Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014
ASSETS:
Investments	$ 8,936.3		$ 8,571.2
Cash and cash equivalents	584.5		608.0	$ 661.8		$ 589.3
Insurance balances receivable	961.0		745.9
Funds held	279.1		640.8
Reinsurance recoverable	1,516.7	$ 1,512.0	1,480.0	1,433.1	$ 1,350.3	1,340.3
Reinsurance recoverable on paid losses	73.8		96.4
Net deferred acquisition costs	168.9		165.2
Goodwill and intangible assets	506.1		504.7
Balances receivable on sale of investments	114.6		36.9
Investments in subsidiaries	0.0		0.0
Due from subsidiaries	0.0		0.0
Other assets	726.3		662.8
Total assets	13,867.3		13,511.9
LIABILITIES:
Reserve for losses and loss expenses	6,623.2	$ 6,575.1	6,456.2	6,363.9	$ 5,905.1	5,881.2
Unearned premiums	1,822.1		1,683.3
Reinsurance balances payable	227.7		214.4
Balances due on purchases of investments	155.0		125.1
Senior notes	1,293.7		1,292.9
Other long-term debt	23.1		23.0
Due to subsidiaries	0.0		0.0
Other liabilities	138.0		184.5
Total liabilities	10,282.8		9,979.4
Total shareholders’ equity	3,584.5		3,532.5	3,624.7		3,778.2
Total liabilities and shareholders’ equity	$ 13,867.3		13,511.9
Percentage ownership of Subsidiary Issuer	100.00%
Consolidating Adjustments [Member]
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
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	(7,572.0)		(7,743.3)
Due from subsidiaries	(473.2)		(226.3)
Other assets	0.0		0.0
Total assets	(8,045.2)		(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	(473.2)		(226.4)
Other liabilities	0.0		0.0
Total liabilities	(473.2)		(226.4)
Total shareholders’ equity	(7,572.0)		(7,743.2)
Total liabilities and shareholders’ equity	(8,045.2)		(7,969.6)
Allied World Switzerland (Parent Guarantor) [Member]
ASSETS:
Investments	0.0		0.0
Cash and cash equivalents	22.6		21.8	29.2		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	3,381.1		3,347.0
Due from subsidiaries	186.3		173.1
Other assets	1.5		1.8
Total assets	3,591.5		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	3.6		8.6
Other liabilities	3.4		2.6
Total liabilities	7.0		11.2
Total shareholders’ equity	3,584.5		3,532.5
Total liabilities and shareholders’ equity	3,591.5		3,543.7
Allied World Bermuda (Subsidiary Issuer) [Member]
ASSETS:
Investments	0.0		0.0
Cash and cash equivalents	1.1		1.0	1.9		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	4,190.9		4,396.3
Due from subsidiaries	275.6		36.4
Other assets	0.8		0.1
Total assets	4,468.4		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	1,293.7		1,292.9
Other long-term debt	0.0		0.0
Due to subsidiaries	7.7		8.3
Other liabilities	21.4		22.2
Total liabilities	1,322.8		1,323.4
Total shareholders’ equity	3,145.6		3,110.4
Total liabilities and shareholders’ equity	4,468.4		4,433.8
Other Allied World Subsidiaries [Member]
ASSETS:
Investments	8,936.3		8,571.2
Cash and cash equivalents	560.8		585.2	$ 630.7		$ 555.0
Insurance balances receivable	961.0		745.9
Funds held	279.1		640.8
Reinsurance recoverable	1,516.7		1,480.0
Reinsurance recoverable on paid losses	73.8		96.4
Net deferred acquisition costs	168.9		165.2
Goodwill and intangible assets	506.1		504.7
Balances receivable on sale of investments	114.6		36.9
Investments in subsidiaries	0.0		0.0
Due from subsidiaries	11.3		16.8
Other assets	724.0		660.9
Total assets	13,852.6		13,504.0
LIABILITIES:
Reserve for losses and loss expenses	6,623.2		6,456.2
Unearned premiums	1,822.1		1,683.3
Reinsurance balances payable	227.7		214.4
Balances due on purchases of investments	155.0		125.1
Senior notes	0.0		0.0
Other long-term debt	23.1		23.0
Due to subsidiaries	461.9		209.5
Other liabilities	113.2		159.7
Total liabilities	9,426.2		8,871.2
Total shareholders’ equity	4,426.4		4,632.8
Total liabilities and shareholders’ equity	$ 13,852.6		$ 13,504.0