjustment expenses, prior accident years	79,951		12,938		21,449
Transaction costs and other acquisition-related expenses			-49,050	[2]
Amortization of policy acquisition costs	-110,289		-45,494
Prospective adoption of FASB ASU No. 2010-26					-1,463	[3]
Other amortization of policy acquisition costs					-10,913
Other operating expenses	-201,673		-183,817		-19,933
Underwriting profit (loss)	38,900		-60,982		26,764
Net investment income	0		0		0
Net realized investment gains	0		0		0
Other revenues (insurance)	2,696		5,432		0
Other expenses (insurance)	-1,847		0		0
Segment profit (loss)	39,749		-55,550		26,764
U.S. GAAP combined ratio	96.00%	[4]	109.00%	[4]	77.00%	[4]
Other Insurance (Discontinued Lines) [Member]
Segment Reporting Information [Line Items]
Gross premium volume	-1,441		40		-4
Net written premiums	-371		41		-5
Earned premiums	-12		40		-5
Losses and loss adjustment expenses, current accident year	0		0		0
Losses and loss adjustment expenses, prior accident years	-27,578		-30,582		-21,075
Transaction costs and other acquisition-related expenses			0	[2]
Amortization of policy acquisition costs	0		0
Prospective adoption of FASB ASU No. 2010-26					0	[3]
Other amortization of policy acquisition costs					0
Other operating expenses	-381		112		-203
Underwriting profit (loss)	-27,971		-30,430		-21,283
Net investment income	0		0		0
Net realized investment gains	0		0		0
Other revenues (insurance)	1,631		1,130		0
Other expenses (insurance)	-37,132		-28,126		0
Segment profit (loss)	-63,472		-57,426		-21,283
U.S. GAAP combined ratio - not meaniningful	NM	[4],[5]	NM	[4],[6]	NM	[4],[6]
Investing [Member]
Segment Reporting Information [Line Items]
Gross premium volume	0		0		0
Net written premiums	0		0		0
Earned premiums	0		0		0
Losses and loss adjustment expenses, current accident year	0		0		0
Losses and loss adjustment expenses, prior accident years	0		0		0
Transaction costs and other acquisition-related expenses			0	[2]
Amortization of policy acquisition costs	0		0
Prospective adoption of FASB ASU No. 2010-26					0	[3]
Other amortization of policy acquisition costs					0
Other operating expenses	0		0		0
Underwriting profit (loss)	0		0		0
Net investment income	363,230		317,373		282,107
Net realized investment gains	46,000		63,152		31,593
Other revenues (insurance)	0		0		0
Other expenses (insurance)	0		0		0
Segment profit (loss)	$409,230		$380,525		$313,700
[1]	In connection with the acquisition of Alterra, the Company incurred transaction costs of $16.0 million for the year ended December 31, 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 $14.8 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 transaction costs of $16.0 million for the year ended DecemberB 31, 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 $14.8 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.
[4]	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.
[5]	NM b Ratio is not meaningful
[6]	NM b Ratio is not meaningful.

Segment_Reporting_Disclosures_4

Segment Reporting Disclosures (Summary Of Deferred Policy Acquisition Costs, Unearned Premiums And Unpaid Losses And Loss Adjustment Expenses) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	$353,410	$260,967	$157,465	$194,674
Unearned premiums	2,245,690	2,127,115
Unpaid losses and loss adjustment expenses	10,404,152	10,262,056	5,371,426
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	165,333	151,015
Unearned premiums	1,110,910	1,058,529
Unpaid losses and loss adjustment expenses	3,577,166	3,572,667
International Insurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	47,618	50,425
Unearned premiums	491,708	516,689
Unpaid losses and loss adjustment expenses	3,353,417	3,399,228
Reinsurance [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	140,459	59,527
Unearned premiums	643,072	551,897
Unpaid losses and loss adjustment expenses	2,818,792	2,811,362
Other Insurance (Discontinued Lines) [Member]
Segment Reporting Information [Line Items]
Deferred policy acquisition costs	0	0
Unearned premiums	0	0
Unpaid losses and loss adjustment expenses	$654,777	$478,799

