Condensed Consolidated Guarantor Financial Statements (Schedule Of Condensed Consolidating Income Statement) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Condensed Income Statements, Captions [Line Items]
Net premiums earned	$ 544.9	$ 580.1
Net investment income	52.3	53.3
Net realized investment gains	40.7	18.9
Other income	1.3	0.6
Net losses and loss expenses	(359.0)	(372.4)
Acquisition costs	(77.1)	(88.3)
General and administrative expenses	(104.1)	(96.4)
Other expense	(7.1)	(1.1)
Amortization of intangible assets	(2.3)	(2.5)
Interest expense	(10.4)	(20.0)
Foreign exchange gain (loss)	(1.4)	3.0
Income tax (expense) benefit	2.5	(1.1)
Equity in earnings of consolidated subsidiaries	0.0	0.0
NET INCOME	80.3	74.1
Other comprehensive income (loss)	5.7	3.1
COMPREHENSIVE INCOME (LOSS)	86.0	77.2
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	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 expense	0.0	0.0
Amortization of intangible assets	0.0	0.0
Interest expense	0.0	0.0
Foreign exchange gain (loss)	0.0	0.0
Income tax (expense) benefit	0.0	0.0
Equity in earnings of consolidated subsidiaries	(197.7)	(169.7)
NET INCOME	(197.7)	(169.7)
Other comprehensive income (loss)	(5.7)	(3.1)
COMPREHENSIVE INCOME (LOSS)	(203.4)	(172.8)
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	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	(9.1)	0.6
Other expense	(0.9)	0.0
Amortization of intangible assets	0.0	0.0
Interest expense	0.0	0.0
Foreign exchange gain (loss)	0.0	0.0
Income tax (expense) benefit	(0.9)	(0.3)
Equity in earnings of consolidated subsidiaries	91.2	73.8
NET INCOME	80.3	74.1
Other comprehensive income (loss)	5.7	3.1
COMPREHENSIVE INCOME (LOSS)	86.0	77.2
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	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	(0.3)	0.5
Other expense	(0.1)	0.0
Amortization of intangible assets	0.0	0.0
Interest expense	(9.8)	(19.4)
Foreign exchange gain (loss)	0.0	0.0
Income tax (expense) benefit	0.0	0.0
Equity in earnings of consolidated subsidiaries	106.5	95.9
NET INCOME	96.3	77.0
Other comprehensive income (loss)	0.0	0.0
COMPREHENSIVE INCOME (LOSS)	96.3	77.0
Other Allied World Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Net premiums earned	544.9	580.1
Net investment income	52.3	53.3
Net realized investment gains	40.7	18.9
Other income	1.3	0.6
Net losses and loss expenses	(359.0)	(372.4)
Acquisition costs	(77.1)	(88.3)
General and administrative expenses	(94.7)	(97.5)
Other expense	(6.1)	(1.1)
Amortization of intangible assets	(2.3)	(2.5)
Interest expense	(0.6)	(0.6)
Foreign exchange gain (loss)	(1.4)	3.0
Income tax (expense) benefit	3.4	(0.8)
Equity in earnings of consolidated subsidiaries	0.0	0.0
NET INCOME	101.4	92.7
Other comprehensive income (loss)	5.7	3.1
COMPREHENSIVE INCOME (LOSS)	$ 107.1	$ 95.8