Guarantor Financial Information - Condensed consolidating statement of cash flows (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016
Operating Activities
Net Cash Provided by (Used in) Operating Activities		$ 554,067	$ 541,224
Investing Activities
Purchases of fixed maturity investments		(19,899,326)	(17,541,731)
Purchases of equity securities		(400,155)	(212,678)
Purchases of other investments		(883,704)	(650,613)
Proceeds from sales of fixed maturity investments		19,611,680	16,978,549
Proceeds from sales of equity securities		473,064	337,619
Proceeds from sales, redemptions and maturities of other investments		614,494	636,535
Proceeds from redemptions and maturities of fixed maturity investments		447,941	370,980
Net settlements of derivative instruments		(5,984)	45,174
Net (purchases) sales of short-term investments		(445,203)	(304,460)
Change in cash collateral related to securities lending		175,693	(18,715)
Contributions to subsidiaries		0	0
Issuance of intercompany loans		0
Repayment of intercompany loans		0
Acquisitions, net of cash		0	(1,460)
Purchases of fixed assets		(11,103)	(8,284)
Other		(13,792)	13,416
Net Cash Provided By (Used For) Investing Activities		(336,395)	(355,668)
Financing Activities
Purchases of common shares under share repurchase program		0	(75,256)
Proceeds from common shares issued, net		(6,838)	(1,487)
Proceeds From Intercompany Loans Financing Activities		0
Proceeds from borrowings		0	46,000
Repayments of Intercompany Debt Financing Activities		0
Repayments of borrowings		(72,000)	(179,171)
Change in cash collateral related to securities lending		(175,693)	18,715
Dividends paid to redeemable noncontrolling interests		(8,994)	(8,994)
Dividends paid to parent	[1]	0	0
Other		(41,698)	(2,223)
Preferred dividends paid		(22,567)	(10,969)
Net Cash Provided By (Used For) Financing Activities		(327,790)	(213,385)
Effects of exchange rate changes on foreign currency cash		7,496	(8,906)
Increase (decrease) in cash		(102,622)	(36,735)
Cash beginning of year		842,942	553,326
Cash end of period		740,320	516,591
Reportable Legal Entities | ACGL (Parent Guarantor)
Operating Activities
Net Cash Provided by (Used in) Operating Activities		7,483	89,499
Investing Activities
Purchases of fixed maturity investments		0	0
Purchases of equity securities		0	0
Purchases of other investments		0	0
Proceeds from sales of fixed maturity investments		0	0
Proceeds from sales of equity securities		0	0
Proceeds from sales, redemptions and maturities of other investments		0	0
Proceeds from redemptions and maturities of fixed maturity investments		0	0
Net settlements of derivative instruments		0	0
Net (purchases) sales of short-term investments		2,354	(76)
Change in cash collateral related to securities lending		0	0
Contributions to subsidiaries		20,641	(3,300)
Issuance of intercompany loans		0
Repayment of intercompany loans		0
Acquisitions, net of cash			0
Purchases of fixed assets		(18)	(8)
Other		0	2,000
Net Cash Provided By (Used For) Investing Activities		22,977	(1,384)
Financing Activities
Purchases of common shares under share repurchase program			(75,256)
Proceeds from common shares issued, net		(6,838)	(1,487)
Proceeds From Intercompany Loans Financing Activities		0
Proceeds from borrowings			0
Repayments of Intercompany Debt Financing Activities		0
Repayments of borrowings		0	0
Change in cash collateral related to securities lending		0	0
Dividends paid to redeemable noncontrolling interests		0	0
Dividends paid to parent	[1]	0	0
Other		0	0
Preferred dividends paid		(22,567)	(10,969)
Net Cash Provided By (Used For) Financing Activities		(29,405)	(87,712)
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash		1,055	403
Cash beginning of year		1,687	6,809
Cash end of period		2,742	7,212
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Operating Activities
Net Cash Provided by (Used in) Operating Activities		27,853	10,732
Investing Activities
Purchases of fixed maturity investments		0	0
Purchases of equity securities		0	0
Purchases of other investments		0	0
Proceeds from sales of fixed maturity investments		0	0
Proceeds from sales of equity securities		0	0
Proceeds from sales, redemptions and maturities of other investments		0	0
Proceeds from redemptions and maturities of fixed maturity investments		0	41,500
Net settlements of derivative instruments		0	0
Net (purchases) sales of short-term investments		(27,896)	(53,729)
Change in cash collateral related to securities lending		0	0
Contributions to subsidiaries		(72,900)	0
Issuance of intercompany loans		0
Repayment of intercompany loans		47,000
Acquisitions, net of cash			0
