Guarantor Financial Information - Condensed consolidating balance sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Percentage ownership of subsidiary	100.00%
Assets
Total investments	$ 24,402,246	$ 21,745,284
Cash	726,230	646,556	$ 606,199
Investment in subsidiaries	0	0
Due from subsidiaries and affiliates	0	0
Premiums receivable	1,778,717	1,299,150
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	4,346,816	2,919,372
Contractholder receivables	2,119,460	2,079,111
Ceded unearned premiums	1,234,683	975,469
Deferred acquisition costs	633,400	569,574
Goodwill and intangible assets	738,083	634,920
Other assets	1,905,726	1,348,893
Total assets	37,885,361	32,218,329	32,051,658
Liabilities
Reserve for losses and loss adjustment expenses	13,891,842	11,853,297	11,383,792	$ 10,200,960
Unearned premiums	4,339,549	3,753,636
Reinsurance balances payable	667,072	393,107
Contractholder payables	2,119,460	2,079,111
Collateral held for insured obligations	206,698	236,630
Senior notes	1,871,626	1,733,528
Revolving credit agreement borrowings	484,287	455,682
Due to subsidiaries and affiliates	0	0
Other liabilities	1,989,275	1,275,659
Total liabilities	25,569,809	21,780,650	21,805,723
Redeemable noncontrolling interests	55,404	206,292	205,922	$ 205,553
Shareholders’ Equity
Total shareholders' equity available to Arch	11,497,371	9,439,827	9,196,602
Non-redeemable noncontrolling interests	762,777	791,560	843,411
Total shareholders' equity	12,260,148	10,231,387	$ 10,040,013
Total liabilities, noncontrolling interests and shareholders' equity	37,885,361	32,218,329
Reportable legal entities | Arch Capital (Parent Guarantor)
Assets
Total investments	42	104
Cash	18,113	6,125
Investment in subsidiaries	11,786,861	9,735,256
Due from subsidiaries and affiliates	17	9
Premiums receivable	0	0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0	0
Contractholder receivables	0	0
Ceded unearned premiums	0	0
Deferred acquisition costs	0	0
Goodwill and intangible assets	0	0
Other assets	20,461	12,588
Total assets	11,825,494	9,754,082
Liabilities
Reserve for losses and loss adjustment expenses	0	0
Unearned premiums	0	0
Reinsurance balances payable	0	0
Contractholder payables	0	0
Collateral held for insured obligations	0	0
Senior notes	297,254	297,150
Revolving credit agreement borrowings	0	0
Due to subsidiaries and affiliates	0	0
Other liabilities	30,869	17,105
Total liabilities	328,123	314,255
Redeemable noncontrolling interests	0	0
Shareholders’ Equity
Total shareholders' equity available to Arch	11,497,371	9,439,827
Non-redeemable noncontrolling interests	0	0
Total shareholders' equity	11,497,371	9,439,827
Total liabilities, noncontrolling interests and shareholders' equity	11,825,494	9,754,082
Reportable legal entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	692,606	452,674
Cash	54,518	5,940
Investment in subsidiaries	4,347,806	3,999,243
Due from subsidiaries and affiliates	200,635	2
Premiums receivable	0	0
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	0	0
Contractholder receivables	0	0
Ceded unearned premiums	0	0
Deferred acquisition costs	0	0
Goodwill and intangible assets	0	0
Other assets	32,187	80,949
Total assets	5,327,752	4,538,808
Liabilities
Reserve for losses and loss adjustment expenses	0	0
Unearned premiums	0	0
Reinsurance balances payable	0	0
Contractholder payables	0	0
Collateral held for insured obligations	0	0
Senior notes	494,831	494,723
Revolving credit agreement borrowings	0	0
Due to subsidiaries and affiliates	536,805	536,805
Other liabilities	33,267	26,270
Total liabilities	1,064,903	1,057,798
Redeemable noncontrolling interests	0	0
Shareholders’ Equity
Total shareholders' equity available to Arch	4,262,849	3,481,010
Non-redeemable noncontrolling interests	0	0
Total shareholders' equity	4,262,849	3,481,010
Total liabilities, noncontrolling interests and shareholders' equity	5,327,752	4,538,808
Reportable legal entities | Other Arch Capital Subsidiaries
Assets
Total investments	23,748,432	21,307,206
Cash	653,599	634,491
Investment in subsidiaries	0	0
Due from subsidiaries and affiliates	1,901,865	1,802,686
Premiums receivable	2,547,130	1,834,389
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	9,711,773	8,618,660
Contractholder receivables	2,119,460	2,079,111
Ceded unearned premiums	2,230,164	1,730,262
Deferred acquisition costs	675,917	618,535
Goodwill and intangible assets	738,083	634,920
Other assets	1,981,598	1,466,438
Total assets	46,308,021	40,726,698
Liabilities
Reserve for losses and loss adjustment expenses	19,028,538	17,345,142
Unearned premiums	5,335,030	4,508,429
Reinsurance balances payable	1,435,485	928,346
Contractholder payables	2,119,460	2,079,111
Collateral held for insured obligations	206,698	236,630
Senior notes	1,114,541	941,655
Revolving credit agreement borrowings	484,287	455,682
Due to subsidiaries and affiliates	1,565,712	1,265,892
Other liabilities	2,324,793	1,699,768
Total liabilities	33,614,544	29,460,655
Redeemable noncontrolling interests	58,882	220,992
Shareholders’ Equity
Total shareholders' equity available to Arch	11,871,818	10,253,491
Non-redeemable noncontrolling interests	762,777	791,560
Total shareholders' equity	12,634,595	11,045,051
Total liabilities, noncontrolling interests and shareholders' equity	46,308,021	40,726,698
Consolidating adjustments and eliminations
Assets
Total investments	(38,834)	(14,700)
Cash	0	0
Investment in subsidiaries	(16,134,667)	(13,734,499)
Due from subsidiaries and affiliates	(2,102,517)	(1,802,697)
Premiums receivable	(768,413)	(535,239)
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	(5,364,957)	(5,699,288)
Contractholder receivables	0	0
Ceded unearned premiums	(995,481)	(754,793)
Deferred acquisition costs	(42,517)	(48,961)
Goodwill and intangible assets	0	0
Other assets	(128,520)	(211,082)
Total assets	(25,575,906)	(22,801,259)
Liabilities
Reserve for losses and loss adjustment expenses	(5,136,696)	(5,491,845)
Unearned premiums	(995,481)	(754,793)
Reinsurance balances payable	(768,413)	(535,239)
Contractholder payables	0	0
Collateral held for insured obligations		0
Senior notes	(35,000)	0
Revolving credit agreement borrowings	0	0
Due to subsidiaries and affiliates	(2,102,517)	(1,802,697)
Other liabilities	(399,654)	(467,484)
Total liabilities	(9,437,761)	(9,052,058)
Redeemable noncontrolling interests	(3,478)	(14,700)
Shareholders’ Equity
Total shareholders' equity available to Arch	(16,134,667)	(13,734,501)
Non-redeemable noncontrolling interests	0	0
Total shareholders' equity	(16,134,667)	(13,734,501)
Total liabilities, noncontrolling interests and shareholders' equity	$ (25,575,906)	$ (22,801,259)