Guarantor Financial Information - Condensed consolidating balance sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Total investments	$ 21,931,881		$ 19,719,651
Cash	862,361		842,942	$ 578,816		$ 553,326
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	0		0
Premiums receivable	1,269,678		1,072,435
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,506,015		2,114,138
Contractholder receivables	1,864,348		1,717,436
Ceded unearned premiums	947,135		859,567
Deferred acquisition costs	531,196		447,560
Goodwill and intangible assets	684,405		781,553	90,941
Other assets	2,027,778		1,816,827
Total assets	32,624,797		29,372,109
Liabilities
Reserve for losses and loss adjustment expenses	11,351,267	$ 10,520,511	10,200,960	9,610,189	$ 9,471,647	9,125,250
Unearned premiums	3,751,550		3,406,870
Reinsurance balances payable	352,006		300,407
Contractholder payables	1,864,348		1,717,436
Collateral held for insured obligations	345,726		301,406
Deposit accounting liabilities			22,150
Senior notes	1,732,726		1,732,258
Revolving credit agreement borrowings	826,242		756,650
Due to subsidiaries and affiliates	0		0
Other liabilities	2,423,061		1,622,847
Total liabilities	22,646,926		20,060,984
Redeemable noncontrolling interests	205,829		205,553
Shareholders' Equity
Total shareholders’ equity available to Arch	8,911,144		8,253,718	7,313,983
Non-redeemable noncontrolling interests	860,898		851,854	834,808
Total shareholders’ equity	9,772,042		9,105,572	8,148,791
Total liabilities, noncontrolling interests and shareholders’ equity	32,624,797		29,372,109
Reportable Legal Entities | ACGL (Parent Guarantor)
Assets
Total investments	402		2,612
Cash	4,868		1,687	2,041		6,809
Investments in subsidiaries	9,214,178		8,660,586
Due from subsidiaries and affiliates	81		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	13,357		15,725
Total assets	9,232,886		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	297,030		296,957
Revolving credit agreement borrowings	0		100,000
Due to subsidiaries and affiliates	41		26,270
Other liabilities	24,671		17,962
Total liabilities	321,742		441,189
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	8,911,144		8,253,718
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	8,911,144		8,253,718
Total liabilities, noncontrolling interests and shareholders’ equity	9,232,886		8,694,907
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Assets
Total investments	69,671		41,672
Cash	88,818		71,955	19,392		17,023
Investments in subsidiaries	4,028,493		3,716,681
Due from subsidiaries and affiliates	117		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	59,734		49,244
Total assets	4,246,833		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	494,596		494,525
Revolving credit agreement borrowings	0		0
Due to subsidiaries and affiliates	542,103		535,584
Other liabilities	100,278		54,823
Total liabilities	1,136,977		1,084,932
Redeemable noncontrolling interests	0		0
Shareholders' Equity
Total shareholders’ equity available to Arch	3,109,856		2,845,918
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	3,109,856		2,845,918
Total liabilities, noncontrolling interests and shareholders’ equity	$ 4,246,833		3,930,850
Percentage Ownership of Subsidiary	100.00%
Reportable Legal Entities | Other ACGL Subsidiaries
Assets
Total investments	$ 21,876,508		19,690,067
Cash	768,675		769,300	557,383		529,494
Investments in subsidiaries	0		0
Due from subsidiaries and affiliates	1,923,941		1,866,681
Premiums receivable	1,863,203		1,579,865
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	7,023,836		6,114,518
Contractholder receivables	1,864,348		1,717,436
Ceded unearned premiums	2,240,129		1,985,311
Deferred acquisition costs	677,567		577,461
Goodwill and intangible assets	684,405		781,553
Other assets	2,146,993		1,901,786
Total assets	41,069,605		36,983,978
Liabilities
Reserve for losses and loss adjustment expenses	15,836,263		14,164,191
Unearned premiums	5,044,544		4,532,614
Reinsurance balances payable	945,530		807,837
Contractholder payables	1,864,348		1,717,436
Collateral held for insured obligations	345,726		301,406
Deposit accounting liabilities			22,150
Senior notes	941,100		940,776
Revolving credit agreement borrowings	826,242		656,650
Due to subsidiaries and affiliates	1,381,996		1,370,422
Other liabilities	2,669,614		1,867,040
Total liabilities	29,855,363		26,380,522
Redeemable noncontrolling interests	220,529		220,253
Shareholders' Equity
Total shareholders’ equity available to Arch	10,132,815		9,531,349
Non-redeemable noncontrolling interests	860,898		851,854
Total shareholders’ equity	10,993,713		10,383,203
Total liabilities, noncontrolling interests and shareholders’ equity	41,069,605		36,983,978
Consolidating Adjustments and Eliminations
Assets
Total investments	(14,700)		(14,700)
Cash	0		0	$ 0		$ 0
Investments in subsidiaries	(13,242,671)		(12,377,267)
Due from subsidiaries and affiliates	(1,924,139)		(1,932,276)
Premiums receivable	(593,525)		(507,430)
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	(4,517,821)		(4,000,380)
Contractholder receivables	0		0
Ceded unearned premiums	(1,292,994)		(1,125,744)
Deferred acquisition costs	(146,371)		(129,901)
Goodwill and intangible assets	0		0
Other assets	(192,306)		(149,928)
Total assets	(21,924,527)		(20,237,626)
Liabilities
Reserve for losses and loss adjustment expenses	(4,484,996)		(3,963,231)
Unearned premiums	(1,292,994)		(1,125,744)
Reinsurance balances payable	(593,524)		(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	(1,924,140)		(1,932,276)
Other liabilities	(371,502)		(316,978)
Total liabilities	(8,667,156)		(7,845,659)
Redeemable noncontrolling interests	(14,700)		(14,700)
Shareholders' Equity
Total shareholders’ equity available to Arch	(13,242,671)		(12,377,267)
Non-redeemable noncontrolling interests	0		0
Total shareholders’ equity	(13,242,671)		(12,377,267)
Total liabilities, noncontrolling interests and shareholders’ equity	$ (21,924,527)		$ (20,237,626)