Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 584.0	$ 650.7	$ 1,767.1	$ 1,865.6
Net investment income	50.6	45.7	159.7	133.0
Net realized investment gains (losses)	10.7	(113.6)	104.0	(88.8)
Other income	1.8	0.7	7.6	2.5
Net losses and loss expenses	(376.3)	(416.9)	(1,114.1)	(1,173.6)
Acquisition costs	(82.0)	(100.1)	(257.7)	(279.4)
General and administrative expenses	(104.2)	(105.8)	(305.3)	(311.3)
Other expense	(2.2)	(1.3)	(5.0)	(4.3)
Amortization of intangible assets	(2.5)	(2.6)	(7.5)	(6.1)
Interest expense	(13.5)	(14.5)	(53.3)	(43.2)
Foreign exchange gain (loss)	(1.0)	0.8	4.9	(10.4)
Income tax (expense) benefit	3.2	5.3	(4.3)	(1.8)
Equity in earnings of consolidated subsidiaries	0.0	0.0	0.0	0.0
NET INCOME (LOSS)	68.6	(51.6)	296.1	82.2
Other comprehensive income (loss)	0.6	(1.0)	3.9	(4.3)
COMPREHENSIVE INCOME (LOSS)	69.2	(52.6)	300.0	77.9
Consolidating Adjustments
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 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 expense	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 gain (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	(168.3)	80.7	(661.2)	(248.5)
NET INCOME (LOSS)	(168.3)	80.7	(661.2)	(248.5)
Other comprehensive income (loss)	(0.6)	1.0	(3.9)	4.3
COMPREHENSIVE INCOME (LOSS)	(168.9)	81.7	(665.1)	(244.2)
Allied World Switzerland (Parent Guarantor) [Member] | Reportable Legal Entities
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 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	(9.5)	(7.4)	(7.0)	(27.1)
Other expense	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 gain (loss)	0.0	0.0	(0.1)	0.0
Income tax (expense) benefit	0.5	(0.5)	(0.8)	(0.6)
Equity in earnings of consolidated subsidiaries	77.6	(43.7)	304.0	109.9
NET INCOME (LOSS)	68.6	(51.6)	296.1	82.2
Other comprehensive income (loss)	0.6	(1.0)	3.9	(4.3)
COMPREHENSIVE INCOME (LOSS)	69.2	(52.6)	300.0	77.9
Allied World Bermuda (Subsidiary Issuer) [Member] | Reportable Legal Entities
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0	0.0	0.0
Net investment income	0.1	0.0	0.1	0.0
Net realized investment gains (losses)	0.0	0.0	0.0	0.0
Other 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	(0.2)	0.6	0.3	0.4
Other expense	0.0	0.0	0.0	0.0
Amortization of intangible assets	0.0	0.0	0.0	0.0
Interest expense	(13.0)	(13.9)	(51.8)	(41.6)
Foreign exchange gain (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	90.7	(37.0)	357.2	138.6
NET INCOME (LOSS)	77.6	(50.3)	305.8	97.4
Other comprehensive income (loss)	0.0	0.0	0.0	0.0
COMPREHENSIVE INCOME (LOSS)	77.6	(50.3)	305.8	97.4
Other Allied World Subsidiaries [Member] | Reportable Legal Entities
Condensed Income Statements, Captions [Line Items]
Net premiums earned	584.0	650.7	1,767.1	1,865.6
Net investment income	50.5	45.7	159.6	133.0
Net realized investment gains (losses)	10.7	(113.6)	104.0	(88.8)
Other income	1.8	0.7	7.6	2.5
Net losses and loss expenses	(376.3)	(416.9)	(1,114.1)	(1,173.6)
Acquisition costs	(82.0)	(100.1)	(257.7)	(279.4)
General and administrative expenses	(94.5)	(99.0)	(298.6)	(284.6)
Other expense	(2.2)	(1.3)	(5.0)	(4.3)
Amortization of intangible assets	(2.5)	(2.6)	(7.5)	(6.1)
Interest expense	(0.5)	(0.6)	(1.5)	(1.6)
Foreign exchange gain (loss)	(1.0)	0.8	5.0	(10.4)
Income tax (expense) benefit	2.7	5.8	(3.5)	(1.2)
Equity in earnings of consolidated subsidiaries	0.0	0.0	0.0	0.0
NET INCOME (LOSS)	90.7	(30.4)	355.4	151.1
Other comprehensive income (loss)	0.6	(1.0)	3.9	(4.3)
COMPREHENSIVE INCOME (LOSS)	$ 91.3	$ (31.4)	$ 359.3	$ 146.8