Segment_Reporting_Disclosures_5

Segment Reporting Disclosures (Summary Of Segment Earned Premiums) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Earned premiums	$3,840,912	$3,231,616	$2,147,128
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	2,022,860	1,727,766	1,360,229
U.S. Insurance [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	491,645	431,798	354,003
U.S. Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	321,005	268,203	216,234
U.S. Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	266,019	190,530	141,568
U.S. Insurance [Member] | Personal Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums	299,442	185,935	86,217
U.S. Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Earned premiums	244,216	205,004	135,037
U.S. Insurance [Member] | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Earned premiums	263,164	250,790	241,208
U.S. Insurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	137,369	195,506	185,962
International Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	909,679	833,984	672,405
International Insurance [Member] | General Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	146,178	128,171	102,875
International Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Earned premiums	285,300	252,816	174,590
International Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	76,691	91,497	48,483
International Insurance [Member] | Marine And Energy [Member]
Segment Reporting Information [Line Items]
Earned premiums	287,263	287,745	288,605
International Insurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	114,247	73,755	57,852
Reinsurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	908,385	669,826	114,499
Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Earned premiums	270,461	227,394	71,172
Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Earned premiums	323,390	244,981	476
Reinsurance [Member] | Auto [Member]
Segment Reporting Information [Line Items]
Earned premiums	152,645	84,042	21,163
Reinsurance [Member] | Other Products [Member]
Segment Reporting Information [Line Items]
Earned premiums	161,889	113,409	21,688
Other Insurance (Discontinued Lines) [Member]
Segment Reporting Information [Line Items]
Earned premiums	($12)	$40	($5)

Segment_Reporting_Disclosures_6

Segment Reporting Disclosures (Segment Assets To The Company's Consolidated Balance Sheets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Segment Reporting Information [Line Items]
Assets	$25,200,357	$23,955,511	$12,556,588
Non-Insurance Operations [Member]
Segment Reporting Information [Line Items]
Assets	1,246,762	936,448	891,586
Segment Assets [Member]
Segment Reporting Information [Line Items]
Assets	23,953,595	23,019,063	11,665,002
Segment Assets [Member] | Investing [Member]
Segment Reporting Information [Line Items]
Assets	18,531,150	17,550,332	9,277,697
Segment Assets [Member] | Underwriting [Member]
Segment Reporting Information [Line Items]
Assets	$5,422,445	$5,468,731	$2,387,305

Other_Revenues_And_Other_Expen2

Other Revenues And Other Expenses (Summary Of Other Revenues And Other Expenses By Component) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Revenues and Other Expenses [Line Items]
Other revenues	$883,525	$710,942	$539,284
Other expenses	854,871	663,528	478,248
Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	28,632	24,494	49,932
Other expenses	62,834	50,278	45,292
Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	854,893	686,448	489,352
Other expenses	792,037	613,250	432,956
Managing General Agent Operations [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	23,324	17,399	48,056
Other expenses	22,527	20,382	43,069
Life And Annuity [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	1,631	1,130	0
Other expenses	37,132	28,126	0
Other Insurance [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	3,677	5,965	1,876
Other expenses	3,175	1,770	2,223
Manufacturing [Member] | Non-Insurance Operations [Member] | Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	575,353	495,138	366,886
Other expenses	513,668	437,712	328,484
Non-Manufacturing [Member] | Non-Insurance Operations [Member] | Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	262,767	191,310	122,466
Other expenses	261,551	175,538	104,472
Other Non-Insurance [Member] | Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	16,773	0	0
Other expenses	$16,818	$0	$0

Employee_Benefit_Plans_Narrati

Employee Benefit Plans (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	funds
Defined Benefit Plan Disclosure [Line Items]
Expenses relating to defined contribution plans	$27,200,000	$24,300,000	$19,100,000
Consecutive number of years	3 years
Number of years preceding retirement	10 years
Tax benefit of the net actuarial pension loss recognized as a component of accumulated other comprehensive income	16,200,000	12,400,000
Percentage of reasonable rate of return on fixed maturities	4.50%
Expected rate at which equity securities outperform fixed maturities in plan assets	3.50%
Amortization of the net actuarial pension loss included as a component of accumulated other comprehensive income	2,400,000
Percentage of equity securities invested in UK companies	30.00%
Percentage of equity securities invested in other markets	70.00%
Number of fixed maturity mutual funds	5
Accumulated benefit obligation	166,900,000	146,000,000
Expected plan contributions in 2015	0
Expected benefits to be paid in 2015	4,000,000
Expected benefits to be paid in 2016	4,100,000
Expected benefits to be paid in 2017	4,100,000
Expected benefits to be paid in 2018	4,200,000
Expected benefits to be paid in 2019	4,300,000
Aggregate benefits expected to be paid from 2020 to 2024	$23,100,000
United Kingdom [Member] | Foreign Governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds	2
United Kingdom [Member] | Corporate Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds	1
Other Foreign Governments [Member] | Foreign Governments [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds	1
United Kingdom And Other Foreign Governments [Member] | Corporate Bond Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of fixed maturity mutual funds	1
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percent	46.00%
Actual asset allocation, percent	43.00%	63.00%
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation, percent	54.00%
Actual asset allocation, percent	57.00%	37.00%

