Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Balance Sheet) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
ASSETS:
Investments	$ 7,694.9	$ 7,942.3
Cash and cash equivalents	1,284.9	720.9	$ 772.0	$ 608.0
Insurance balances receivable	862.2	784.0
Funds held	297.1	466.8
Reinsurance recoverable	1,725.6	1,625.0	1,512.0	1,480.0
Reinsurance recoverable on paid losses	119.9	104.4
Net deferred acquisition costs	148.9	121.1
Goodwill and intangible assets	492.8	494.4
Balances receivable on sale of investments	23.9	114.7
Investments in subsidiaries	0.0	0.0
Due from subsidiaries	0.0	0.0
Other assets	777.3	805.4
Total assets	13,427.5	13,179.0
LIABILITIES:
Reserve for losses and loss expenses	6,762.7	6,639.2	6,575.1	6,456.2
Unearned premiums	1,813.2	1,688.1
Reinsurance balances payable	202.9	223.3
Balances due on purchases of investments	58.2	79.7
Senior notes	794.4	794.2
Other long-term debt	22.6	22.0
Due to subsidiaries	0.0	0.0
Other liabilities	135.2	180.7
Total liabilities	9,789.2	9,627.2
Total shareholders’ equity	3,638.3	3,551.8	3,535.4
Total liabilities and shareholders’ equity	$ 13,427.5	13,179.0
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	(6,676.3)	(7,453.8)
Due from subsidiaries	(607.3)	(145.5)
Other assets	0.0	0.0
Total assets	(7,283.6)	(7,599.3)
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	(607.3)	(145.6)
Other liabilities	0.0	0.0
Total liabilities	(607.3)	(145.6)
Total shareholders’ equity	(6,676.3)	(7,453.7)
Total liabilities and shareholders’ equity	(7,283.6)	(7,599.3)
Allied World Switzerland (Parent Guarantor) [Member]
ASSETS:
Investments	0.0	0.0
Cash and cash equivalents	14.8	13.1	18.5	21.8
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	3,068.0	3,433.7
Due from subsidiaries	567.4	115.6
Other assets	1.1	1.8
Total assets	3,651.3	3,564.2
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	9.4	10.0
Other liabilities	3.6	2.4
Total liabilities	13.0	12.4
Total shareholders’ equity	3,638.3	3,551.8
Total liabilities and shareholders’ equity	3,651.3	3,564.2
Allied World Bermuda (Subsidiary Issuer) [Member]
ASSETS:
Investments	0.0	0.0
Cash and cash equivalents	25.8	0.8	1.4	1.0
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	3,608.3	4,020.1
Due from subsidiaries	22.9	0.0
Other assets	0.8	0.8
Total assets	3,657.8	4,021.7
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	794.4	794.2
Other long-term debt	0.0	0.0
Due to subsidiaries	7.6	20.0
Other liabilities	15.3	5.7
Total liabilities	817.3	819.9
Total shareholders’ equity	2,840.5	3,201.8
Total liabilities and shareholders’ equity	3,657.8	4,021.7
Other Allied World Subsidiaries [Member]
ASSETS:
Investments	7,694.9	7,942.3
Cash and cash equivalents	1,244.3	707.0	$ 752.1	$ 585.2
Insurance balances receivable	862.2	784.0
Funds held	297.1	466.8
Reinsurance recoverable	1,725.6	1,625.0
Reinsurance recoverable on paid losses	119.9	104.4
Net deferred acquisition costs	148.9	121.1
Goodwill and intangible assets	492.8	494.4
Balances receivable on sale of investments	23.9	114.7
Investments in subsidiaries	0.0	0.0
Due from subsidiaries	17.0	29.9
Other assets	775.4	802.8
Total assets	13,402.0	13,192.4
LIABILITIES:
Reserve for losses and loss expenses	6,762.7	6,639.2
Unearned premiums	1,813.2	1,688.1
Reinsurance balances payable	202.9	223.3
Balances due on purchases of investments	58.2	79.7
Senior notes	0.0	0.0
Other long-term debt	22.6	22.0
Due to subsidiaries	590.3	115.6
Other liabilities	116.3	172.6
Total liabilities	9,566.2	8,940.5
Total shareholders’ equity	3,835.8	4,251.9
Total liabilities and shareholders’ equity	$ 13,402.0	$ 13,192.4