Guarantor Financial Information - Condensed consolidating balance sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Assets
Total investments	$ 20,699,152		$ 19,719,651
Cash	740,320		842,942	$ 516,591		$ 553,326
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	0		0
Premiums receivable	1,314,564		1,072,435
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,155,107		2,114,138
Contractholder receivables	1,826,966		1,717,436
Ceded unearned premiums	961,330		859,567
Deferred acquisition costs	506,748		447,560
Goodwill and intangible assets	712,975		781,553	88,327
Other assets	1,945,477		1,816,827
Total assets	30,862,639		29,372,109
Liabilities
Reserve for losses and loss adjustment expenses	10,520,511	$ 10,296,821	10,200,960	9,471,647	$ 9,378,987	9,125,250
Unearned premiums	3,679,651		3,406,870
Reinsurance balances payable	361,000		300,407
Contractholder payables	1,826,966		1,717,436
Collateral held for insured obligations	338,737		301,406
Deposit accounting liabilities			22,150
Senior notes	1,732,570		1,732,258
Revolving credit agreement borrowings	686,452		756,650
Due to subsidiaries and affiliates	0		0
Other liabilities	1,734,673		1,622,847
Total liabilities	20,880,560		20,060,984
Redeemable noncontrolling interests	205,736		205,553
Shareholders' Equity
Total shareholders’ equity available to Arch	8,898,887		8,253,718	6,665,583
Non-redeemable noncontrolling interests	877,456		851,854	788,589
Total shareholders’ equity	9,776,343		9,105,572	7,454,172
Total liabilities, noncontrolling interests and shareholders’ equity	30,862,639		29,372,109
Reportable Legal Entities | ACGL (Parent Guarantor)
Assets
Total investments	258		2,612
Cash	2,742		1,687	7,212		6,809
Investments in subsidiaries	9,292,102		8,660,586
Due from subsidiaries and affiliates	27		14,297
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	14,101		15,725
Total assets	9,309,230		8,694,907
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
Deposit accounting liabilities			0
Senior notes	297,007		296,957
Revolving credit agreement borrowings	100,000		100,000
Due to subsidiaries and affiliates	301		26,270
Other liabilities	13,035		17,962
Total liabilities	410,343		441,189
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	8,898,887		8,253,718
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	8,898,887		8,253,718
Total liabilities, noncontrolling interests and shareholders’ equity	9,309,230		8,694,907
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	69,569		41,672
Cash	46,002		71,955	15,726		17,023
Investments in subsidiaries	3,967,022		3,716,681
Due from subsidiaries and affiliates	248		51,298
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	65,626		49,244
Total assets	4,148,467		3,930,850
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
Deposit accounting liabilities			0
Senior notes	494,572		494,525
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	536,831		535,584
Other liabilities	60,878		54,823
Total liabilities	1,092,281		1,084,932
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	3,056,186		2,845,918
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	3,056,186		2,845,918
Total liabilities, noncontrolling interests and shareholders’ equity	$ 4,148,467		3,930,850
Percentage Ownership of Subsidiary	100.00%
Reportable Legal Entities | Other ACGL Subsidiaries
Assets
Total investments	$ 20,644,025		19,690,067
Cash	691,576		769,300	493,653		529,494
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	1,888,499		1,866,681
Premiums receivable	1,948,630		1,579,865
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	6,390,582		6,114,518
Contractholder receivables	1,826,966		1,717,436
Ceded unearned premiums	2,218,529		1,985,311
Deferred acquisition costs	651,295		577,461
Goodwill and intangible assets	712,975		781,553
Other assets	2,042,421		1,901,786
Total assets	39,015,498		36,983,978
Liabilities
Reserve for losses and loss adjustment expenses	14,728,976		14,164,191
Unearned premiums	4,936,850		4,532,614
Reinsurance balances payable	995,065		807,837
Contractholder payables	1,826,966		1,717,436
Collateral held for insured obligations	338,737		301,406
Deposit accounting liabilities			22,150
Senior notes	940,991		940,776
Revolving credit agreement borrowings	586,452		656,650
Due to subsidiaries and affiliates	1,352,313		1,370,422
Other liabilities	2,008,318		1,867,040
Total liabilities	27,714,668		26,380,522
Redeemable noncontrolling interests	220,436		220,253
Shareholders' Equity
Total shareholders’ equity available to Arch	10,202,938		9,531,349
Non-redeemable noncontrolling interests	877,456		851,854
Total shareholders’ equity	11,080,394		10,383,203
Total liabilities, noncontrolling interests and shareholders’ equity	39,015,498		36,983,978
Consolidating Adjustments and Eliminations
Assets
Total investments	(14,700)		(14,700)
Cash	0		0	$ 0		$ 0
Investments in subsidiaries	(13,259,124)		(12,377,267)
Due from subsidiaries and affiliates	(1,888,774)		(1,932,276)
Premiums receivable	(634,066)		(507,430)
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	(4,235,475)		(4,000,380)
Contractholder receivables	0		0
Ceded unearned premiums	(1,257,199)		(1,125,744)
Deferred acquisition costs	(144,547)		(129,901)
Goodwill and intangible assets	0		0
Other assets	(176,671)		(149,928)
Total assets	(21,610,556)		(20,237,626)
Liabilities
Reserve for losses and loss adjustment expenses	(4,208,465)		(3,963,231)
Unearned premiums	(1,257,199)		(1,125,744)
Reinsurance balances payable	(634,065)		(507,430)
Contractholder payables	0		0
Collateral held for insured obligations			0
Deposit accounting liabilities			0
Senior notes	0		0
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	(1,889,445)		(1,932,276)
Other liabilities	(347,558)		(316,978)
Total liabilities	(8,336,732)		(7,845,659)
Redeemable noncontrolling interests	(14,700)		(14,700)
Shareholders' Equity
Total shareholders’ equity available to Arch	(13,259,124)		(12,377,267)
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	(13,259,124)		(12,377,267)
Total liabilities, noncontrolling interests and shareholders’ equity	$ (21,610,556)		$ (20,237,626)