Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
ASSETS:
Investments	$ 8,710.4	$ 8,571.2
Cash and cash equivalents	772.0	608.0	$ 515.0	$ 589.3
Insurance balances receivable	885.4	745.9
Funds held	353.2	640.8
Reinsurance recoverable	1,512.0	1,480.0	1,350.3	1,340.3
Reinsurance recoverable on paid losses	82.7	96.4
Net deferred acquisition costs	186.9	165.2
Goodwill and intangible assets	505.4	504.7
Balances receivable on sale of investments	24.6	36.9
Investments in subsidiaries	0.0	0.0
Due from subsidiaries	0.0	0.0
Other assets	695.4	662.8
Total assets	13,728.0	13,511.9
LIABILITIES:
Reserve for losses and loss expenses	6,575.1	6,456.2	5,905.1	5,881.2
Unearned premiums	1,796.9	1,683.3
Reinsurance balances payable	221.6	214.4
Balances due on purchases of investments	148.0	125.1
Senior notes	1,293.3	1,292.9
Other long-term debt	23.3	23.0
Due to subsidiaries	0.0	0.0
Other liabilities	134.4	184.5
Total liabilities	10,192.6	9,979.4
Total shareholders’ equity	3,535.4	3,532.5	3,829.1	3,778.2
Total liabilities and shareholders’ equity	$ 13,728.0	13,511.9
Percentage ownership of Subsidiary Issuer	100.00%
Consolidation, Eliminations [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	(7,916.1)	(7,743.3)
Due from subsidiaries	(125.1)	(226.3)
Other assets	0.0	0.0
Total assets	(8,041.2)	(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
Due to subsidiaries	(125.1)	(226.4)
Other liabilities	0.0	0.0
Total liabilities	(125.1)	(226.4)
Total shareholders’ equity	(7,916.1)	(7,743.2)
Total liabilities and shareholders’ equity	(8,041.2)	(7,969.6)
Allied World Switzerland (Parent Guarantor) [Member]
ASSETS:
Investments	0.0	0.0
Cash and cash equivalents	18.5	21.8	36.4	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	3,422.4	3,347.0
Due from subsidiaries	97.8	173.1
Other assets	2.4	1.8
Total assets	3,541.1	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	2.4	8.6
Other liabilities	3.3	2.6
Total liabilities	5.7	11.2
Total shareholders’ equity	3,535.4	3,532.5
Total liabilities and shareholders’ equity	3,541.1	3,543.7
Allied World Bermuda (Subsidiary Issuer) [Member]
ASSETS:
Investments	0.0	0.0
Cash and cash equivalents	1.4	1.0	2.1	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	4,493.7	4,396.3
Due from subsidiaries	16.8	36.4
Other assets	0.0	0.1
Total assets	4,511.9	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	1,293.3	1,292.9
Other long-term debt	0.0	0.0
Due to subsidiaries	8.1	8.3
Other liabilities	21.6	22.2
Total liabilities	1,323.0	1,323.4
Total shareholders’ equity	3,188.9	3,110.4
Total liabilities and shareholders’ equity	4,511.9	4,433.8
Other Allied World Subsidiaries [Member]
ASSETS:
Investments	8,710.4	8,571.2
Cash and cash equivalents	752.1	585.2	$ 476.5	$ 555.0
Insurance balances receivable	885.4	745.9
Funds held	353.2	640.8
Reinsurance recoverable	1,512.0	1,480.0
Reinsurance recoverable on paid losses	82.7	96.4
Net deferred acquisition costs	186.9	165.2
Goodwill and intangible assets	505.4	504.7
Balances receivable on sale of investments	24.6	36.9
Investments in subsidiaries	0.0	0.0
Due from subsidiaries	10.5	16.8
Other assets	693.0	660.9
Total assets	13,716.2	13,504.0
LIABILITIES:
Reserve for losses and loss expenses	6,575.1	6,456.2
Unearned premiums	1,796.9	1,683.3
Reinsurance balances payable	221.6	214.4
Balances due on purchases of investments	148.0	125.1
Senior notes	0.0	0.0
Other long-term debt	23.3	23.0
Due to subsidiaries	114.6	209.5
Other liabilities	109.5	159.7
Total liabilities	8,989.0	8,871.2
Total shareholders’ equity	4,727.2	4,632.8
Total liabilities and shareholders’ equity	$ 13,716.2	$ 13,504.0