Employee_Benefit_Plans_Schedul

Employee Benefit Plans (Schedule Of Amounts Recognized On The Consolidated Balance Sheets) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of period	$163,010	$151,327
Interest cost	7,572	6,533	6,815
Plan amendments	495	0
Benefits paid	-4,424	-3,542
Actuarial loss	29,609	5,459
Effect of foreign currency rate changes	-10,706	3,233
Projected benefit obligation at end of year	185,556	163,010	151,327
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	189,437	164,090
Actual gain on plan assets	22,395	19,430
Employer contributions	5,610	5,338
Benefits paid	-4,424	-3,542
Effect of foreign currency rate changes	-11,619	4,121
Fair value of plan assets at end of year	201,399	189,437	164,090
Funded status of the plan	15,843	26,427
Net actuarial pension loss	61,378	42,941
Total	$77,221	$69,368

Employee_Benefit_Plans_Schedul1

Employee Benefit Plans (Schedule of Defined Benefit Plan Amounts Recognized in Accumulated Other Comprehensive Income) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Tax benefit (expense)	$3,687	($1,015)	($1,991)
Total other comprehensive income (loss)	614,752	178,468	250,417
Net Actuarial Pension Loss [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	-20,521	3,146	6,065
Net actuarial loss	1,589	1,934	2,590
Prior service costs	495	0	0
Tax benefit (expense)	3,687	-1,015	-1,991
Total other comprehensive income (loss)	($14,750)	$4,065	$6,664

Employee_Benefit_Plans_Schedul2

Employee Benefit Plans (Schedule Of Net Periodic Benefit Income And Weighted Average Assumptions) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Service cost	$0	$0	$361
Interest cost	7,572	6,533	6,815
Expected return on plan assets	-12,812	-10,825	-9,788
Amortization of prior service cost	495	0	0
Amortization of net actuarial pension loss	1,589	1,934	2,590
Net periodic benefit income	($3,156)	($2,358)	($22)
Discount rate	3.80%	4.70%	4.50%
Expected return on plan assets, percent	6.00%	6.60%	6.60%
Rate of compensation increase	2.90%	3.20%	3.10%

Employee_Benefit_Plans_Summary

Employee Benefit Plans (Summary Of Fair Value Of Plan Assets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$201,399	$189,437	$164,090
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	201,399	189,437
Level 1 [Member] | Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	114,243	70,997
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	87,148	118,431
Level 1 [Member] | Cash And Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$8	$9

Markel_Corporation_Parent_Comp2

Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Balance Sheets) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Condensed Financial Statements, Captions [Line Items]
Fixed maturities	$10,422,882	$10,142,536
Equity securities	4,137,576	3,251,798
Short-term investments (estimated fair value approximates cost)	1,594,849	1,452,288
Total Investments	16,155,307	14,846,622
Cash and cash equivalents	1,960,169	1,978,526	863,766	703,259
Restricted cash and cash equivalents	522,225	786,926
Other assets	985,834	1,067,922
Total Assets	25,200,357	23,955,511	12,556,588
Net deferred tax liability	310,547
Other liabilities	1,051,931	777,850
Total Liabilities	17,537,307	17,205,318
Total Shareholders' Equity	7,594,818	6,673,577
Total Liabilities and Equity	25,200,357	23,955,511
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities	48,807	67,363
Equity securities	434,714	392,123
Short-term investments (estimated fair value approximates cost)	764,953	654,971
Total Investments	1,248,474	1,114,457
Cash and cash equivalents	243,702	207,352	237,816	280,604
Restricted cash and cash equivalents	959	1,010
Receivables	16,110	14,326
Investments in consolidated subsidiaries	7,560,862	6,826,790
Notes receivable from subsidiaries	212,631	168,611
Income taxes receivable	10,951	5,320
Other assets	93,434	102,193
Total Assets	9,387,123	8,440,059
Senior long-term debt	1,635,173	1,633,873
Net deferred tax liability	74,534	40,443
Other liabilities	82,598	92,166
Total Liabilities	1,792,305	1,766,482
Total Shareholders' Equity	7,594,818	6,673,577
Total Liabilities and Equity	$9,387,123	$8,440,059

