Guarantor Financial Information - Condensed consolidating balance sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Total investments	$ 23,127,963		$ 21,745,284
Cash	605,316		646,556
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	0		0
Premiums receivable	1,606,040		1,299,150
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	3,171,257		2,919,372
Contractholder receivables	2,102,544		2,079,111
Ceded unearned premiums	1,136,728		975,469
Deferred acquisition costs	600,740		569,574
Goodwill and intangible assets	641,010		634,920	$ 593,008
Other assets	1,860,072		1,348,893
Total assets	34,851,670		32,218,329
Liabilities
Reserve for losses and loss adjustment expenses	12,230,316	$ 12,010,041	11,853,297	11,424,337	$ 11,496,205	$ 11,383,792
Unearned premiums	4,056,860		3,753,636
Reinsurance balances payable	531,990		393,107
Contractholder payables	2,102,544		2,079,111
Collateral held for insured obligations	237,056		236,630
Senior notes	1,733,865		1,733,528
Revolving credit agreement borrowings	491,006		455,682
Due to subsidiaries and affiliates	0		0
Other liabilities	1,648,859		1,275,659
Total liabilities	23,032,496		21,780,650
Redeemable noncontrolling interests	206,475		206,292
Shareholders' Equity
Total shareholders’ equity available to Arch	10,757,352		9,439,827	9,163,755
Non-redeemable noncontrolling interests	855,347		791,560	861,153
Total shareholders’ equity	11,612,699		10,231,387	$ 10,024,908
Total liabilities, noncontrolling interests and shareholders’ equity	34,851,670		32,218,329
Reportable Legal Entities | Arch Capital (Parent Guarantor)
Assets
Total investments	40		104
Cash	10,750		6,125
Investments in subsidiaries	11,045,465		9,735,256
Due from subsidiaries and affiliates	332		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	22,950		12,588
Total assets	11,079,537		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	297,201		297,150
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	9		0
Other liabilities	24,975		17,105
Total liabilities	322,185		314,255
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	10,757,352		9,439,827
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	10,757,352		9,439,827
Total liabilities, noncontrolling interests and shareholders’ equity	11,079,537		9,754,082
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	483,904		452,674
Cash	47,875		5,940
Investments in subsidiaries	4,493,198		3,999,243
Due from subsidiaries and affiliates	2		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	40,345		80,949
Total assets	5,065,324		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	494,776		494,723
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	536,747		536,805
Other liabilities	64,956		26,270
Total liabilities	1,096,479		1,057,798
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	3,968,845		3,481,010
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	3,968,845		3,481,010
Total liabilities, noncontrolling interests and shareholders’ equity	$ 5,065,324		4,538,808
Percentage Ownership of Subsidiary	100.00%
Reportable Legal Entities | Other Arch Capital Subsidiaries
Assets
Total investments	$ 22,693,719		21,307,206
Cash	546,691		634,491
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	1,881,929		1,802,686
Premiums receivable	2,291,360		1,834,389
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	8,618,279		8,618,660
Contractholder receivables	2,102,544		2,079,111
Ceded unearned premiums	1,984,219		1,730,262
Deferred acquisition costs	658,992		618,535
Goodwill and intangible assets	641,010		634,920
Other assets	1,953,732		1,466,438
Total assets	43,372,475		40,726,698
Liabilities
Reserve for losses and loss adjustment expenses	17,430,847		17,345,142
Unearned premiums	4,904,351		4,508,429
Reinsurance balances payable	1,217,310		928,346
Contractholder payables	2,102,544		2,079,111
Collateral held for insured obligations	237,056		236,630
Senior notes	941,888		941,655
Revolving credit agreement borrowings	491,006		455,682
Due to subsidiaries and affiliates	1,345,507		1,265,892
Other liabilities	2,055,626		1,699,768
Total liabilities	30,726,135		29,460,655
Redeemable noncontrolling interests	221,175		220,992
Shareholders' Equity
Total shareholders’ equity available to Arch	11,569,818		10,253,491
Non-redeemable noncontrolling interests	855,347		791,560
Total shareholders’ equity	12,425,165		11,045,051
Total liabilities, noncontrolling interests and shareholders’ equity	43,372,475		40,726,698
Consolidating Adjustments and Eliminations
Assets
Total investments	(49,700)		(14,700)
Cash	0		0
Investments in subsidiaries	(15,538,663)		(13,734,499)
Due from subsidiaries and affiliates	(1,882,263)		(1,802,697)
Premiums receivable	(685,320)		(535,239)
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	(5,447,022)		(5,699,288)
Contractholder receivables	0		0
Ceded unearned premiums	(847,491)		(754,793)
Deferred acquisition costs	(58,252)		(48,961)
Goodwill and intangible assets	0		0
Other assets	(156,955)		(211,082)
Total assets	(24,665,666)		(22,801,259)
Liabilities
Reserve for losses and loss adjustment expenses	(5,200,531)		(5,491,845)
Unearned premiums	(847,491)		(754,793)
Reinsurance balances payable	(685,320)		(535,239)
Contractholder payables	0		0
Collateral held for insured obligations			0
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	(1,882,263)		(1,802,697)
Other liabilities	(496,698)		(467,484)
Total liabilities	(9,112,303)		(9,052,058)
Redeemable noncontrolling interests	(14,700)		(14,700)
Shareholders' Equity
Total shareholders’ equity available to Arch	(15,538,663)		(13,734,501)
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	(15,538,663)		(13,734,501)
Total liabilities, noncontrolling interests and shareholders’ equity	$ (24,665,666)		$ (22,801,259)