Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 602.9	$ 646.4	$ 1,183.0	$ 1,214.9
Net investment income	55.8	42.8	109.1	87.3
Net realized investment gains (losses)	74.5	(20.2)	93.4	24.8
Other insurance-related income	5.2	0.9	5.7	1.9
Net losses and loss expenses	(365.4)	(431.5)	(737.8)	(756.7)
Acquisition costs	(87.4)	(100.6)	(175.7)	(179.3)
General and administrative expenses	(104.7)	(108.4)	(201.1)	(205.5)
Other insurance-related expenses	(1.7)	(1.2)	(2.8)	(3.1)
Amortization of intangible assets	(2.5)	(2.8)	(5.0)	(3.4)
Interest expense	(19.9)	(14.5)	(39.8)	(28.8)
Foreign exchange loss	2.9	(1.3)	5.9	(11.2)
Income tax (expense) benefit	(6.3)	(0.1)	(7.4)	(7.1)
Equity in earnings of consolidated subsidiaries	0.0	0.0	0.0	0.0
NET INCOME	153.4	9.5	227.5	133.8
Other comprehensive loss	0.1	(3.3)	3.3	(3.3)
COMPREHENSIVE INCOME (LOSS)	153.5	6.2	230.8	130.5
Consolidating Adjustments [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net realized investment gains (losses)	0.0	0.0	0.0	0.0
Other insurance-related income	0.0	0.0	0.0	0.0
Net losses and loss expenses	0.0	0.0	0.0	0.0
Acquisition costs	0.0	0.0	0.0	0.0
General and administrative expenses	0.0	0.0	0.0	0.0
Other insurance-related expenses	0.0	0.0	0.0	0.0
Amortization of intangible assets	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Foreign exchange loss	0.0	0.0	0.0	0.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Equity in earnings of consolidated subsidiaries	(323.1)	(51.0)	(492.8)	(329.1)
NET INCOME	(323.1)	(51.0)	(492.8)	(329.1)
Other comprehensive loss	(0.1)	3.3	(3.3)	3.3
COMPREHENSIVE INCOME (LOSS)	(323.2)	(47.7)	(496.1)	(325.8)
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net realized investment gains (losses)	0.0	0.0	0.0	0.0
Other insurance-related income	0.0	0.0	0.0	0.0
Net losses and loss expenses	0.0	0.0	0.0	0.0
Acquisition costs	0.0	0.0	0.0	0.0
General and administrative expenses	2.0	(10.2)	2.6	(19.6)
Other insurance-related expenses	0.0	0.0	0.0	0.0
Amortization of intangible assets	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Foreign exchange loss	(0.1)	0.0	(0.1)	0.0
Income tax (expense) benefit	(1.0)	0.6	(1.3)	(0.1)
Equity in earnings of consolidated subsidiaries	152.5	19.1	226.3	153.5
NET INCOME	153.4	9.5	227.5	133.8
Other comprehensive loss	0.1	(3.3)	3.3	(3.3)
COMPREHENSIVE INCOME (LOSS)	153.5	6.2	230.8	130.5
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Net investment income	0.0	0.0	0.0	0.0
Net realized investment gains (losses)	0.0	0.0	0.0	0.0
Other insurance-related income	0.0	0.0	0.0	0.0
Net losses and loss expenses	0.0	0.0	0.0	0.0
Acquisition costs	0.0	0.0	0.0	0.0
General and administrative expenses	(0.1)	(0.4)	0.4	(0.2)
Other insurance-related expenses	0.0	0.0	0.0	0.0
Amortization of intangible assets	0.0	0.0	0.0	0.0
Interest expense	(19.4)	(13.9)	(38.8)	(27.7)
Foreign exchange loss	0.0	0.0	0.0	0.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Equity in earnings of consolidated subsidiaries	170.6	31.9	266.5	175.6
NET INCOME	151.1	17.6	228.1	147.7
Other comprehensive loss	0.0	0.0	0.0	0.0
COMPREHENSIVE INCOME (LOSS)	151.1	17.6	228.1	147.7
Other Allied World Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	602.9	646.4	1,183.0	1,214.9
Net investment income	55.8	42.8	109.1	87.3
Net realized investment gains (losses)	74.5	(20.2)	93.4	24.8
Other insurance-related income	5.2	0.9	5.7	1.9
Net losses and loss expenses	(365.4)	(431.5)	(737.8)	(756.7)
Acquisition costs	(87.4)	(100.6)	(175.7)	(179.3)
General and administrative expenses	(106.6)	(97.8)	(204.1)	(185.7)
Other insurance-related expenses	(1.7)	(1.2)	(2.8)	(3.1)
Amortization of intangible assets	(2.5)	(2.8)	(5.0)	(3.4)
Interest expense	(0.5)	(0.6)	(1.0)	(1.1)
Foreign exchange loss	3.0	(1.3)	6.0	(11.2)
Income tax (expense) benefit	(5.3)	(0.7)	(6.1)	(7.0)
Equity in earnings of consolidated subsidiaries	0.0	0.0	0.0	0.0
NET INCOME	172.0	33.4	264.7	181.4
Other comprehensive loss	0.1	(3.3)	3.3	(3.3)
COMPREHENSIVE INCOME (LOSS)	$ 172.1	$ 30.1	$ 268.0	$ 178.1