Markel_Corporation_Parent_Comp3

Markel Corporation (Parent Company Only) Financial Information (Schedule of Condensed Balance Sheets - Parenthetical) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	$9,929,137	$10,129,141
Equity securities, cost	1,951,658	1,566,553
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Fixed maturities, amortized cost	47,346	66,154
Equity securities, cost	$193,864	$197,549

Markel_Corporation_Parent_Comp4

Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Statements Of Income And Comprehensive Income) (Details) (USD $)	3 Months Ended												12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net investment income													$363,230	$317,373	$282,107
Other-than-temporary impairment losses													-4,784	-4,706	-12,078
Net realized investment gains, excluding other-than-temporary impairment losses													50,784	67,858	43,671
Net realized investment gains													46,000	63,152	31,593
Other													883,525	710,942	539,284
Total Operating Revenues	1,335,755	1,299,286	1,258,971	1,239,655	1,279,785	1,191,665	1,031,769	819,864	807,955	765,775	693,247	733,135	5,133,667	4,323,083	3,000,112
Interest expense													117,442	114,004	92,762
Other expenses													854,871	663,528	478,248
Total Operating Expenses													4,575,847	3,847,336	2,595,300
Income tax benefit													116,690	77,898	53,802
Net Income to Shareholders	117,595	75,803	40,068	87,716	98,764	65,599	27,756	88,902	56,792	49,653	89,687	57,253	321,182	281,021	253,385
Net holding gains arising during the period													687,735	225,545	266,425
Reclassification adjustments for net gains (losses) included in net income to shareholders													-26,161	-40,830	-24,051
Change in net unrealized gains on investments, net of taxes													661,747	184,574	242,214
Change in foreign currency translation adjustments, net of taxes													-32,241	-10,143	1,534
Total Other Comprehensive Income													614,756	178,496	250,412
Comprehensive Income to Shareholders	418,571	36,502	250,588	230,273	206,450	144,409	-149,054	257,684	76,987	147,454	73,416	205,945	935,934	459,489	503,802
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net investment income													5,354	21,946	30,619
Dividends on common stock of consolidated subsidiaries													217,121	806,233	337,585
Other-than-temporary impairment losses													-120	-15	-38
Net realized investment gains, excluding other-than-temporary impairment losses													3,873	67,232	14,926
Net realized investment gains													3,753	67,217	14,888
Other													0	1	3
Total Operating Revenues													226,228	895,397	383,095
Interest expense													94,097	92,743	87,391
Other expenses													2,685	2,617	1,166
Total Operating Expenses													96,782	95,360	88,557
Income Before Equity in Undistributed Earnings of Consolidated Subsidiaries and Income Taxes													129,446	800,037	294,538
Equity in undistributed earnings of consolidated subsidiaries													163,341	-520,323	-61,663
Income tax benefit													-28,395	-1,307	-20,510
Net Income to Shareholders													321,182	281,021	253,385
Net holding gains arising during the period													32,118	66,623	10,897
Consolidated subsidiaries' net holding gains arising during the period													655,617	158,922	255,528
Consolidated subsidiaries' change in unrealized other-than-temporary impairment losses on fixed maturities arising during the period													173	-141	-160
Reclassification adjustments for net gains (losses) included in net income to shareholders													-1,874	-43,220	11,847
Consolidated subsidiaries' reclassification adjustments for net gains (losses) included in net income to shareholders													-24,287	2,390	-35,898
Change in net unrealized gains on investments, net of taxes													661,747	184,574	242,214
Change in foreign currency translation adjustments, net of taxes													1,949	-2,670	-242
Consolidated subsidiaries' change in foreign currency translation adjustments, net of taxes													-34,194	-7,501	1,781
Consolidated subsidiaries' change in net actuarial pension loss, net of taxes													-14,750	4,065	6,664
Total Other Comprehensive Income													614,752	178,468	250,417
Comprehensive Income to Shareholders													$935,934	$459,489	$503,802

