Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 580.1	$ 568.5
Net investment income	53.3	44.6
Net realized investment gains (losses)	18.9	45.0
Other income	0.6	0.9
Net losses and loss expenses	(372.4)	(325.2)
Acquisition costs	(88.3)	(78.7)
General and administrative expenses	(96.4)	(97.1)
Other insurance-related expenses	(1.1)	(1.8)
Amortization of intangible assets	(2.5)	(0.6)
Interest expense	(20.0)	(14.4)
Foreign exchange loss	3.0	(9.9)
Income tax (expense) benefit	(1.1)	(6.9)
Equity in earnings of consolidated subsidiaries	0.0	0.0
NET INCOME	74.1	124.4
Other comprehensive income	3.1	0.0
COMPREHENSIVE INCOME (LOSS)	77.2	124.4
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0
Net investment income	0.0	0.0
Net realized investment gains (losses)	0.0	0.0
Other income	0.0	0.0
Net losses and loss expenses	0.0	0.0
Acquisition costs	0.0	0.0
General and administrative expenses	0.0	0.0
Other insurance-related expenses	0.0	0.0
Amortization of intangible assets	0.0	0.0
Interest expense	0.0	0.0
Foreign exchange loss	0.0	0.0
Income tax (expense) benefit	0.0	0.0
Equity in earnings of consolidated subsidiaries	(169.7)	(278.1)
NET INCOME	(169.7)	(278.1)
Other comprehensive income	(3.1)	0.0
COMPREHENSIVE INCOME (LOSS)	(172.8)	(278.1)
Allied World Switzerland (Parent Guarantor) [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0
Net investment income	0.0	0.0
Net realized investment gains (losses)	0.0	0.0
Other income	0.0	0.0
Net losses and loss expenses	0.0	0.0
Acquisition costs	0.0	0.0
General and administrative expenses	0.6	(9.4)
Other insurance-related expenses	0.0	0.0
Amortization of intangible assets	0.0	0.0
Interest expense	0.0	0.0
Foreign exchange loss	0.0	0.0
Income tax (expense) benefit	(0.3)	(0.6)
Equity in earnings of consolidated subsidiaries	73.8	134.4
NET INCOME	74.1	124.4
Other comprehensive income	3.1	0.0
COMPREHENSIVE INCOME (LOSS)	77.2	124.4
Allied World Bermuda (Subsidiary Issuer) [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	0.0	0.0
Net investment income	0.0	0.0
Net realized investment gains (losses)	0.0	0.0
Other income	0.0	0.0
Net losses and loss expenses	0.0	0.0
Acquisition costs	0.0	0.0
General and administrative expenses	0.5	0.2
Other insurance-related expenses	0.0	0.0
Amortization of intangible assets	0.0	0.0
Interest expense	(19.4)	(13.9)
Foreign exchange loss	0.0	0.0
Income tax (expense) benefit	0.0	0.0
Equity in earnings of consolidated subsidiaries	95.9	143.7
NET INCOME	77.0	130.0
Other comprehensive income	0.0	0.0
COMPREHENSIVE INCOME (LOSS)	77.0	130.0
Other Allied World Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	580.1	568.5
Net investment income	53.3	44.6
Net realized investment gains (losses)	18.9	45.0
Other income	0.6	0.9
Net losses and loss expenses	(372.4)	(325.2)
Acquisition costs	(88.3)	(78.7)
General and administrative expenses	(97.5)	(87.9)
Other insurance-related expenses	(1.1)	(1.8)
Amortization of intangible assets	(2.5)	(0.6)
Interest expense	(0.6)	(0.5)
Foreign exchange loss	3.0	(9.9)
Income tax (expense) benefit	(0.8)	(6.3)
Equity in earnings of consolidated subsidiaries	0.0	0.0
NET INCOME	92.7	148.1
Other comprehensive income	3.1	0.0
COMPREHENSIVE INCOME (LOSS)	$ 95.8	$ 148.1