Guarantor Financial Information - Condensed consolidating balance sheet (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2013		Dec. 31, 2012
Assets
Total investments	$ 13,843,497		$ 13,611,734
Cash	1,569,605		434,057		356,767		371,041
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	0		0
Premiums receivable	1,008,375		753,924
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,790,025		1,804,330
Contractholder receivables	1,118,991		1,064,246
Prepaid reinsurance premiums	349,077		328,343
Deferred acquisition costs, net	384,294		342,314
Other assets	1,736,494		1,227,146
Total assets	21,800,358		19,566,094		18,757,061
Liabilities
Reserve for losses and loss adjustment expenses	8,938,958		8,824,696
Unearned premiums	2,148,475		1,896,365
Reinsurance balances payable	201,794		196,167
Contractholder payables	1,118,991		1,064,246
Deposit accounting liabilities	409,080		421,297
Senior notes	800,000		800,000
Revolving credit agreement borrowings	100,000		100,000
Due to subsidiaries and affiliates	0		0
Other liabilities	1,178,197		615,827
Total liabilities	14,895,495		13,918,598		13,421,531
Redeemable noncontrolling interests	219,234	[1]	0	[1]
Shareholders' Equity
Total shareholders' equity available to Arch	5,892,133		5,647,496		5,335,530
Non-redeemable noncontrolling interests	793,496	[1]	0	[1]	0	[1]
Total shareholders' equity	6,685,629		5,647,496		5,335,530
Total liabilities, noncontrolling interests and shareholders' equity	21,800,358		19,566,094
Reportable Legal Entities | ACGL (Parent Guarantor)
Assets
Total investments	2,546		2,530
Cash	12,353		3,223		3,614		6,417
Investments in subsidiaries	6,288,035		6,046,060
Due from subsidiaries and affiliates	503		2,251
Premiums receivable	0		0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		0
Contractholder receivables	0		0
Prepaid reinsurance premiums	0		0
Deferred acquisition costs, net	0		0
Other assets	6,309		6,598
Total assets	6,309,746		6,060,662
Liabilities
Reserve for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Contractholder payables	0		0
Deposit accounting liabilities	0		0
Senior notes	300,000		300,000
Revolving credit agreement borrowings	100,000		100,000
Due to subsidiaries and affiliates	2,091		18
Other liabilities	15,522		13,148
Total liabilities	417,613		413,166
Redeemable noncontrolling interests	0	[1]	0
Shareholders' Equity
Total shareholders' equity available to Arch	5,892,133		5,647,496
Non-redeemable noncontrolling interests	0	[1]	0
Total shareholders' equity	5,892,133		5,647,496
Total liabilities, noncontrolling interests and shareholders' equity	6,309,746		6,060,662
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	48,689		408,957
Cash	45,472		509		356		612
Investments in subsidiaries	1,598,732		1,258,889
Due from subsidiaries and affiliates	0		0
Premiums receivable	0		0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0		0
Contractholder receivables	0		0
Prepaid reinsurance premiums	0		0
Deferred acquisition costs, net	0		0
Other assets	62,500		60,342
Total assets	1,755,393		1,728,697
Liabilities
Reserve for losses and loss adjustment expenses	0		0
Unearned premiums	0		0
Reinsurance balances payable	0		0
Contractholder payables	0		0
Deposit accounting liabilities	0		0
Senior notes	500,000		500,000
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	1,167		10,250
Other liabilities	46,040		33,206
Total liabilities	547,207		543,456
Redeemable noncontrolling interests	0	[1]	0
Shareholders' Equity
Total shareholders' equity available to Arch	1,208,186		1,185,241
Non-redeemable noncontrolling interests	0	[1]	0
Total shareholders' equity	1,208,186		1,185,241
Total liabilities, noncontrolling interests and shareholders' equity	1,755,393		1,728,697
Reportable Legal Entities | Other ACGL Subsidiaries
Assets
Total investments	13,792,262		13,200,247
Cash	1,511,780		430,325		352,797		364,012
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	401,987		405,110
Premiums receivable	1,399,825		1,085,369
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	5,617,877		5,645,156
Contractholder receivables	1,118,991		1,064,246
Prepaid reinsurance premiums	1,293,460		1,109,312
Deferred acquisition costs, net	384,294		342,314
Other assets	2,211,353		1,714,651
Total assets	27,731,829		24,996,730
Liabilities
Reserve for losses and loss adjustment expenses	12,749,908		12,625,766
Unearned premiums	3,092,858		2,677,334
Reinsurance balances payable	742,630		662,394
Contractholder payables	1,118,991		1,064,246
Deposit accounting liabilities	735,993		758,490
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	399,232		397,093
Other liabilities	1,200,906		691,699
Total liabilities	20,040,518		18,877,022
Redeemable noncontrolling interests	219,234	[1]	0
Shareholders' Equity
Total shareholders' equity available to Arch	6,678,581		6,119,708
Non-redeemable noncontrolling interests	793,496	[1]	0
Total shareholders' equity	7,472,077		6,119,708
Total liabilities, noncontrolling interests and shareholders' equity	27,731,829		24,996,730
Consolidating Adjustments and Eliminations
Assets
Total investments	0		0
Cash	0		0		0		0
Investments in subsidiaries	(7,886,767)		(7,304,949)
Due from subsidiaries and affiliates	(402,490)		(407,361)
Premiums receivable	(391,450)		(331,445)
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	(3,827,852)		(3,840,826)
Contractholder receivables	0		0
Prepaid reinsurance premiums	(944,383)		(780,969)
Deferred acquisition costs, net	0		0
Other assets	(543,668)		(554,445)
Total assets	(13,996,610)		(13,219,995)
Liabilities
Reserve for losses and loss adjustment expenses	(3,810,950)		(3,801,070)
Unearned premiums	(944,383)		(780,969)
Reinsurance balances payable	(540,836)		(466,227)
Contractholder payables	0		0
Deposit accounting liabilities	(326,913)		(337,193)
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	(402,490)		(407,361)
Other liabilities	(84,271)		(122,226)
Total liabilities	(6,109,843)		(5,915,046)
Redeemable noncontrolling interests	0	[1]	0
Shareholders' Equity
Total shareholders' equity available to Arch	(7,886,767)		(7,304,949)
Non-redeemable noncontrolling interests	0	[1]	0
Total shareholders' equity	(7,886,767)		(7,304,949)
Total liabilities, noncontrolling interests and shareholders' equity	$ (13,996,610)		$ (13,219,995)

[1] See Note 4.