Markel_Corporation_Parent_Comp5

Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Statements Of Cash Flows) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net income to shareholders	$321,182	$281,021	$253,385
Net Cash Provided By Operating Activities	716,792	745,521	392,533
Proceeds from sales of fixed maturities and equity securities	1,286,871	879,564	336,548
Proceeds from maturities, calls and prepayments of fixed maturities	1,420,817	1,475,938	510,697
Cost of fixed maturities and equity securities purchased	-3,153,055	-1,651,397	-426,439
Net change in short-term investments	-129,164	-470,423	-428,292
Cost of equity method investments	-16,081	-38,018	-40,650
Change in restricted cash and cash equivalents	264,701	-263,014	-37,642
Additions to property and equipment	-82,132	-47,725	-45,519
Other	-2,368	1,103	-2,158
Net Cash Provided (Used) By Investing Activities	-622,205	187,387	-377,130
Additions to senior long-term debt	89,480	547,214	492,792
Repurchases of common stock	-26,053	-57,388	-16,873
Issuance of common stock	5,691	24,518	9,145
Other	-21,357	-23	-19,485
Net Cash Provided (Used) By Financing Activities	-67,124	175,367	141,962
Increase (decrease) in cash and cash equivalents	-18,357	1,114,760	160,507
Cash and cash equivalents at beginning of year	1,978,526	863,766	703,259
CASH AND CASH EQUIVALENTS AT END OF YEAR	1,960,169	1,978,526	863,766
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income to shareholders	321,182	281,021	253,385
Adjustments to reconcile net income to shareholders to net cash provided by operating activities	-218,396	186,574	-153,773
Net Cash Provided By Operating Activities	102,786	467,595	99,612
Proceeds from sales of fixed maturities and equity securities	9,306	142,259	149,314
Proceeds from maturities, calls and prepayments of fixed maturities	15,710	2,819	64,340
Cost of fixed maturities and equity securities purchased	-687	-23,412	-139,681
Net change in short-term investments	-109,728	10,251	-374,801
Securities received from subsidiaries as dividends or repayment of notes receivable	89,996	249,996	260,088
Decrease in notes receivable due from subsidiaries	28,506	5,302	66,802
Capital contributions to subsidiaries	-74,788	-67,878	-198,349
Acquisitions	0	-1,017,988	-100,409
Cost of equity method investments	0	-5,291	-38,250
Change in restricted cash and cash equivalents	51	-348	-204
Additions to property and equipment	-342	-3,653	-9,437
Other	-2,150	3,207	-4,369
Net Cash Provided (Used) By Investing Activities	-44,126	-704,736	-324,956
Additions to senior long-term debt	0	491,235	347,207
Repayments and retirement of senior long-term debt	0	-246,665	-157,359
Repurchases of common stock	-26,053	-57,388	-16,873
Issuance of common stock	5,691	24,518	9,145
Other	-1,948	-5,023	436
Net Cash Provided (Used) By Financing Activities	-22,310	206,677	182,556
Increase (decrease) in cash and cash equivalents	36,350	-30,464	-42,788
Cash and cash equivalents at beginning of year	207,352	237,816	280,604
CASH AND CASH EQUIVALENTS AT END OF YEAR	$243,702	$207,352	$237,816

Quarterly_Financial_Informatio2

Quarterly Financial Information (Details) (USD $)	3 Months Ended												12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$1,335,755	$1,299,286	$1,258,971	$1,239,655	$1,279,785	$1,191,665	$1,031,769	$819,864	$807,955	$765,775	$693,247	$733,135	$5,133,667	$4,323,083	$3,000,112
Net income	118,222	76,824	41,141	87,501	98,939	66,967	28,676	89,263	58,093	51,674	90,768	57,713	323,688	283,845	258,248
Net income to shareholders	117,595	75,803	40,068	87,716	98,764	65,599	27,756	88,902	56,792	49,653	89,687	57,253	321,182	281,021	253,385
Comprehensive income (loss) to shareholders	$418,571	$36,502	$250,588	$230,273	$206,450	$144,409	($149,054)	$257,684	$76,987	$147,454	$73,416	$205,945	$935,934	$459,489	$503,802
Basic net income per share (dollars per share)	$8.10	$5.33	$2.67	$6.28	$6.98	$4.69	$2.24	$9.53	$6.25	$5.33	$8.44	$5.94	$22.38	$22.57	$25.96
Diluted net income per share (dollars per share)	$8.05	$5.30	$2.66	$6.25	$6.95	$4.67	$2.24	$9.50	$6.23	$5.32	$8.42	$5.92	$22.27	$22.48	$25.89
Common stock price, high (dollars per share)	$707.36	$666	$655.75	$596.87	$582.59	$549.09	$546.94	$510.05	$502.20	$459.90	$453.50	$451.90
Common stock price, low (dollars per share)	$632.65	$623.90	$593.76	$527.17	$511.06	$506.64	$501.76	$434.98	$425.17	$420	$421	$398.65