Purchases of fixed assets		(10)	0
Other		0	0
Net Cash Provided By (Used For) Investing Activities		(53,806)	(12,229)
Financing Activities
Purchases of common shares under share repurchase program			0
Proceeds from common shares issued, net		0	0
Proceeds From Intercompany Loans Financing Activities		0
Proceeds from borrowings			0
Repayments of Intercompany Debt Financing Activities		0
Repayments of borrowings		0	0
Change in cash collateral related to securities lending		0	0
Dividends paid to redeemable noncontrolling interests		0	0
Dividends paid to parent	[1]	0	0
Other		0	200
Preferred dividends paid		0	0
Net Cash Provided By (Used For) Financing Activities		0	200
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash		(25,953)	(1,297)
Cash beginning of year		71,955	17,023
Cash end of period		46,002	15,726
Reportable Legal Entities | Other ACGL Subsidiaries
Operating Activities
Net Cash Provided by (Used in) Operating Activities		976,969	588,067
Investing Activities
Purchases of fixed maturity investments		(19,899,326)	(17,541,731)
Purchases of equity securities		(400,155)	(212,678)
Purchases of other investments		(883,704)	(650,613)
Proceeds from sales of fixed maturity investments		19,611,680	16,978,549
Proceeds from sales of equity securities		473,064	337,619
Proceeds from sales, redemptions and maturities of other investments		614,494	636,535
Proceeds from redemptions and maturities of fixed maturity investments		447,941	329,480
Net settlements of derivative instruments		(5,984)	45,174
Net (purchases) sales of short-term investments		(419,661)	(250,655)
Change in cash collateral related to securities lending		175,693	(18,715)
Contributions to subsidiaries		(342,950)	(2,779)
Issuance of intercompany loans		(47,000)
Repayment of intercompany loans		0
Acquisitions, net of cash			(1,460)
Purchases of fixed assets		(11,075)	(8,276)
Other		6,849	11,416
Net Cash Provided By (Used For) Investing Activities		(680,134)	(348,134)
Financing Activities
Purchases of common shares under share repurchase program			0
Proceeds from common shares issued, net		395,209	6,079
Proceeds From Intercompany Loans Financing Activities		47,000
Proceeds from borrowings			46,000
Repayments of Intercompany Debt Financing Activities		(47,000)
Repayments of borrowings		(72,000)	(179,171)
Change in cash collateral related to securities lending		(175,693)	18,715
Dividends paid to redeemable noncontrolling interests		(9,632)	(9,632)
Dividends paid to parent	[1]	(457,600)	(146,436)
Other		(62,339)	(2,423)
Preferred dividends paid		0	0
Net Cash Provided By (Used For) Financing Activities		(382,055)	(266,868)
Effects of exchange rate changes on foreign currency cash		7,496	(8,906)
Increase (decrease) in cash		(77,724)	(35,841)
Cash beginning of year		769,300	529,494
Cash end of period		691,576	493,653
Consolidating Adjustments and Eliminations
Operating Activities
Net Cash Provided by (Used in) Operating Activities		(458,238)	(147,074)
Investing Activities
Purchases of fixed maturity investments		0	0
Purchases of equity securities		0	0
Purchases of other investments		0	0
Proceeds from sales of fixed maturity investments		0	0
Proceeds from sales of equity securities		0	0
Proceeds from sales, redemptions and maturities of other investments		0	0
Proceeds from redemptions and maturities of fixed maturity investments		0	0
Net settlements of derivative instruments		0	0
Net (purchases) sales of short-term investments		0	0
Change in cash collateral related to securities lending		0	0
Contributions to subsidiaries		395,209	6,079
Issuance of intercompany loans		47,000
Repayment of intercompany loans		(47,000)
Acquisitions, net of cash			0
Purchases of fixed assets		0	0
Other		(20,641)	0
Net Cash Provided By (Used For) Investing Activities		374,568	6,079
Financing Activities
Purchases of common shares under share repurchase program			0
Proceeds from common shares issued, net		(395,209)	(6,079)
Proceeds From Intercompany Loans Financing Activities		(47,000)
Proceeds from borrowings			0
Repayments of Intercompany Debt Financing Activities		47,000
Repayments of borrowings		0	0
Change in cash collateral related to securities lending		0	0
Dividends paid to redeemable noncontrolling interests		638	638
Dividends paid to parent	[1]	457,600	146,436
Other		20,641	0
Preferred dividends paid		0	0
Net Cash Provided By (Used For) Financing Activities		83,670	140,995
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash		0	0
Cash beginning of year		0	0
Cash end of period		$ 0	$ 0

[1] Dividends received by parent are included in net cash provided by (used for) operating activities