Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
ASSETS:
Investments	$ 8,485.1		$ 8,571.2
Cash and cash equivalents	773.8		608.0	$ 603.8		$ 589.3
Insurance balances receivable	893.3		745.9
Funds held	263.9		640.8
Reinsurance recoverable	1,550.7	$ 1,516.7	1,480.0	1,449.8	$ 1,433.1	1,340.3
Reinsurance recoverable on paid losses	129.1		96.4
Net deferred acquisition costs	157.7		165.2
Goodwill and intangible assets	503.9		504.7
Balances receivable on sale of investments	20.3		36.9
Investments in subsidiaries	0.0		0.0
Due from subsidiaries	0.0		0.0
Other assets	764.6		662.8
Total assets	13,542.4		13,511.9
LIABILITIES:
Reserve for losses and loss expenses	6,665.8	$ 6,623.2	6,456.2	6,436.6	$ 6,363.9	5,881.2
Unearned premiums	1,785.2		1,683.3
Reinsurance balances payable	258.4		214.4
Balances due on purchases of investments	242.4		125.1
Senior notes	794.0		1,292.9
Other long-term debt	23.3		23.0
Due to subsidiaries	0.0		0.0
Other liabilities	157.4		184.5
Total liabilities	9,926.5		9,979.4
Total shareholders’ equity	3,615.9		3,532.5	3,555.4		3,778.3
Total liabilities and shareholders’ equity	$ 13,542.4		13,511.9
Percentage ownership of Subsidiary Issuer	100.00%
Consolidating Adjustments
ASSETS:
Investments	$ 0.0		0.0
Cash and cash equivalents	0.0		0.0	0.0		0.0
Insurance balances receivable	0.0		0.0
Funds held	0.0		0.0
Reinsurance recoverable	0.0		0.0
Reinsurance recoverable on paid losses	0.0		0.0
Net deferred acquisition costs	0.0		0.0
Goodwill and intangible assets	0.0		0.0
Balances receivable on sale of investments	0.0		0.0
Investments in subsidiaries	(7,504.8)		(7,743.3)
Due from subsidiaries	(169.2)		(226.3)
Other assets	0.0		0.0
Total assets	(7,674.0)		(7,969.6)
LIABILITIES:
Reserve for losses and loss expenses	0.0		0.0
Unearned premiums	0.0		0.0
Reinsurance balances payable	0.0		0.0
Balances due on purchases of investments	0.0		0.0
Senior notes	0.0		0.0
Other long-term debt	0.0		0.0
Due to subsidiaries	(169.2)		(226.4)
Other liabilities	0.0		0.0
Total liabilities	(169.2)		(226.4)
Total shareholders’ equity	(7,504.8)		(7,743.2)
Total liabilities and shareholders’ equity	(7,674.0)		(7,969.6)
Allied World Switzerland (Parent Guarantor) [Member] | Reportable Legal Entities
ASSETS:
Investments	0.0		0.0
Cash and cash equivalents	14.8		21.8	30.6		32.6
Insurance balances receivable	0.0		0.0
Funds held	0.0		0.0
Reinsurance recoverable	0.0		0.0
Reinsurance recoverable on paid losses	0.0		0.0
Net deferred acquisition costs	0.0		0.0
Goodwill and intangible assets	0.0		0.0
Balances receivable on sale of investments	0.0		0.0
Investments in subsidiaries	3,466.4		3,347.0
Due from subsidiaries	145.0		173.1
Other assets	2.1		1.8
Total assets	3,628.3		3,543.7
LIABILITIES:
Reserve for losses and loss expenses	0.0		0.0
Unearned premiums	0.0		0.0
Reinsurance balances payable	0.0		0.0
Balances due on purchases of investments	0.0		0.0
Senior notes	0.0		0.0
Other long-term debt	0.0		0.0
Due to subsidiaries	9.7		8.6
Other liabilities	2.7		2.6
Total liabilities	12.4		11.2
Total shareholders’ equity	3,615.9		3,532.5
Total liabilities and shareholders’ equity	3,628.3		3,543.7
Allied World Bermuda (Subsidiary Issuer) [Member] | Reportable Legal Entities
ASSETS:
Investments	0.0		0.0
Cash and cash equivalents	0.9		1.0	1.4		1.7
Insurance balances receivable	0.0		0.0
Funds held	0.0		0.0
Reinsurance recoverable	0.0		0.0
Reinsurance recoverable on paid losses	0.0		0.0
Net deferred acquisition costs	0.0		0.0
Goodwill and intangible assets	0.0		0.0
Balances receivable on sale of investments	0.0		0.0
Investments in subsidiaries	4,038.4		4,396.3
Due from subsidiaries	6.9		36.4
Other assets	0.8		0.1
Total assets	4,047.0		4,433.8
LIABILITIES:
Reserve for losses and loss expenses	0.0		0.0
Unearned premiums	0.0		0.0
Reinsurance balances payable	0.0		0.0
Balances due on purchases of investments	0.0		0.0
Senior notes	794.0		1,292.9
Other long-term debt	0.0		0.0
Due to subsidiaries	7.7		8.3
Other liabilities	15.3		22.2
Total liabilities	817.0		1,323.4
Total shareholders’ equity	3,230.0		3,110.4
Total liabilities and shareholders’ equity	4,047.0		4,433.8
Other Allied World Subsidiaries [Member] | Reportable Legal Entities
ASSETS:
Investments	8,485.1		8,571.2
Cash and cash equivalents	758.1		585.2	$ 571.8		$ 555.0
Insurance balances receivable	893.3		745.9
Funds held	263.9		640.8
Reinsurance recoverable	1,550.7		1,480.0
Reinsurance recoverable on paid losses	129.1		96.4
Net deferred acquisition costs	157.7		165.2
Goodwill and intangible assets	503.9		504.7
Balances receivable on sale of investments	20.3		36.9
Investments in subsidiaries	0.0		0.0
Due from subsidiaries	17.3		16.8
Other assets	761.7		660.9
Total assets	13,541.1		13,504.0
LIABILITIES:
Reserve for losses and loss expenses	6,665.8		6,456.2
Unearned premiums	1,785.2		1,683.3
Reinsurance balances payable	258.4		214.4
Balances due on purchases of investments	242.4		125.1
Senior notes	0.0		0.0
Other long-term debt	23.3		23.0
Due to subsidiaries	151.8		209.5
Other liabilities	139.4		159.7
Total liabilities	9,266.3		8,871.2
Total shareholders’ equity	4,274.8		4,632.8
Total liabilities and shareholders’ equity	$ 13,541.1		$ 13,504.0